Professor McGill University Montreal, Quebec, Canada
Background/Question/Methods
Dispersive patterns are often thought to be driven by individual body size, sex, and density, yet such proximal factors rarely account for most of the variation present among dispersive movements in nature. We assessed the amount of variation in movement distances that could be accounted for by potential predictors of dispersal using a 20-year empirical dataset of movements performed by Fowler’s Toads (Anaxyrus fowleri) on the northern shore of Lake Erie at Long Point, Ontario. These toads are individually identifiable and undertake dispersive movements parallel to the shoreline as they forage at night. We used linear mixed-effect models (LMMs) to explore the impact of intrinsic and extrinsic predictors on dispersive movements, and linear regressions to explore the impact of environmental stochasticity as the annual standard variation in three environmental variables: mean daily air temperature, total daily precipitation, and maximum monthly lake water level. If any intrinsic or extrinsic predictors are responsible for the dispersive movements made by these toads, then they should account for significant amounts of the variation seen in the toads’ movement distances. Alternatively, if the toads are nomadic, then environmental variation should be of greater significance than any potential proximal predictors, whether intrinsic or extrinsic.
Results/Conclusions
We found that all potential predictors of dispersive movements of these animals were, at best, weak predictors, as they accounted for virtually none of the variation observed among movement distances (LMM marginal R2 = 0.014). However, when exploring the impact of environmental stochasticity on dispersive movements, we identified a strong positive correlation between the distribution of toad movement distances and variability in lake water level (linear regression R2 = 0.380). As there exists a conceptual gap between the fields of animal movement ecology and animal dispersal, the concept of nomadism can bridge the gap between the indeterministic short-scale movement distances and stochastic large-scale dispersal. Our results from studying a very simple system of small amphibians moving at will to and fro along a lakeshore do not exclude the possibility that dispersal in other organisms in other environments may be significantly driven by any combination of internal and/or extrnal factors. We do show, however, that this need not necessarily always be true. We therefore conclude that deterministic proximal factors, whether intrinsic or extrinsic, are not necessary to drive dispersive movements, and that variation in dispersive movements can be ascribed, instead, to stochastic environmental unpredictability, consistent with nomadic movement patterns.
