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Rodda G.H.,U.S. Geological Survey | Adams A.A.Y.,U.S. Geological Survey | Campbell E.W.,Ohio Cooperative Fish and Wildlife Research Unit | Fritts T.H.,U.S. Geological Survey
Journal of Herpetology | Year: 2015

Accurate estimation of a species' size distribution is a key component of characterizing its ecology, evolution, physiology, and demography. We compared the body size distributions of five Pacific lizards (Carlia ailanpalai, Emoia caeruleocauda, Gehyra mutilata, Hemidactylus frenatus, and Lepidodactylus lugubris) from general herpetological collecting (including visual surveys and glue boards) with those from complete censuses obtained by total removal. All species exhibited the same pattern: general herpetological collecting undersampled juveniles and oversampled mid-sized adults. The bias was greatest for the smallest juveniles and was not statistically evident for newly maturing and very large adults. All of the true size distributions of these continuously breeding species were skewed heavily toward juveniles, more so than the detections obtained from general collecting. A strongly skewed size distribution is not well characterized by the mean or maximum, though those are the statistics routinely reported for species' sizes. We found body mass to be distributed more symmetrically than was snout-vent length, providing an additional rationale for collecting and reporting that size measure. © Copyright 2015 Society for the Study of Amphibians and Reptiles. Source


Campbell III E.W.,Ohio Cooperative Fish and Wildlife Research Unit | Yackel Adams A.A.,U.S. Geological Survey | Converse S.J.,U.S. Geological Survey | Fritts T.H.,U.S. Geological Survey | Rodda G.H.,U.S. Geological Survey
Ecology | Year: 2012

The effect of predators on the abundance of prey species is a topic of ongoing debate in ecology; the effect of snake predators on their prey has been less debated, as there exists a general consensus that snakes do not negatively influence the abundance of their prey. However, this viewpoint has not been adequately tested. We quantified the effect of brown treesnake (Boiga irregularis) predation on the abundance and size of lizards on Guam by contrasting lizards in two 1-ha treatment plots of secondary forest from which snakes had been removed and excluded vs. two 1-ha control plots in which snakes were monitored but not removed or excluded. We removed resident snakes from the treatment plots with snake traps and hand capture, and snake immigration into these plots was precluded by electrified snake barriers. Lizards were sampled in all plots quarterly for a year following snake elimination in the treatment plots. Following the completion of this experiment, we used total removal sampling to census lizards on a 100-m2 subsample of each plot. Results of systematic lizard population monitoring before and after snake removal suggest that the abundance of the skink, Carlia ailanpalai, increased substantially and the abundance of two species of gekkonids, Lepidodactylus lugubris and Hemidactylus frenatus, also increased on snake-free plots. No treatment effect was observed for the skink Emoia caeruleocauda. Mean snout-vent length of all lizard species only increased following snake removal in the treatment plots. The general increase in prey density and mean size was unexpected in light of the literature consensus that snakes do not control the abundance of their prey species. Our findings show that, at least where alternate predators are lacking, snakes may indeed affect prey populations. © 2012 by the Ecological Society of America. Source


Rodda G.H.,U.S. Geological Survey | Dean-Bradley K.,U.S. Geological Survey | Campbell E.W.,Ohio Cooperative Fish and Wildlife Research Unit | Fritts T.H.,U.S. Geological Survey | And 3 more authors.
Journal of Herpetology | Year: 2015

To obtain quantitative information about population dynamics from counts of animals, the per capita detectabilities of each species must remain constant over the course of monitoring. We characterized lizard detection constancy for four species over 17 yr from a single site in northern Guam, a relatively benign situation because detection was relatively easy and we were able to hold constant the site, habitat type, species, season, and sampling method. We monitored two species of diurnal terrestrial skinks (Carlia ailanpalai [Curious Skink], Emoia caeruleocauda [Pacific Bluetailed Skink]) using glueboards placed on the ground in the shade for 3 h on rainless mornings, yielding 10,286 skink captures. We additionally monitored two species of nocturnal arboreal geckos (Hemidactylus frenatus [Common House Gecko]; Lepidodactylus lugubris [Mourning Gecko]) on the basis of 15,212 sightings. We compared these count samples to a series of complete censuses we conducted from four or more total removal plots (everything removed to mineral soil) totaling 400 m (about 1% of study site) in each of the years 1995, 1999, and 2012, providing time-stamped quantification of detectability for each species. Unfortunately, the actual population trajectories taken by the four species were masked by unexplained variation in detectability. This observation of debilitating latent variability in lizard detectability under nearly ideal conditions undercuts our trust in population estimation techniques that fail to quantify venue-specific detectability, rely on pooled detection probability estimates, or assume that modulation in predefined environmental covariates suffices for estimating detectability. © Copyright 2015 Society for the Study of Amphibians and Reptiles. Source

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