Accounting for detectability improves estimates of species richness in tropical bat surveys
1. 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 0·76 (range 0·57–1·00) and was significantly greater for phytophagous than for animalivorous species. Species-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 0·4 and was highly heterogeneous across 232 species, with estimates ranging from 0·03 to 0·84. 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.
Document Type: Research Article
Affiliations: 1: Departamento de Zoologia, Universidade de Brasília, Brasília, Distrito Federal 70910-900, Brazil 2: Departamento de Ciências Biológicas, Universidade Estadual de Santa Cruz, CEP 45650-000 Ilhéus, Bahia, Brazil 3: Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen, AB24 2TZ, UK 4: INRA, UMR CBGP, Campus International de Baillarguet CS 30016, F-34988 Montferrier-sur-Lez, France 5: Department of Biology, McGill University, 1205 Docteur Penfield, Montréal, Canada H3A 1B1 6: Institute of Experimental Ecology, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany 7: INRA, UMR 406 Abeilles & Environnement, Site Agroparc, F-84914 Avignon, France 8: Department of Biological Sciences, Texas Tech University, Lubbock, TX 79409-3131, USA 9: Department of Biology, Center for Ecology and Conservation Biology, Boston University, Boston, MA 02215, USA 10: Centro de Estudios en Biología Teórica y Aplicada, Programa para la Conservación de los Murciélagos de Bolivia, La Paz, Bolivia 11: Department of Zoology, Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago IL 60605-2496, USA 12: UMR 7205, Department Systématique et Evolution, Muséum National d’Histoire Naturelle, 55 Rue Buffon, BP51, F-75005 Paris, France 13: Centre for Ecology and Conservation, University of Exeter in Cornwall, Penryn, TR10 9EZ, UK 14: Leibniz-Institute for Zoo and Wildlife Research, Alfred-Kowalke-Str. 17, 10315 Berlin, Germany 15: Instituto de Biología, Universidad de Antioquia, A.A. 1226, Medellín, Colombia 16: Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de México, Morelia, Apartado Postal 27-3, 58089 Morelia, Michoacán, Mexico 17: Bat Conservation International, P.O. Box 162603, Austin, TX 78716, USA
Publication date: June 1, 2011