Wednesday, 28 March 2012

Railways facilitate dispersal of plants as well as people

Just as railway lines help people move from one place to another, so too do they appear to facilitate migration of plants, according to the findings of four collaborators from France's Museum National d'Histoire Naturelle. The researchers investigated whether railways--or, more specifically, railway edges--help connect plant communities in urban areas by providing routes of dispersal through inhospitable city environments.

Their work was conducted along two railway lines running through densely populated areas just south of Paris. The researchers surveyed vegetation at a total of 71 focal sites that were otherwise similar in their habitat features. They first looked for differences in plant community composition between the two lines; next, they focused within each single line in order to assess the similarity of plant communities across neighboring sites with and without "spatial breaks" (overpasses or train stations) between them. If railways help keep communities connected, then there should be greater plant community similarity among sites on a line than between the two different lines. Further, if this connectivity is disrupted by structures such as stations and overpasses, there should be greater similarity between sites that are not separated by these features.

The researchers also recognized that variations in plant dispersal technique might influence species' abilities to capitalize on the presence of railways--for instance, these routes might be more useful to species whose seeds are distributed by the wind than to those whose seeds simply drop straight down beneath the parent plant. Thus, all species were categorized as being highly mobile (wind pollination), moderately mobile (insect pollination, movement of seeds by animals), or poorly mobile (self-pollinating, seeds falling to ground beneath parent). Finally, each plant was assigned an "Ellenberg indicator value," which indicates its preference for particular light, temperature, moisture, and chemical conditions; since these vary with intensity of urbanization, the Ellenberg values help reveal species' sensitivities to anthropogenically disturbed environments.

(Some of the many methods of plant dispersal)

Across all sites, the scientists found 186 plant species, 95% of which were familiar from larger-scale biodiversity studies across the region. In general, floral communities were more similar within than between train lines, indicating that the railways were helping plants disperse amongst connected sites. Differences between and among lines were similar for invasive and wind-pollinated plants and for those whose seeds are transported in the guts of animals--likely because these species are already so mobile that the presence of the train route does not further improve their ability to spread.

Train stations did not have much of an effect on connectivity, but the presence of overpasses did: These structures generally reduced connectedness of neighboring sites. Again, wind-pollinated species and those whose seeds are dispersed in animal droppings were not significantly isolated by railway breaks, highlighting how effective these dispersal methods are at bridging the gap between distant habitat patches.

The degree of urbanization at each site was not a significant predictor of plant species richness or diversity. However, urban areas were more likely to have a higher number of invasive species, as well as having plants with Ellenberg values indicating a preference for high temperature and lots of nitrogen in the soil; urban areas also had fewer moisture-tolerant species. These results reflect the fact that urban habitats tend to be warmer, drier, and inundated with chemical runoff from anthropogenic products; species that can tolerate these conditions tend to do much better in disturbed environments like those along train tracks.

(A railway overpass)

On the whole, these results indicate that railway edges improve connectivity for urban plant species by allowing dispersal through hostile habitats. In fact, they appear to be especially helpful for much-beleaguered meadow species such as grasses and herbs. Surprisingly, and contrary to previous results, railways did not seem to promote the spread of invasive species; thus, managers may not have to worry about this negative side effect while reaping the rewards of railway connectivity. The benefit of edge habitats was dampened by the presence of overpasses, suggesting that urban planners might try to minimize use of these structures if they wish to improve conditions for floral communities in cities.
Alternatively, they might focus on modification techniques that could allow overpasses to become more useful. For instance, the authors suggest that these structures might be widened, deliberately planted with vegetation, or fitted with retaining walls so that seeds do not fall off before becoming implanted.

Penone, C., Machon, N., Julliard, R., and Le Viol, I. 2012. Do railway edges provide functional connectivity for plant communities in an urban context? Biological Conservation 148:126-133.

Thanks to the following websites for providing the images used in this post:

Saturday, 24 March 2012

Human-elephant conflict in southwestern India

Human-wildlife conflict is a rapidly expanding area of research, with conservationists working hard to understand the circumstances under which tensions are highest between people and their wild animal neighbors. A number of factors contribute to rate and intensity of conflict, including population densities of both humans and wildlife, habitat structure, weather, time of year, and animal life histories, to name just a few. As a result, separate assessments are required for each "problem" species in each area at risk; thus, while recent findings about elephant crop raiding behaviors in Africa may be illuminating for managers trying to grapple with similar activities among Asian elephants (Elephas maximus), it is still important for researchers to conduct local assessments in order to fully understand the correlates of conflict in elephant hotspots such as southern India.

(Asian elephants, Elephas maximus)

Indians can petition the government for compensation after crop-raiding and attacks by wildlife, and the resulting paper trail provides information about a number of potential correlates of conflict--including the location, intensity, and frequency of crop raids, the efficacy of deterrents (if any are installed), and the speed and utility of the compensation process. Further, data from the claims can be coupled with meteorological and geographical information in order to provide an indication of whether, and how, wildlife encounters are impacted by weather and distance to animal preserves.

Indian conservationist Sanjay Gubbi recognized the potential of this latent dataset, prompting him to sort through hundreds of reports filed over a 3-year period in and around Nagarhole National Park in southwest India. The park is contiguous with several other important wildlife habitats, including Bandipur National Park, Mudumalai and Waynaard Wildlife Sanctuaries, and several reserved forests; cumulatively, these areas cover about 5,000 square kilometers of prime elephant habitat. Although locals occasionally experience attacks by tigers (Panthera tigris) and crop-raiding by gaur (Bos gaurus), elephants are the major source of stress in the area--accounting for 99.9% of the 1955 crop loss incidents for which the government dispensed compensation during Gubbi's 3-year study period.

Elephants damaged or consumed 26 different types of crop, but five major species bore the brunt of the activity: finger millet (Eleusine coracana), maize (Zea mays), cotton (Gossypium spp.), rice (Oryza sativa), and sugarcane (Saccharum officinarum). The temporal pattern of human-elephant conflicts--peaking between August-November but remaining low throughout the rest of the year--appears to refute the myth that crop-raiding is more common in the dry months because elephants are desperate for food. Rather, conflicts peaked just after the monsoon season, when vegetation should be most lush. These peaks coincide with the ripening of tasty crops such as millet, maize, and rice. Thus, it appears that the elephants may be using their keen sense of smell to detect and target potential food sources when they become available. This may also explain why conflicts were most common in a band 3.1-5 km away from the Nagarhole National Park boundary rather than right next to it, as one might predict: Perhaps the best crops are those several kilometers away, and the elephants use their olfactory prowess to bypass mediocre fields and target only those with the best food.

Although previous researchers have suggested that elephants are particularly fond of sugarcane, Gubbi's results indicated much heavier activity in millet, maize, and rice fields. Thus, he posits that the elephants utilize sugarcane opportunistically when it is encountered near more satisfying crops. Inedible cotton, on the other hand, is probably only damaged when it gets in the way of hungry elephants commuting to nearby fields with tastier products.

 (Finger millet, Eleusine coracana)

Conflicts were rare outside of a 6-km band around Nagarhole National Park. Elephant activity was more likely in areas that shared longer borders with protected habitat; conversely, they it was slightly less common in places with more unirrigated land. Overall, most villages (49.3%) suffered only 1-10 conflicts, but a surprising number (36.7%) experienced 11-50. Few saw more than this, though some areas did experience over 100 visits by raiding elephants.
Over the course of the 3-year study period, claimants in these areas received a total of $52,026 to cover losses resulting from elephant raids; even worse, elephants were responsible for 10 human deaths and 8 injuries. Individual farmers had to wait an average of 114 days to receive the relatively small sum of approximately $31 per conflict.

A recently-completed study on elephants in the same region reported only 806 conflicts during a 12-year period--or approximately 67 conflicts per year. This contrasts markedly with the ~652 conflicts per year reported here. Although some of the difference may be the result of an increase in claim filing, it also appears likely that both human and elephant populations have swelled enough to increase the likelihood of unpleasant encounters between the two species. It may be possible to reduce these encounters using simple mitigation techniques such as the installation of fences and ditches around fields, or even high-tech solutions such as chemical sprays that mask the enticing smells of ripening crops.

(Elephant in Nagarhole National Park)

The author feels that his work refutes previous claims about elephant behavior and exposes some suggested management strategies as useless. For instance, elephants do not, as previously believed, fixate on sugarcane fields or irrigated areas; thus, efforts to minimize these near preserves would not likely reduce conflict. Likewise, he says, the animals do not raid crops more during the dry seasons, and so the creation of water holes and the cultivation of fodder crops inside the park would probably just waste precious management resources. Instead, Gubbi recommends erecting physical barriers around park boundaries, incorporating nearby protected areas into the park so they can serve as buffers, and concentrating conflict-prevention efforts on the August-November period when crop raiding is most common. When and if these measures fail, he encourages the government to provide better compensation to farmers, or at least to do so more quickly; this should prevent retaliatory behavior against elephants. Finally, the author also suggests a state-wide study on human-elephant conflicts in order to provide more perspective so that long-term elephant management plans can be coordinated throughout the country.

Gubbi, Sanjay. 2012. Patterns and correlates of human-elephant conflict around a south Indian reserve. Biological Conservation 148:88-95.

Thanks to the following websites for providing the images used in this post:

Friday, 23 March 2012

Follow-up on a previous post: Bird collisions and power lines

One of last September's posts looked at the results of a meta-analysis investigating whether avian collisions with power lines were reduced by the installation of bird flight diverters. The researchers who performed the study indicated that one drawback of their analysis was its reliance on previous work that often had small sample sizes, covered relatively small habitat fragments, or both. In order to address these deficiencies, the researchers have since completed their own field experiments examining the utility of both large and small spiral diverters, attached to either transmission or distribution lines.

(The large (a) and small (b) diverters used in the current experiment)

Their field work was conducted in central Spain and focused on 5 regions classified as "important bird areas;" these also happen to be the main dry cereal farmland centers near the city of Madrid. Although the researchers were interested in investigating deaths of any bird species, they were especially concerned about mortality in threatened species such as the great bustard (Otis tarda), little bustard (Tetrax tetrax), pin-tailed sandgrouse (Pterocles alchata), and black-bellied sandgrouse (Pterocles orientalis), all of which are common in the region and are particularly susceptible to collisions.

Bird collisions were surveyed monthly between August 2001 and December 2010, after which 15 of the 22 focal power lines were designated as experimental. All lines were monitored for an additional year ("before treatment"), fitted with spirals or left bare to act as the control treatment, and then monitored for a second year ("after treatment"); monitoring consisted of patrolling each power line on foot and looking for carcasses within a 25-m band on either side of the line. Some of the power lines were "transmission lines" (carrying electricity from a power plant to a receiving station), while others were "distribution lines" (carrying electricity from a receiving station to customers). All transmission lines were fitted with large spiral diverters, while distribution lines were fitted with either large or small spirals. This allowed the researchers to determine how mortality was impacted by three major factors: type of power line, presence/absence of diverter, and type of diverter. They ran separate analyses for total numbers of species observed, casualties observed, estimated collisions (using previously developed formulas that extrapolate from survey data), and collisions involving the most common species in the area (doves, great bustards, and little bustards).

(Schematic showing the difference between transmission and distribution lines)

Over the study period, the researchers located 521 carcasses of 45 bird species--19 of which were species of conservation concern. Across the 15 treatment lines, 11 had a general decrease in mortality post-marking, while only 4 experienced an increase. Overall, this resulted in 88 fewer bird deaths, or a 47% reduction in mortality. The 521 carcasses found here are estimated to represent a whopping 14,282 collisions, or an average of 8.2 per month per km of power line. Analyses of marker efficacy using these collision data, instead of the absolute numbers of carcasses, again suggested that diverters are successful at keeping birds away from lines; 316 fewer birds, or a 9.6% reduction in mortality, were estimated across the cumulative length of treated lines. Treated lines had significantly lower mortality rates than control lines, further highlighting the usefulness of this mitigation technique.

Additional statistical analyses revealed that large spirals were similarly useful when installed on both transmission and distribution power lines. Likewise, small and large spirals had similar efficacy on distribution lines. However, species-specific analyses indicated that some species did notice differences in line and marker type, even if birds in general did not. For instance, while little bustard mortality was reduced by any type of wire marking, irrespective of location or size, great bustards responded more positively to marking of transmission lines, and to the use of large spiral diverters; dove mortality, on the other hand, did not vary as a result of either of these factors.

(Little bustard, Tetrax tetrax--a species that responded similar to all types of wire marking, regardless of location or size)

On the whole, it appears that installation of bird flight diverters is a useful technique for reducing avian mortality at power lines. Although a 9.6% reduction in collisions may not seem like much, the authors of the current paper stress that this result could be highly biologically significant--especially for species of conservation concern, such as the great bustard, or in particularly sensitive habitats, such as migration corridors or areas where power lines happen to be located next to important roosting or staging sites. Another pleasant surprise was that small diverters were, for the most part, as effective as large diverters. Because the smaller spirals are lighter, they are less likely to cause power lines to come down (a problem that is particularly common in inclement weather).

Despite this general good news, the researchers caution land managers to keep in mind that the current results might not be broadly applicable to all systems. Species' responses to power lines vary as a result of many factors, including wing loading, aspect, and visual field, and so different species assemblages will vary in their susceptibility to power lines; environmental factors, such as habitat openness and weather conditions, will also play a role in determining mortality. Further work is still needed to understand whether collision rates are impacted by other human disturbances (such as loud noises and moving distractions), and whether spiral color impacts the extent to which birds are diverted away from lines. Perhaps most importantly, the authors suggest that researchers need to identify collision hotspots in order to determine the most useful place to install diverters of all sizes and colors.

Barrientos, R., Ponce, C., Palacin, C., Martin, C.A., Martin, B., and Alonso, J.C. 2012. Wire marking results in small but significant reduction in avian mortality at power lines: a BACI designed study. PLoS ONE 7(3):e32569.

Thanks to the following websites for providing the images used in this post:

(Under)estimating wildlife responses to environmental disasters

Many important questions were raised by the 2010 Deepwater Horizon disaster. While some of these focused on oil drilling, sources of energy, and disaster relief services in the US, many biologists concentrated on the efficacy of ecosystem-wide emergency response services. In particular, the catastrophe spurred researchers to ponder our ability to adequately measure the intensity of environmental catastrophes--and then respond appropriately.

(Northern gannets, Morus bassanus)

One outcome of that pondering is a report from an international team of collaborators who have found that traditional methods of measuring oil spill-related wildlife fatalities may underestimate the number of animals affected.The results were produced from a research project on northern gannets (Morus bassanus), the largest sea bird breeding in the North Atlantic and the only bird species of all-Canadian origin to be impacted by the Deepwater Horizon spill. After breeding on the Canadian coast, gannets migrate south and winter along the US shoreline from Maine to Texas; many that were feeding in and around the Gulf of Mexico in late April 2010 were the unlucky casualties of the recent oil disaster.

In the current study, a comparison of mark-recapture data (collected from the recovery of bands from dead birds along the shoreline) and bird-borne tracking data (collected from global location sensors and satellite tags attached to the birds) revealed that the two techniques estimated very different oil-related mortality rates. While band data suggest that 2.2% of adults and 12.8% of immature gannets use the Gulf of Mexico over the course of a year, tracking data indicate that these numbers may be as high as 28.3% and 22.2% in the non-breeding season alone. To put that into perspective, that is the difference between ~13,300 and ~66,100 adult gannets, or ~42,000 and ~52,509 juveniles, respectively. Further, the tracking data allow scientists to assess when the birds arrive in, and depart from, the Gulf. Both age groups begin appearing as of the first week in November, with adults generally heading back north by the first week of April. Juveniles, on the other hand, remain in the south much longer, which means that this was the main age class in the Gulf during the time of the spill.

(The Deepwater Horizon during some of its final moments above the water's surface)

Overall, the results indicate that post-spill mortality estimates were probably very conservative. There may have been several thousand more gannet deaths than previously realized--particularly among younger age classes--but the implications of this are not yet understood. Most northern gannets do not attempt to breed until they are 5-7 years old, which means that the population-level impacts of the Deepwater Horizon event may not be noticeable for several more years--when the birds that were killed during the disaster would have begun their first nesting attempts. On the other hand, the researchers state that gannets may be "buffered by age-related life-history processes," and not suffer any serious population declines. Presumably, the positive outcomes of this scenario would be related to reduced competition pressure (as a result of the recent die-offs), leading to improved juvenile survivorship among chicks produced by surviving adults.

(Nesting northern gannets)

Either way, only time will tell. To date, the only hints of the outcome are 2011 photographs of birds breeding in the Cape St. Mary's colony; the plumage of several birds was covered in a dark substance that appeared to be oil, though this could not be confirmed. Continued monitoring--not just of population numbers but also of survival, physiology, reproductive success, and migratory movements--will be used to assess the health of this species.

Although these gannet-specific results are interesting and important, the researchers stress that one of the most important take-home messages from their work is the disconnect between banding and tracking data. The latter likely underestimate mortality rates because they do not account for the many birds that die at sea and sink to the ocean floor rather than washing up on, or flying to, beaches; this suggests that post-pollution conservation and management proposals may not be suitably tailored to effect a comeback for negatively impacted animals. Further, this is probably an issue for species other than gannets, and, indeed, for non-avian wildlife as well.

Montevecchi, W., Fifield, D., Burke, C., Garthe, S., Hedd, A., Rail, J.-F., and Robertson, G. 2012. Tracking long-distance migration to assess marine pollution impact. Biology Letters 8:218-221.

Thanks to the following websites for providing the images used in this post:

Wednesday, 21 March 2012

Bioacoustics and conservation in the popular press!

It’s not often that popular science books focus on bioacoustics. Recent notable examples include Don Kroodmsa’s The Singing Life of Birds and Birdsong of the Seasons, both of which receive rave reviews from readers and, as is obvious from the titles, focus on the amazing diversity of sounds produced by our feathered friends. But birds are only one of many acoustically communicating species, and animals in general are only one source of sound on our planet. Others include abiotic environmental sources such as wind, waves, and geological activity, and, as has been receiving much attention recently in the scientific literature, noises that can ultimately be traced back to humans.

As most regular Anthrophysis readers already know, noise—or, to be more precise, “unwanted sound”—is a serious problem in most contemporary habitats, where it can disrupt sleep patterns, act as a distraction, cause stress leading to other physical and emotional problems, and obscure the acoustic signals of acoustically communicating species. The history of noise, and all its pernicious effects, was recently cataloged by Hillel Schwartz in the book Making Noise: From Babel to the Big Bang and Beyond.

Cover of the American version of the book

But what about more pleasant cacophonies, such as the explosion of sound that emanates from a rainforest  as dozens of species of birds, insects, amphibians, and mammals all vocalize simultaneously? What about the sound of a crashing thunderstorm as its rain pummels leaves in the tree canopy and its winds cause the branches to creak? These natural symphonies are the subject of a new book released this week to US audiences: Bernie Krause’s The Great Animal Orchestra: Finding the Origins of Music in the World’s Wild Places (available to UK readers in April).

Krause’s book fills the gap between Kroodsma’s and Schwwartz’s, examining the sorts of complex, multi-taxa sounds that do more than simply provide ambience, but are not so overbearing that most of us would classify them as “noise.” Rather, these acoustic stimuli are, Krause argues, the origins of human music. After all, emerging in an era with no televisions, radios, or stereos, our first musically inclined ancestors had only one source of inspiration other than their own voices: the world around them. Thanks to the huge variety of sounds in their environment—each differing in pitch, amplitude, tempo, and timing of production—these early humans were by no means at a loss for acoustic stimulation.

Cover of the British version of the book

The Great Animal Orchestra covers a quite a bit of ground for a book that is only ~250 pages long. The author, a former professional musician and Hollywood sound expert, frequently describes his own experiences with nature’s music and his growing interest in “soundscapes,” or complete collections of sounds that characterize a given habitat. Over the course of a lifetime, Krause has collected an audio archive comprising tens of thousands of samples, which cumulatively could play for about 1,500 hours straight (or two solid months). His personal acoustic journey serves almost as a parallel to that of the average reader making his/her way through the book—beginning as a relative novice to the world of biotic sounds and gradually becoming acquainted with the richness of natural soundscapes. The reader first learns about the sources of noise in the environment, and how they fit together to make “the great animal orchestra,” and then is shown how this acoustic display is a vital part of the human experience. Almost inevitably, the final chapters of the book deal with habitat loss, declining biodiversity, and noise pollution—the factors that, along with our own apathy, threaten to render many soundscapes extinct.

Although Krause does not shy away from scientific explanations of relevant topics—how sound propagates, what makes an octave, how to read a sonogram—he makes them accessible to lay readers while avoiding a level of simplicity that would bore a fellow bioacoustician. Throughout, Krause’s enthusiasm and wonderment are infectious. Some of the best passages are those in which Krause describes acoustic scenes he has experienced over the years—as in the following passage about Arctic glaciers:

                  The ice mass shatters as it is compressed under great pressure and undergoes periods of melting and snow accumulation, and in addition to the startling popping and groaning of the ice and the ever-present wind and frequent storms, calving glaciers release huge walls of frozen water into the shorelines of rivers, fjords, and seacoasts with a volatile, thunderous burst of sound, the fallen accumulation generating huge waves in the water below. Then there is the sound of the glacier’s own movement: a slight, ominous oscillation caused by its relentless progression overland—a slow, creeping sensation more felt than heard.

Dr. Bernie Krause

I have to admit that, while I found every bit of the book fascinating, I did not always understand the logic behind its organization. Some themes were revisited multiple times in a way that seemed scattered rather than purposeful; several sections could have been better rooted in the central theme—which, for that matter, could have been more clearly delineated and reinforced throughout. Altogether, I feel that the book suffers from an identity crisis that is never quite resolved; it is part autobiography (of an unarguably interesting person), part history of music, part explanation of bioacoustics, and part E.O. Wilson-style argument for the need for conservation. There is no fundamental reason why all of these disparate topics should not exist in one place; they are complementary, as the book does make clear. My main complaint is that these ideas never quite meshed into a single flowing narrative. However, I should stress that these critiques reflect my own artistic preferences for a certain style of organization, and do not have any bearing on the worthiness of the ideas discussed here, since these are certainly worth the cost of the book and the time required to read it.

Indeed, the bioacoustician, nature-lover, and conservationist in me all concur that The Great Animal Orchestra is a wonderful introduction to the wild world of sound and the amazing things you can hear if you stop to listen. I agree with Krause that the world is a richer place when you are aware of its acoustic textures. Like the author, I find it unbearable to think that we are on the cusp of forever losing certain sounds from our acoustic experience. Thanks to habitat fragmentation and other anthropogenic influences, I may never again hear bobwhites or whip-poor-wills calling behind my childhood home, and I am acutely aware of how their loss alters the feel of the habitat. Similar losses, often at much larger scales and involving many more species simultaneously, are occurring around the world. Krause’s book is an elegant explanation of how these losses threaten to mute the “great animal orchestra,” why each of us should care, and what we can do to both enjoy the soundscapes that remain, and attempt to preserve them for future generations.

To watch a Magic of Science episode with Dr. Bernie Krause, click here.

The US hardcover edition of the book, published by Little, Brown and Company, is 288 pages long and is available at Amazon.com from 19 March 2012.

The UK hardcover edition of the book, published by Profile Books, is also 288 pages long and is available at Amazon.co.uk from 1 April 2012.

Tuesday, 20 March 2012

Independence of amplitude and frequency shifts in an urban songbird's vocalizations

Songbirds in urban areas are known to vocalize at a higher pitch to avoid being "masked" by anthropogenic sounds; they are also known to vocalize more loudly, an acoustic alteration known as the Lombard effect. But do these patterns represent two separate responses to human noise, or can they both be traced back to the same source--a general constriction of the muscles used when producing songs and calls?

(An Oregon dark-eyed junco, Junco hyemalis thurberi)

For the last couple of years, this question has been a source of debate among bioacousticians, but a new paper from an international pair of collaborators seeks to end the controversy. The duo made use of a database of more than 1000 recordings of 151 male dark-eyed juncos (Junco hyemalis thurberi) from southern California. Like many songbirds, juncos do not sing at a consistent volume (more scientifically referred to as amplitude) throughout each vocal performance; within each bout, some notes and syllables are invariably quieter than others. The researchers reasoned that these natural fluctuations in volume could be used as a proxy for the different amplitudes characterizing urban and rural songs. If frequency hikes are merely a side effect of singing louder, then louder songs should be characterized by higher frequencies--or, more specifically, higher minimum frequency, the characteristic most consistently impacted in anthropogenic noise studies. On the other hand, if these two song traits are independently controlled, any relationship between the two variables would be possible--for instance, frequency might remain consistent or even decrease during the performance of higher-amplitude vocalizations.

In the current study, the amplitude-frequency relationship was examined at two different levels. First, the researchers compared the loudest and softest songs within individual males' song bouts (which consist of several songs sung in a row). Although junco songs usually consist of a single syllable, some comprise two or more; where this is the case, the first and last syllable are usually quieter than those in the middle. Thus, the second analysis compared the loudest and quietest syllables within a single song.

(UCSD campus, where researchers have studied urban dark-eyed juncos that produce vocalizations notably different from those of their more rural brethren; city birds also look different from their country cousins.)

Results of both analyses were opposite to the trend that would be expected if high frequency were a byproduct of greater amplitude. Specifically, louder songs and syllables had lower, not higher, peak frequencies. The researchers worried that these patterns may have been influenced by random variations in bird behavior--subtle shifts in singing posture that cause the birds to look away from the microphone and therefore sound quieter than they actually are. However, additional analyses revealed that measurement error was not likely to have been responsible for these results. Thus, it appears that, in juncos at least, frequency and amplitude shifts in noisy environments are two separate mechanisms that the birds use to avoid masking by anthropogenic sounds.

While this result contradicts a popular current theory, the authors indicate that they do not find it altogether surprising. The avian vocal system, they argue, maintains a separation between amplitude and frequency: The former is manipulated by the contraction of respiratory muscles, while the latter is regulated by specialized syrinx (or "vocal cord") muscles. Further, previous work has revealed that many birds experience seasonal shifts in song frequency (presumably as a result of hormonal fluctuations), but not of song amplitude, indicating that there is no reason to assume that these two characteristics should necessarily be linked.

(Diagram of a bird's vocal morphology)

However, the researchers point out that their results do not completely rule out coordinated fluctuations in these two song traits. For instance, if there were some physiological or morphological response to noise that impacted muscles in general, then both respiration and syrinx movement could be altered, thereby causing simultaneous changes in amplitude and frequency. Additionally, the birds themselves might judge dual adjustments to be necessary in some situations.

Other studies have shown that amplitude modulations are more successful than frequency modulations at combating anthropogenic noise. If frequency shifts are not merely a side effect of amplitude adjustments, this leaves the question of why urban songbirds should rely so heavily on this less-than-perfect response to anthropogenic soundscapes. According to the researchers, it all boils down to  energy: Singing louder is probably more demanding than singing at a higher pitch, so unless a bird is in peak physical condition and has lots of energy to spare, he may be more likely to pick the latter technique even if it is not as effective.

Cardoso, G.C., and Atwell, J.W. 2011. On the relation between loudness and the increased song frequency of urban birds. Animal Behaviour 82:831-836.

Thanks to the following websites for providing the images used in this post:

Sunday, 18 March 2012

Do power line corridors provide good shrubland bird habitat?

Straight on the heels of the news that gravel pits provide excellent butterfly habitat, researchers from Connecticut College have revealed that open corridors along power lines--also called right-of-ways, or ROWs--may offer shrubland bird species a place to nest.

(Powerline right-of-way)

This finding resulted from a three-year study of 93 ROW plots in southeastern Connecticut. Selective removal of trees from the plots keeps the power lines and their maintenance crews safe from potentially dangerous branches, while also preserving a perpetual early-successional shrubland habitat. Since many shrub-loving bird species have been declining in North America over the past several decades--often as a result of habitat loss--the designers of the current study hypothesized that ROWs might provide some much-needed breeding and feeding grounds.

Indeed, point counts detected a total of 65 different species, including two, the yellow-breasted chat (Icteria virens) and brown thrasher (Toxostoma rufum), that are species of conservation concern in Connecticut. Among of the five most common species (eastern towhee, Pipilo erythrophthalmus; gray catbird (Dumetella carolinensis; brown-headed cowbird, Molothrus ater; prairie warbler, Dendroica discolor; and field sparrow, Spizella pusilla), three are associated predominantly with shrublands and are therefore in particular need of habitat supplementation.

 (Yellow-breasted chat, Icteria virens: a Connecticut-listed endangered species)

Superficially, this seemed like good news. However, the researchers wanted to dig a bit deeper and investigate whether ROWs offer a safe and productive environment in which to breed, or whether they might instead act as population sinks--habitats that are sustained only through the continued immigration of outsiders. To do this, the scientists located and monitored nests of the three most common shrubland-obligate species: eastern towhees, prairie warblers, and field sparrows. The results put a very different spin on their previous findings. Individual field sparrow nests had, at 20.6%, the highest observed chance of fledging young; this value dropped to 19% for prairie warbler nests and 15.5% for eastern towhee nests in the best years, and even lower at other times. The silver lining in the cloud was that, thanks to successful re-nesting attempts, individual females had up to a 71% chance of fledging at least one chick during the breeding season. All the same, reproductive rates of both species were too low to "balance losses from mortality," indicating that the ROW populations could only persist if fed by migrants from elsewhere.

Although the main reason for nest failure was predation, probably by animals such as eastern chipmunks (Tamias striatus), blue jays (Cyanocitta cristata), and American crows (Corvus brachyrhynchos), another important factor was brood parasitism by brown-headed cowbirds. This was particularly common in nests located nearer to roads and buildings--an association that emphasized the importance of broader habitat and landscape variables. These were examined in more detail via vegetation surveys and computer-based landcover analyses. Some relationships were species-specific, but the researchers also found some enlightening general trends. For example, shrubland bird diversity and abundances were generally lower in areas that had higher levels of developed and agricultural habitat in the surrounding landscape. Additionally, many species were less common in narrower ROWs.

(Blue jay, Cyanocitta cristata, a potential predator of shrubland-nesting species--or, more specifically, their eggs and young)

A range of fledging rates have been reported for ROWs studied in the past; these range from values comparable to those reported here to near complete nesting success. In other words, although ROWs can provide good habitat for declining shrubland bird species, they are not guaranteed to do so. It appears that an important determinant of success is the type of land cover in neighboring habitats. Specifically, ROWs in the midst of more anthropogenically altered habitat are lower quality than those in more forested areas. This is because potential predators and brood parasites will be found at lower densities in ROWs when they have alternative habitats to choose from, but will descend on shrubland areas if these are the only available places to occupy.

On the whole, the researchers feel that their results show the potential usefulness of selective tree removal as a shrubland-creating management technique that can
promote biodiversity. Although this method is commonly used in Australia and North America, it has yet to gain much traction in Europe and East Asia, where shrubland birds are also on the decline; corridors in the middle of protected forests would make particularly good targets. However, the authors recommend that managers keep a close eye on ROW habitats in order to assess whether they are a source or sink for local birds. Sinks are not necessarily a bad thing--for instance, where neighboring shrubland habitats are producing an overabundance of individuals whose settlement in ROWs can perpetuate the local population--but could become detrimental if there are no nearby sources of immigrants. Perhaps most importantly, the researchers advise against the creation of any new corridors that would fragment, or further fragment, surrounding forested areas.

For supplemental images associated with this post, visit the Anthrophysis pin board at Pinterest.

Askins, R.A., Folsom-O'Keefe, C.M., and Hardy, M.C. 2012. Effects of vegetation, corridor width and regional land use on early successional birds on powerline corridors. PLoS ONE 7(2):e31520.

Thanks to the following websites for providing the images used in this post:

Thursday, 15 March 2012

Gravel pits are an unexpected butterfly paradise

Gravel pits: To the average human they may be nothing more than an eyesore, but to many species of butterfly they are home sweet home. That's the finding of a project by a group of Polish researchers looking to examine the potential ecological value of this increasingly common habitat. In Poland alone, excavation activities doubled from 2000 to 2006, leaving behind hundreds of gravel pits waiting to be occupied by wildlife. Previous work had shown that these areas, characterized by high plant diversity and open pools, are happily utilized by waterbirds; the current study was undertaken to determine whether insects might also benefit from this anthropogenic habitat.

(Sand and gravel pit)

Butterfly surveys were conducted at a total of 100 sites: 50 gravel pits in southern Poland, and 50 nearby grassland areas representing a typical alternative butterfly habitat. Count data were used to quantify butterfly numbers, abundance, diversity, and commonness (which reflects how often a species was seen across all surveys). Local and landscape-level habitat variables, including amount and type of vegetation, amount of nearby water, and distance to features such as forests and human infrastructure, were also quantified at each site in order to explore, more specifically, what factors might influence butterfly habitat choice.

(The grizzled skipper, or Pyrgus malvae, was one of 21 species more common in gravel pits than in nearby grassland areas)

Although butterfly abundances were similar in grassland sites and gravel pits, the latter had higher richness and diversity; further, species found in gravel pits were often locally rare, whereas those in grasslands were more likely to be common and widespread. Landscape characteristics did not explain butterfly diversity or commonness particularly well, but they did predict but richness and abundance. In particular, these variables were negatively impacted by isolation (or distance from other suitable habitats), but positively affected by age of gravel pit and shrub density. The former pattern suggests that neighboring habitat patches are used as "stepping stones" to reach gravel pits; the latter result indicates that butterflies benefit from the presence of shrubs that emerge after extensive mining efforts have ended and the gravel pits are left to go "feral."

(The small skipper, or Thymelicus sylvestris, is one of 13 butterfly species found at equal densities in both gravel pits and grasslands)

The authors are quick to point out that too much shrub growth can be a bad thing, since butterflies prefer open habitats to closed, highly vegetated areas. Thus, it may be necessary to periodically manage gravel pits in order to prevent complete succession (or, in other words, the growth of new forests). Although this would require some effort, it is still less work (and money) than needed to reclaim and restore old gravel pit land to its pre-excavation state. While it remains to be seen whether such a plan would be equally beneficial to other local species, it seems likely that some butterfly-friendly habitat features--such as small islets that could be installed in gravel pit pools to encourage the insects to migrate across open water--would be useful for species such as nesting waterbirds.

 (The peacock, or Inachis io, is one of 15 species found to be more abundant in grasslands than in gravel pits)

These results provide evidence that at least some "man-made habitats can be remarkably good for biodiversity." In addition to acting as a final destination for some migration species, these could also be
important elements in "ecological networks," which the authors define as "landscape-scale systems of interconnecting corridors, nodes, and patches designed to mitigate effects of habitat fragmentation." Thus, even when they are not the final destination, these patches could help wildlife reach suitable natural feeding and breeding areas even in landscapes dominated by anthropogenic features such as busy roads, housing developments, and shopping strips.While gravel pits appear to be potentially useful in this regard, other former industrial sites--such as quarries and coal mines--are likely to offer similar benefits.

Lenda, M., Skorka, P., Moron, D., Rosin, Z.M., and Tryjanowski, P. 2012. The importance of the gravel excavation industry for the conservation of grassland butterflies. Biological Conservation 148:180-190.

Thanks to the following websites for providing the images used in this post:

Monday, 12 March 2012

Watch where you step!

Researchers from Australia's Sunshine Coast have found that beach-dwelling invertebrates are negatively impacted by the presence of people--probably because of heavy foot traffic that harms both the habitat and the animals themselves. Although these species are small, they are an important part of the ecosystem, linking the terrestrial and marine environment by cycling nutrients and serving as prey. Thus, in order to preserve the ecosystem function of beach habitats, we may need to develop regulations limiting certain aspects of beach use.

Map of Australia's Sunshine Coast

The study was conducted south of Noosa, Southern Queensland, on the east coast of Australia. The scientists picked out four sampling plots at each of four beaches; each beach contained two reference and two impact plots evenly divided between the upper and lower shore. Impact plots were those where sand disturbance showed signs of clear human activity, while reference plots were less intensively trafficked, if at all, by beach-goers. Multiple cores were collected from each plot so the researchers could measure species abundance, richness, diversity, and community composition, as well as grain size and moisture content of the sand.

Although the "morphodynamic beach conditions"--in other words, characteristics such as slope and distance from low water mark--were similar between reference and impact treatments, there were clear differences in disturbance level. There were many more footprints in the impact plots along the lower shore, and these were found at a higher density than in reference areas. Human-trafficked areas had coarser sand and approximately half as much moisture; the latter pattern was probably driven by trampling, which "may enhance water loss from the sediment" by breaking the "thin surface crust" of the sand.

Sunshine Coast near Noosa

Likewise, abundance of benthic (or bottom-dwelling) organisms was reduced by as much as 72% in the seawards impact plots; declines of individual species were as high as 90%. Impacted species included the amphipod Urohaustorius halei, and bloodworms (Glycera spp.). Density and diversity were also lower (by up to 55% and 68%, respectively) in the seawards plots, and overall spatial variation in community structure was significantly related to the intensity of human habitat use at the lower shore.

Dunes are generally thought to be more sensitive to anthropogenic disturbance, while areas closer to the shore are typically perceived as more robust. In particular, dune flora and macrofauna are known to be at risk from human traffic. However, the researchers did not find any significant differences in microfauna between impact and reference plots along the upper beach; species such as the isopod Pseudolana concinna, the most common upper shore benthic animal, could be found at similar levels in all dune-wards plots. Thus, as far as sand-dwelling invertebrates are concerned, shoreline areas are more dangerous than habitats along the upper beach.

Bloodworm (Glycera spp.)

According to the researchers, the most parsimonious explanation for their results is "crushing of organisms" by foot traffic. If so, it would appear that pedestrian activities are in many ways comparable to beach vehicular traffic, in terms of negatively impacting microfauna and disturbing their habitat--a surprising result given that heavier, faster-moving vehicles seem intuitively more dangerous. Unfortunately, the authors list this as only one of several anthropogenic disturbances at beaches; others include removal of "cast and wrack" from the shoreline, installation of infrastructure, and beach grooming, to name a few. Although it seems likely that these various factors work together to harm the ecosystem, it is not yet clear whether their impacts are additive or multiplicative.

In order to minimize the harmful effects of human activities, the researchers suggest three major interventions: limiting the number of beach visitors, using signs and fences to funnel visitors to particular areas, and, where necessary, going one step further and officially zoning the beach into wildlife-only refuges and human-accessible recreational areas. Since the first technique is unlikely to get much support, the authors state that it will be important to focus instead on second two options. Zoning efforts have already seen some success, and can take advantage of visitor interest in particular areas of the beach--for instance, those near facilities, established footpaths, and sites with life guards. Another important step will be preventing the formation of new tracks that take beach-goers too close to sensitive beach wildlife. Hopefully, some combination of these techniques will preserve species assemblages, and also, therefore, the "functional integrity" of beach ecosystems.

Schlacher, T.A. and Thompson, L. 2012. Beach recreation impacts benthic invertebrates on ocean-exposed sandy shores. Biological Conservation 147:123-132.

Thanks to the following websites for providing the images used in this post:

Saturday, 10 March 2012

Preventing wind-turbine related vulture deaths

Wind turbines have long been associated with bird (and bat) mortality, but the extent to which these structures threaten avian populations is unclear; estimates of annual death rates vary depending on species, location, and turbine characteristics. Researchers suspect that collision fatalities could have a particularly negative impact on populations of long-lived species with low reproductive rates, such as hawks, owls, and vultures. Many of these animals are migratory, collecting in or flying through habitats along predictable flyways, often in groups of hundreds and thousands of individuals. Where the flyways overlap with wind farms, birds may fatally collide with turbines unless mitigation techniques are used to reduce risk.

Wind turbines in a Spanish wind farm

One such site of overlap is Cadiz, Spain, used by griffon vultures (Gyps fulvus) as both a breeding area and a spot from which to launch migration into Africa each fall; during the peak of migration, 1,800 vultures have been seen in the area during a single day. Another common sight in the region is wind farms looking to take advantage of the sea breezes. Since the farms were established in 2006-2007, surveillance teams have monitored avian mortality and collected data on factors that might have influenced collision rates. During 2008-2009, surveillance teams were also involved in mitigation efforts using temporary stoppage of turbine blades to try to reduce bird deaths; turbines were shut down, for instance, whenever birds' flight trajectories seemed likely to put the animals in harm's way, or when a group of birds was seen entering air space over a farm. However, only particular turbines--those judged most likely to cause harm--were stopped, while the rest were left running. This is referred to as a "selective stopping protocol."

Using observations made throughout this four-year study period, an international team of collaborators has investigated three major questions. First, are particular turbines more dangerous than others, and, if so, why? Second, does stopping the turbines actually reduce vulture mortality? Finally, is the selective stopping technique economically sustainable?

Griffon vulture, Gyps fulvus, in flight

Overall, the researchers analyzed observations at 296 wind turbines across 13 study farms. All turbines were constructed in a similar style and were arrayed north-south, but varied in height (106-170 m), rotor diameter (56-90 m), and distance to nearest neighboring turbine (115-180 m). During the four-year study period, 221 dead vultures were found across all sites; there was an average mortality rate of 0.186 vultures per turbine per year. Mortality rates varied from one farm to the next, peaking at 0.727 deaths/turbine/year but going as low as 0.078 deaths/turbine/year. Likewise, within individual farms, certain turbines were more deadly than others; although most turbines were associated with no mortalities at all, a few were responsible for many deaths. Fatalities spiked in October and November, when migratory birds descended upon the Strait of Gibraltar before passing from Spain into Africa.

The top ten deadliest turbines, which were distributed across six wind farms, were chosen as the focus of the selective-stopping efforts. Surveillance was increased at these turbines so that the blades could be stilled whenever they appeared most threatening to passing vultures. These efforts dropped the mortality rate from 0.224 to 0.114 deaths/turbine/year--an almost 50% reduction. The seasonal spike in vulture deaths vanished after the selective stopping program was initiated. This was particularly good news for juvenile griffon vultures, who accounted for three-quarters of all dead birds that could be aged (though this could be related to high numbers of dispersing juveniles rather than a particular susceptibility of young birds).

Griffon vultures migrating over the Strait of Gibraltar

The 2-year selective stopping period was associated with a grand total of 4,408 turbine stops, or an average of 18.06 stops per turbine. Analyses of a subset of these stops indicated that turbines were stilled for an average of just over 22 minutes apiece, or a total of 6 hours and 20 minutes per year. This accounts for only 0.07% of annual total energy production.

According to the researchers, the vulture death rates reported here are some of the highest on record in association with wind farms; however, they are on par with estimates at power lines and other power structures. The authors feel that the differences in mortality rates, both among different farms and among turbines within a single farm, indicate that careful site selection could reduce collisions. In particular, they suggest that variations in local microhabitat features, such as topography, altitude, and wind patterns, could be responsible for making some turbines deadlier than others. Further research will be needed to explore the importance of these characteristics and potentially design protocols for identifying the safest spots for turbine installation.

Juvenile griffon vulture

Results of the study emphasize how important it is to consider individual species' life history information when conducting ecological assessments and developing management plan. Here, seasonal shifts in habitat use were found to influence mortality rates, and younger birds were found to be especially susceptible to turbine-related death. The authors worry that previous turbine risk assessments may not have considered such factors, and therefore may not have accurately reported the likely outcomes of turbine installation. Other species-specific traits may also play an important role in the frequency of collisions. For instance, studies of griffon vulture visual fields suggest that the animals may have a blind spot that renders them "sightless in the direction of travel"--an unfortunate characteristic that may increase the likelihood of collision.

Luckily, the current study indicates that selective stopping can be quite effective at reducing collisions, whether caused by unfortunate biological characteristics or suboptimal placement of turbines. Additionally, this technique is likely to simultaneously benefit multiple species of birds, simply because it reduces the total amount of time the blades are spinning. The researchers suggest that, in the future, selective stopping protocols could be coupled with automatic systems that determine the trajectories of flying birds; this could achieve the same conservation goal, but with less manpower.

de Lucas, M., Ferrer, M., Bechard, M.J., and Munoz, A.R. 2012. Griffon vulture mortality at wind farms in southern Spain: distribution of fatalities and active mitigation measures. Biological Conservation 147:184-189.

Thanks to the following websites for providing the images used in this post:

Friday, 9 March 2012

The role of sex in conservation

Most of us would agree that there are fundamental differences between the sexes, beyond even the obvious anatomical variations. Some of these differences have been documented by researchers investigating how gender impacts attitudes toward ecological and conservation issues. Previous studies have shown, for instance, that men are more likely than women to condone hunting, and women are more likely than men to think that human-wildlife contact is dangerous. Variations like these may impact how successful scientists are at selling conservation plans to communities, and also how faithfully communities adhere to plans that have previously been introduced.

(Map of Namibia, where the current study was conducted. The long, thin strip between Zambia and Botswana is the Caprivi region.)

In order to investigate the presence and potential impacts of gender differences related to conservation, two researchers from Michigan State University recently studied male and female residents of Namibia's Caprivi region. There, livelihoods are dependent on natural resources that are often threatened by the region's multitude of wildlife, which are thought to cause as much as $770,000 in damage each year to livestock and crops. This human-wildlife conflict (HWC) was the main focus of the current study, as it is important not just in Namibia, but around the world; HWC has been associated with hardships for both humans (e.g., property damage, injury, death) and animals (e.g., death, reduction in habitat, extinction).

The researchers used two techniques, focus groups and interviews, to document gender differences in wildlife-related risk perception. Participants in the focus groups were asked to list risks associated with two topics: wildlife and local livelihoods. They were then asked to decide which risks were related to HWC and to discuss their results as a group. Interviews were used to gain more detailed information about attitudes toward the interface between HWC and risks to wildlife/livelihoods, factors influencing risk perception, HWC-related vulnerability, and adherence to wildlife laws.

(Sunset in Namibia's Caprivi region)

Concept maps were used to visualize patterns and explore gender differences. First, the researchers looked for "overarching risk themes" that were common across all the focus group risk lists. They then identified "theme attributes"  and "descriptions of the attributes." To explain this system of classification, the authors use the example of "lack of access to education," one of the risk themes they identified. One attribute of this theme is "lack of family planning," while a description of the theme is "increased population." Only once these were identified for all major themes did the researchers "gender disaggregate" the data in order to look for differences between the sexes. Concept mapping allowed them to see where men and women overlapped, and where they differed. A separate map was created for each of three HWC-related "dimensions": people, wildlife, and habitat.

Overall, women identified more risks than men did; 81 were related to the wildlife topic (vs. 70 for men), and 99 were related to the livelihood topic (vs. 65 for men). Gender similarities and differences varied depending on both topic and dimension. For instance, in the people dimension of the "local livelihoods" topic, women named a broad range of risks, while men provided more detail on a smaller number; one example is "infrastructure," which was named by both men and women but further defined only by men to include roads, buildings, schools, and clinics. Although both groups frequently overlapped when providing lists of things--such as dangerous animals or activities that cause wildlife-disrupting noise pollution--one group often gave more examples than another. Both sexes, for instance, listed buffaloes, elephants, hyenas, lions, and porcupines as dangerous animals, but only men added hippos and crocodiles to that list. There were also cases where one gender thought of issues or relationships not mentioned by the other. For example, only women emphasized a relationship between wildlife and disease transmission, but only men discussed the importance of resource competition between wildlife and livestock. Other topic-dimension combinations, such as the habitat  dimension of the "wildlife" topic, produced very similar responses from people of both genders. Similarly, results of the interviews suggested that both males and females had similar risk perceptions associated with human-wildlife conflict, though women were significantly more likely to be "highly worried" about the potential impacts of HWC on local livelihoods.

(Spotted hyena, one of the focal species of the Caprivi Carnivore Project, which aims to reduce carnivore-related HWC in the Caprivi region)

The combination of gender similarities and differences documented here suggests that men and women may complement each other nicely when it comes to thinking about conservation issues related to human-wildlife conflict. At the same time, the presence of gender-specific ideas and attitudes indicates that conservationists should try to avoid a "one-size-fits-all" approach to designing, and drumming up support for, new management plans. For instance, men and women in the current study both acknowledged the need for wildlife deterrents. However, women see some deterrents as a source of risk to wildlife and therefore may not support the use of these devices. In situations such as this, alternative techniques could be suggested, or conservationists might provide evidence indicating that there is no need to worry about the desired method. Either way, knowledge of the gender difference could ultimately make the mitigation plan more effective by rendering it more widely attractive throughout the community; over the long run, this could save considerable money, time, and effort.

The researchers also point out the opportunity to combine both men's and women's concerns in order to achieve different goals simultaneously. As an example, they cite the use of fences, which can be used to both reduce disease transmission (a concern of women) and diminish resource competition between wildlife and domestic stock (a concern of men). Techniques such as this allow "knowledge about gender [to be] leveraged to explicitly craft and assess interventions that more purposefully respond to the needs and perceptions of different groups." This, the researchers hope, will allow conservationists to think of gender not merely "as an explanatory variable for understanding how different groups think about conservation issues," but as an opportunity to reduce human-wildlife conflict by improving management plans.

Gore, M.L. and Kahler, J.S. 2012. Gendered risk perceptions associated with  human-wildlife conflict: implications for participatory conservation. PLoS ONE 7(3):e32901.

Thanks to the following websites for providing the images used in this post:

Thursday, 8 March 2012

Do blue whales hear mid-frequency anthropogenic noise?

Although a number of marine acoustic studies have investigated how whales are impacted by masking anthropogenic noise--or sounds that overlap with the whales' own vocalizations--less research has focused on noise that does not seem to directly threaten the animals' abilities to communicate with each other. However, many whales can detect a wide variety of acoustic stimuli that occur outside the frequency range utilized when vocalizing with conspecifics. For instance, foraging blue whales (Balaenoptera musculus) produce low-pitched (<100 Hz) sounds, called D calls, that appear to help them coordinate group feeding behaviors; however, the animals are probably capable of hearing much higher frequencies, such as those characterizing their own echolocation clicks (reportedly reaching frequencies as high as 31 kHz). Thus, it stands to reason that higher-pitched human noises might also elicit behavioral responses in this species.

(Blue whale, Balenoptera musculus)

A team of researchers from the Scripps Institution of Oceanography recently explored this possibility using an acoustic recording device deployed in the Southern California Bight, an important summer feeding ground for the endangered blue whale. The recorder collected continuous acoustic data during four separate 2- to 4-month periods over 2 different summers. From these data, the researchers could isolate occurrences of anthropogenic noises (sonar, ship noise, and explosions--all occurring within the 1-8 kHz frequency band), "natural" noise events (wind, rain, earthquakes, other animals), and blue whale D calling. The researchers calculated the probability of D calling during each of the anthropogenic noise sources, as well as during all "natural" acoustic events. The latter was used as a baseline indicating how the whales should respond to anthropogenic noise events if they are seen as comparable to "natural" sources of noise. Thus, ratios of anthropogenic to "natural" noise should be high for human sounds that don't disturb the whales, but low for human sounds that do.

Ratios were fairly high (0.83) for ship noise, indicating that the whales are not particularly troubled by this sound. However, calling dropped dramatically during explosions (ratio = 0.63) and, especially, sonar (ratio = 0.54). To explore this pattern further, the researchers then investigated D calling behavior with respect to the maximum sound pressure level (or volume) of "natural" and anthropogenic noises throughout the study period. Although whales were equally likely to produce D calls during "natural" noises of all volumes, there was a higher probability of vocalizations with increasingly loud ship noise, and a lower probability with increasingly loud sonar events.

(Transmission patterns for side-scan and multibeam sonar)

The researchers hypothesize that ship noise is associated with increased calling activity because this behavioral shift may enable whales to overcome potential masking--a tactic appropriate for this type of noise pollution because it is a broadband signal that has acoustic energy in the lower pitches at which blue whales communicate. On the other hand, sonar seems to act as a call deterrent. Interestingly, although blue whales in this population typically D call more often during sunset and just before sunrise, they did not appear to be more impacted by anthropogenic noises during these times of day.

Overall, the blue whale behaviors documented here suggest that these animals can hear mid-range (1-8 kHz) anthropogenic noises. Although this seems like an obvious conclusion, it actually isn't; nobody has been able to directly measure the hearing sensitivity of this species, and examinations of their vocalizations have led to hypotheses that the whales hear best around 20 Hz, and decently up to 200 Hz. There have been some doubts as to whether decades-old recordings of high-pitched (4-10 and 21-31 kHz) echolocation clicks are accurate, so researchers have been unsure whether blue whales could detect, and potentially be bothered by higher-pitched sounds such as anthropogenic noises. The current results indicate that the answer to this question is "yes."

In fact, the whales altered their behaviors even after stimuli that were "relatively low intensity." Thus, even isolated noise events that happen several kilometers away could be problematic for these animals. Although the current study found no relationship between time of day and strength of whale response to noise, the researchers still suspect that whales are likely to be most susceptible to noise during their peak vocalization times (at sunset and just prior to sunrise); additional studies that focus on other whale behaviors and obtain an even bigger bigger sample size may be able to detect such patterns.

For supplementary images associated with this post, visit the Anthrophysis pin board at Pinterest.

Melcon, M.L., Cummins, A.J., Kerosky, S.M., Roche, L.K., Wiggins, S.M., and Hildebrand, J.A. 2012. Blue whales respond to anthropogenic noise. PLoS ONE 7(2):e32681.

Thanks to the following websites for providing the images used in this post: