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Are communal nesting counts as useful as mark–recapture data for estimating population size in snakes?

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Snakes are rather difficult subjects for demographic studies. When snakes are not abundant in the field, herpetologists have learnt that a good method for population studies is to rely on mass captures at den sites. In several snake species females also exhibit oviposition at communal nest sites, which are utilized year after year. These oviposition sites may then serve to record individuals for snake population studies. Here, we compared population size estimates generated from a 17-year study of gravid females at a communal nesting site (CNF) with population size estimates from the same snake population across an 8-year traditional capture–mark–recapture (CMR) study. Although in our case only open population methods are appropriate for calculating yearly population sizes, we also used closed population methods in order to highlight an eventual effect of the models used. As a study species, we used the European whip snake (Hierophis viridiflavus) at a site in Mediterranean central Italy. Overall, population size estimates were significantly different between the two methods, with estimates from the CNF samples always higher than those obtained with traditional CMR. This difference was particularly strong with closed population methods, but still evident with open population models when the whole study period was considered. However, there were no statistical differences between population sizes estimated with CNF and CMR when only a subset of years (2002–2009) was used. No statistical relationship between population size estimates with CMR against CNF by year was uncovered, showing that CNF samples did not capture inter-annual variations in population sizes. We conclude that it might not be sound to use population size estimates from CNF samples instead of more traditional CMR studies, although yearly population size variations may at least in part be responsible for the differences between CNF and CMR estimates.
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Keywords: COMPARATIVE ANALYSES; DEMOGRAPHY; FIELD TECHNIQUES; ITALY; SERPENTES; STATISTICS

Document Type: Research Article

Publication date: 2011-01-01

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