Stability of Detectability over 17 Years at a Single Site and other Lizard Detection Comparisons from Guam
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 m2 (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.Abstract
Gecko sighting rates for Leucaena forest on Guam, 1985–2012, with rainfall in the 12 mo preceding the midpoint of each year expressed as a percentage of normal precipitation. Sample sizes varied from 10.3–1,301.3 and totaled 7,020.5 person-hours.
Distribution of mean gecko CPUE rates for 54 searchers for which we had at least 10 person-hours of searching (exclusive of training) from Leucaena forest sites on Guam 1985–2012 (individual searchers varied from 10.8–865.2 and totaled 4,933.7 person-hours).
Road edge versus interior transect gecko detection rates for Leucaena forest in Guam 1985–2012. Sample sizes ranged from 10.5–811.4 and totaled 4,933.7 person-hours.
Gecko detection rates by starting times of nocturnal visual bouts in Leucaena forest on Guam 1985–2012. Local dusk varied from about 1830 to 1930 h. Sample sizes declined from a peak of 2,104.3 person-hours in the 2000 starting hour class to earlier and later time slots (minimum: 13.2 person-hours for the bouts starting after midnight). Total surveys contributing to this figure were 7,014.5 person-hours.
Correspondence between total removal results based on pairwise proportional trends in censuses and relative counts. Each of four species is represented three times. P-value is two-tailed.
Test results between lizard biomass and detectability multiplier, based on 19–34 total removal plots in the Mariana Islands, including all sampled habitats where the focal species were resident 1995–2012. P-value is two-tailed. Note that higher detectability multipliers indicate lower detectability.
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