Road transect surveys do not reveal any consistent effects of a toxic invasive species on tropical reptiles
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ORIGINAL PAPER
Road transect surveys do not reveal any consistent effects of a toxic invasive species on tropical reptiles Gregory P. Brown • Matthew J. Greenlees Benjamin L. Phillips • Richard Shine
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Received: 30 April 2012 / Accepted: 11 October 2012 / Published online: 21 October 2012 Ó Springer Science+Business Media Dordrecht 2012
Abstract Effects of perturbations to wildlife often are measured by changes in rates of encounter with animals during standardised surveys, such as along roads. Previous work has predicted that the invasion of toxic cane toads (Rhinella marina) through the Australian tropics will cause massive mortality of anuran-eating snakes, and influence abundances of other native species. We surveyed three adjacent road transects for nocturnal snakes and lizards, beginning shortly before toads arrived at this site near Darwin, in the Northern Territory. In the wet-seasons of four successive years, we conducted surveys on 591 nights; on 302 of these nights, all three transects were surveyed. We recorded 8,880 live cane toads and 3,365 live reptiles. Toad numbers increased over time on all three transects but encounters with 13 species of native reptiles varied inconsistently. Eight of the 13 species of native reptile showed no significant change in encounter rates following the arrival of toads. Of the five species that did change in encounter rates, only one taxon (the bluetongue skink, Tiliqua scincoides
G. P. Brown M. J. Greenlees B. L. Phillips R. Shine (&) School of Biological Sciences A08, University of Sydney, Sydney, NSW 2006, Australia e-mail: [email protected] Present Address: B. L. Phillips School of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia
intermedia) declined across all three transects. Encounter rates of the other four species often increased on at least one transect but decreased on at least one other. Thus, either the impact of cane toads on counts of reptiles differed between nearby sites, or (more likely) other factors had more influence on reptile numbers. A consistent decrease in reptile numbers on the busiest road over the study period suggests that local snake populations were affected more by road-kill than by invasive toads. Without spatial replication, this decrease could have been interpreted as an impact of toad invasion. Keywords Abundance Alien species Bufo marinus Ecological impact Predator–prey Surveys
Introduction Biodiversity is under threat from a wide array of factors, ranging from local-scale effects (such as habitat degradation) through to landscape-scale processes (such as climate change). Faced with limited resources, managers must allocate their efforts towards mitigating the factors with the greatest potential impact. To do so, they need to measure the magnitude of impact. Measuring changes in abundance of native species as a result of a specific
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threatening process may be relatively easy in some cases, notably with large sedentary organisms (like trees) exposed to local t
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