Environmental unpredictability and stochasticity underlie dispersive
movements of a terrestrial amphibian
Abstract
Dispersive movements are often thought to be multiclausal and driven by
individual body size, sex, conspecific density, environmental variation
and/or other factors. Yet such factors rarely account for most of the
variation present among dispersive movements in nature, leaving open the
possibility that dispersion might be indeterministic and vary in
response to environmental stochasticity. We assessed the amount of
variation in movement distances that could be accounted for by potential
predictors of dispersal with a large empirical dataset of movement
distances performed by Fowler’s Toads (Anaxyrus fowleri) on the northern
shore of Lake Erie at Long Point, Ontario (2002 – 2021, incl.). These
toads are easy to sample repeatedly, can be identified individually and
undertake dispersive movements parallel to the shoreline on a daily
basis as they forage at night. Using a linear mixed-effect model that
incorporated random effect terms to account for sampling variance and
inter-year environmental variation, we found that all potential
predictors of dispersive movements of these animals were, at best, weak
predictors that accounted for virtually none of the variation observed
among movement distances. We also used linear regression models to test
for the impact of environmental stochasticity on dispersive movements
and identified a strong positive correlation between the distribution of
toad movement distances and variability in lake water level. We conclude
that deterministic proximal factors, whether intrinsic or extrinsic,
neither can be shown nor are necessary to drive dispersive movements in
this population. Variation in dispersive movements can be ascribed,
instead, to environmental unpredictability, consistent with nomadism.