Accounting for detectability improves estimates of species richness in tropical bat surveys

Abstract

Summary1. Species richness is a state variable of some interest in monitoring programmes but raw species counts are often biased due to imperfect species detectability. Therefore, monitoring programmes should quantify detectability for target taxa to assess whether it varies over temporal or spatial scales. We assessed the potential for tropical bat monitoring programmes to reliably estimate trends in species richness. 2. Using data from 25 bat assemblages from the Old and New World tropics, we estimated detectability for all species in an assemblage (mean proportion of species detected per sampling plot) and for individual species (species-specific detectability). We further assessed how these estimates of detectability were affected by external sources of variation relating to time, space, survey effort and biological traits. 3. The mean proportion of species detected across 96 sampling plots was estimated at 0AE76 (range 0AE57-1AE00) and was significantly greater for phytophagous than for animalivorous species. Species-*Correspondence author. E-mail: cmeyer@fc.ul.pt 1365-2664.2011.01976.x Ó 2011 The Authors. Journal of Applied Ecology Ó 2011 British Ecological Society averaged detectability for phytophagous species was influenced by the number of surveys and season, whereas the number of surveys and sampling methods [ground-or canopy-level mist nets, harp traps and acoustic sampling (AS)] most strongly affected estimates of detectability for animalivorous bats. Species-specific detectability averaged 0AE4 and was highly heterogeneous across 232 species, with estimates ranging from 0AE03 to 0AE84. Species-level detectability was influenced by a range of external factors such as location, season, or sampling method, suggesting that raw species counts may sometimes be strongly biased. 4. Synthesis and applications. Due to generally high species-specific detection probabilities, Neotropical aerial insectivorous bats proved to be well suited for monitoring using AS. However, for species with low detectability, such as most gleaning animalivores or nectarivores, count data obtained in bat monitoring surveys must be corrected for detection bias. Our results indicate that species-averaged detection probabilities will rarely approach 1 unless many surveys are conducted. Consequently, long-term bat monitoring programmes need to adopt an estimation scheme that corrects for variation in detectability when comparing species richness over time and when making regional comparisons. Similar corrections will be needed for other species-rich tropical taxa. Journal of AppliedEcology 2011, 48, 777-787 doi: 10.1111/j.

Mobility explains the response of aerial insectivorous bats to anthropogenic habitat change in the Neotropics

“…Our overall detectability of 65% in a single survey agrees well with observations from Meyer et al (2011), who found average detectability with acoustic sampling to be 71% for Neotropical AIB. Both values are substantially higher than the 36-40% observed by Duchamp et al (2006) for temperate North America, which could be related to lower overall bat abundance in higher latitudes or to methodological differences, e.g.…”

“…Hence, to calculate species-specific detectability and occupancy, repeated surveys are necessary (MacKenzie et al, 2006). The only acoustic sampling study to date to calculate detectability for Neotropical AIBs found an average detectability of 0.71 (i.e., given its presence, a species is detected in 71% of all sampling events), a value considerably higher than the detectability ranging from 0.25 to 0.58 calculated for other bat trophic guilds through mist netting (Meyer et al, 2011). Thus including detectability can be crucial when estimating the effects of anthropogenic processes on species.…”

“…Points were sampled in three non-consecutive nights for three hours each, starting at sunset. This sampling regime has been shown to yield >90% of all local AIB species (EstradaVillegas et al, 2010;Meyer et al, 2011). We restricted sampling to nights with no or very limited rainfall (<20 mm).…”

Bats (Mammalia: Chiroptera) in a cerrado landscape in São Carlos, southeastern Brazil

“…Even in a better sampled biome such as the Atlantic Forest, most studies found local richness values below 30 species (Stevens 2013), but it is known that the biome harbors much more species than that (Paglia et al 2012). Detectability differs largely among bat species (Meyer et al 2011), mainly when a single sampling method is used. Thus, a combination of mist netting and additional sampling methods such as bioacoustic monitoring would probably increase the recorded richness in the area (as in Sampaio et al 2003).…”

Does environmental suitability explain the relative abundance of the tailed tailless bat, Anoura caudifer?

“…As trophic guilds can differ in the way they move and obtain resources, differences in their detectability are expected. Thus, we estimated the proportion of nectarivore species detected in each study (Meyer et al 2011). Because there is no reliable method to measure the absolute population density of bats, we calculated the relative abundance of the nectarivores according to Arita (1993), dividing the number of captures of A. caudifer by the overall number of nectarivores.…”