Thursday, 1 March 2012

Australian bats show species-specific responses to urbanization

For years, researchers have been trying to figure out which characteristics allow animals to cope with human disturbances and habitats. Although birds have been particularly well studied in this context, the most recent study on this topic focused not on feathered fliers, but on those with fur: bats.

Researchers from the University of New South Wales, the Forest Science Centre, and the University of Sydney collaborated on a project looking to examine how different species of Australian bat respond to variations in habitat characteristics throughout the city of Sydney. They hoped that the results could be used to predict the potential effects of future habitat alterations and also to determine what habitat restoration efforts, if any, could improve bat biodiversity in the city. The researchers were particularly interested in insectivorous bats, which perform the valuable but often underappreciated ecosystem service of keeping insect populations from getting out of hand.

(Sydney, Australia)

The study focused on a portion of the city located within a 60-km radius of its central business district. Automated bat detectors collected data at a total of 113 sites, each of which was sampled during the bat maternity season (October-December), when bats are likely to be most active. The detectors recorded all bat vocalizations throughout each sampling night and the resulting recordings were analyzed by a computer program that was able to identify which species produced each vocal signature.

 (An Anabat recording device)

The study sites were distributed across 29 different 25-square-kilometer landscapes, each of which was classified as urban, suburban, or vegetated. Bat activity was sampled in each of four elements located within each landscape: remnant bushland, riparian areas, open space/parkland, and residential/built. In order to paint a more specific picture of habitat features at each recording site, the researchers established vegetation sampling transects and quantified a number of characteristics, such as the presence and size of trees--including those that could be used as bat roosts--canopy cover, and vegetation structure. Using GIS, the scientists also evaluated landscape variables such as the distances to the nearest water source and native bushland. Finally, insect sampling was performed in order to estimate prey biomass at each site.

All told, the detectors recorded 7,767 bat passes that could be attributed to 17 different groups of bats; 8 of these are currently listed as threatened. Although some detectors recorded as many as 413 passes and 11 taxa per night, others recorded no activity at all. In other words, bat activity varies greatly within an urban environment.

(Gould's wattled bat, Chalinolobus gouldii, classified by the current study as an urban-tolerant bat)

Gould's wattled bat (Chalinolobus gouldii), which was detected three times as often as the next most common species, was present in three-quarters of all sites. Thus, this species appears to be an urban adaptor. The eastern bent-wing bat (Miniopterus schreibersii oceansis) and white-striped free-tailed bat (Tadarida australis) are also quite tolerant of urban habitats. On the other hand, several species were entirely absent from urban areas; these include the little forest bat (Vespadelus vulturnus) and the chocolate wattled bat (Chalinolobus morio). Species assemblages in urban landscapes were different from those in suburban and vegetated areas--a pattern driven primarily by the absence or lower detection of species in urban sites.

Local environmental characteristics explained about 25% of variation in species activity, which may not sound very high, but still suggests that the bats are fairly sensitive to their surroundings. Total moth biomass, open space, and gaps in the vegetation were important features for several species. Taken together, these results indicate that, perhaps unsurprisingly, bat habitat preference is strongly driven by feeding requirements.

(White-striped free-tailed bat, Tadarida australis, classified by the current study as an urban-tolerant species)

If the results of this study are any indication of conditions in other cities, then it appears that urban areas can support an impressively diverse bat fauna. In fact, the authors suggest that bats may be the most diverse group of mammals left in many places. However, activity levels of suburban bats were lower than those found in more "natural" areas, possibly suggesting fewer available prey items. Additionally, bat assemblages in human-modified sites were less complex, and the researchers hypothesize that urban areas may "filter" out some species by removing roosts and altering the acoustic habitat. The latter suggestion stems from observations that city-dwelling bats consistently have lower-frequency echolocation calls.

(A little forest bat, Vespadelus vulturnis, classified by the current study as moderately sensitive to urbanization)

The authors stress that some of the species they recorded in urban areas may not necessarily be urban-tolerant; it is possible that these animals face other threats not measured here. Given the bats clear avoidance of some habitat features, it is possible that individuals in urban environments may sometimes become isolated in patches of "suitable" habitat bordered by "unsuitable" human-modified elements. Additional studies will be needed to ascertain whether bats will use, and therefore potentially benefit from, corridors linking these disparate patches. One technique that might come in handy is the use of radio-tracking technology, which can provide detailed information on where bats are spending their time. In the mean time, it will be important to preserve, and potentially even expand or recreate, patches containing the animals' preferred habitat features.

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

Threlfall, C.G., Law, B., and Banks, P.B. 2012. Sensitivity of insectivorous bats to urbanization: implications for suburban conservation planning. Biological Conservation 146:41-52.

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


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