Trap characteristics and species morphology explain size-biased sampling of two salamander species

in Amphibia-Reptilia
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Demographic studies often depend on sampling techniques providing representative samples from populations. However, the sequence of events leading up to a successful capture or detection is susceptible to biases introduced through individual-level behaviour or physiology. Passive sampling techniques may be especially prone to sampling bias caused by size-related phenomena (e.g., physical limitations on trap entrance). We tested for size-biased sampling among five types of passive traps using a 9-year data set for two species of aquatic salamanders that have a 20 and 61 fold change in length over their ontogeny (Amphiuma means, Siren lacertina). Size-biased trapping was evident for both species, with body size distributions (body length mean and SD) of captured individuals differing among sampling techniques. Because our two species differed in girth at similar lengths, we were able to show that size biases (in length) were most likely caused by girth limitations on trap entry rates, and potentially by differences in retention rates. Accounting for the biases of sampling techniques may be critical when assessing current population status and demographic change.

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Figures
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    Theoretical illustration of the passive trapping process and the generation of a size-biased sample. Population size-frequency histograms are partially shaded to indicate the individuals remaining in a sample. Arrows indicate that a sample of individuals is filtered (dashed rectangle) to yield a particular subset at the next stage of the capture process. In the above example, availability and encounter rates are independent of size, but the capture process excludes large individuals, while the retention process allows small individuals to leave and selects for larger individuals in the final sample.

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    Relationship between snout-vent length (SVL) and mass (A) and girth (B) for S. lacertina (open circles) and A. means (closed circles). Length-mass relationships for both species and length-girth relationships for S. lacertina are taken from live animals in the study population. Length-girth relationships for A. means were collected from museum specimens (see methods). Girth and snout-vent length were strongly correlated for S. lacertina (girth = 0.12 × SVL − 0.10; Adjusted R2 = 0.96) and A. means (girth = 0.07 × SVL − 0.36; Adjusted R2 = 0.92).

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    Capture density plots (proportion of individuals captured across a range of SVL) for Siren lacertina (grey line) and Amphiuma means (black line) from 2006-2014. Also shown are predicted maximum (solid vertical line) and minimum (dashed vertical line) sizes capturable for plastic minnow traps, steel minnow traps, trashcan minnow traps, fyke net, and hoop nets. Fyke and hoop net maxima are absent because they exceed the maximum sizes recorded for both species. No A. means were captured in hoop nets.

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    Estimated mean and standard deviation for snout-vent length of captured Siren lacertina and Amphiuma means for each trap type. Dots indicate point estimates (posterior means) for each parameter while vertical bars depict 95% credible intervals. Different letters among parameter estimates indicate that the 95% CRI for the difference between those parameters (within species) does not overlap zero.

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    Converted values of girth for expected means and standard deviations from fig. 4 using length-girth regression equations. Data plotted for visual reference but not analysed because girth was estimated from girth-length relationships and not directly measured from each individual.

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