Dispersal without drivers: Intrinsic and extrinsic variables have no impact on movement distances in a terrestrial amphibian

Abstract

Dispersive movements are often thought to be multicausal and driven by individual body size, sex, conspecific density, environmental variation, personality, and/or other variables. Yet such variables often do not account for most of the variation among dispersive movements in nature, leaving open the possibility that dispersion may be indeterministic. We assessed the amount of variation in 24 h movement distances that could be accounted for by potential drivers of displacement 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 move parallel to the shoreline as they forage at night, potentially dispersing to new refuge sites. Using a linear mixed‐effect model that incorporated random effect terms to account for sampling variance and inter‐annual variation, we found that all potential intrinsic and extrinsic drivers of movement accounted for virtually none of the variation observed among 24 h distances moved by these animals, whether over short or large spatial scales. We examined the idea of movement personality by testing variance per individual toad and found no evidence of individuality in movement distances. We conclude that deterministic variables, whether intrinsic or extrinsic, neither can be shown to nor are necessary to drive movements in this population over all spatial scales. Stochastic, short time‐scale movements, such as daily foraging movements, can instead accumulate over time to produce large spatial‐scale movements that are dispersive in nature.

Life on the beach: Movements and growth of a coastal amphibian vary with distance to the sea

“…However, we cannot differentiate movements linked to reproduction (migration to ponds) from those expressed during non-breeding activities (resting; foraging), during which individuals could express diverging patterns (Bailey & Muths, 2019;Cayuela et al, 2017;Joly, 2019). Although the spatial extent of movements can depend on individual traits (Cayuela et al, 2020;Hillman et al, 2014;Hust e et al, 2006;Sinsch, 2014;Trochet et al, 2016), we failed to detect any effect of body size and sex on the distances between captures (see also Jreidini & Green, 2022a, 2022bSmith & Green, 2006). Although it is likely that individuals move greater distances to reproduce (Priol, 2015), our results suggest that individuals are able to return to a specific terrestrial site between reproductive events (see also Jreidini & Green, 2022b).…”

“…In this study, we used a capture–mark–recapture design during 3 consecutive years (in autumn, outside the reproductive period of the species) to investigate how distance to the sea influences growth rates and foraging movements in the western spadefoot toad ( Pelobates cultripes ), in three populations from the French Atlantic coast. We posit that (1) growth rates should be higher in juveniles because growth rates decrease with increasing body size (Hota, 1994; Turner, 1962), growth rates should be higher in females because they reach larger body size than males of the same age (Hota, 1994; Wells, 2007), and that growth rates should be higher farther to the seashore according to the physiological constraint of salinity near the coast (Lorrain‐Soligon, Robin, Lelong, Jankovic, Bustamante, et al., 2022), (2) movements (distance traveled) outside the reproductive season (i.e., linked to foraging, Jreidini & Green, 2022a; Jreidini & Green, 2022b; Pittman et al., 2014; Russell et al., 2005) should be higher in juveniles (Bulger et al., 2003; Smith & Green, 2006) and females (Trochet et al., 2016) to sustain higher growth rates (Hota, 1994; Wells, 2007) and should be lower near the seashore if exposure to salinity negatively affects activity (Feder & Londos, 1984; Lorrain‐Soligon, Robin, Lelong, Jankovic, Barbraud, et al., 2022; Titon Jr et al., 2010), and (3) that individuals should express high site fidelity according to their ecology (Priol, 2015).…”

Study methodology impacts density-dependent dispersal observations: a systematic review

“…Numerous species of migrating animals, such as salmon [ 42 ] and salamanders [ 122 ], will alternate between movement away from a breeding site and aggregation as they return to it, with correlated changes in their density. Alternatively, density may have no perceivable effect on dispersive movements (e.g., [ 51 ]), or its effect on dispersal may vary according to a density ‘threshold’ (e.g., [ 7 , 33 , 78 ]) or even appear to be temporary (e.g., [ 17 ]). Individual assemblages may therefore be linked to different types of density-dependent dispersal: positive, negative, neutral, and even non-linear [ 41 ].…”

“…Similarly, density can be measured at different life history stages; for example, natal density can be the number of birds in a nest while breeding density the number of nests [ 14 ]. Studies have also used proxy measures for density, such as habitat carrying capacity [ 55 ], patch area [ 100 ], nearest-neighbor distance [ 51 ], and even habitat quality [ 49 , 57 , 64 ] on the assumption that higher habitat quality should equate to higher population density [ 12 , 14 , 16 , 20 ]. The different density and dispersal metrics therefore could result in incomparable outcomes when it comes to the relationship between density and dispersal.…”