Size‐dependent movement explains why bigger is better in fragmented landscapes

Abstract

Body size is a fundamental trait known to allometrically scale with metabolic rate and therefore a key determinant of individual development, life history, and consequently fitness. In spatially structured environments, movement is an equally important driver of fitness. Because movement is tightly coupled with body size, we expect habitat fragmentation to induce a strong selection pressure on size variation across and within species. Changes in body size distributions are then, in turn, expected to alter food web dynamics. However, no consensus has been reached on how spatial isolation and resource growth affect consumer body size distributions. Our aim was to investigate how these two factors shape the body size distribution of consumers under scenarios of size‐dependent and size‐independent consumer movement by applying a mechanistic, individual‐based resource–consumer model. We also assessed the consequences of altered body size distributions for important ecosystem traits such as resource abundance and consumer stability. Finally, we determined those factors that explain most variation in size distributions. We demonstrate that decreasing connectivity and resource growth select for communities (or populations) consisting of larger species (or individuals) due to strong selection for the ability to move over longer distances if the movement is size‐dependent. When including size‐dependent movement, intermediate levels of connectivity result in increases in local size diversity. Due to this elevated functional diversity, resource uptake is maximized at the metapopulation or metacommunity level. At these intermediate levels of connectivity, size‐dependent movement explains most of the observed variation in size distributions. Interestingly, local and spatial stability of consumer biomass is lowest when isolation and resource growth are high. Finally, we highlight that size‐dependent movement is of vital importance for the survival of populations or communities within highly fragmented landscapes. Our results demonstrate that considering size‐dependent movement is essential to understand how habitat fragmentation and resource growth shape body size distributions—and the resulting metapopulation or metacommunity dynamics—of consumers.

The neglected impact of tracking devices on terrestrial arthropods

“…In flying arthropods, wing loading (ratio of body mass to wing area) can scale positively but also negatively with body mass depending on the species (Boiteau & Colpitts, 2001;Darveau, Hochacka, Welch, Roubik, & Suarez, 2005;Gilchrist & Huey, 2004), implying that larger individuals are not necessarily better fliers. Other differences in ecology and behaviour might be related to body size within a species (Hillaert, Hovestadt, Vandegehuchte, & Bonte, 2018). For instance, in a study on the near-threatened beetle Osmoderma eremita (Nieto & Alexander, 2010), it was observed that radiotagged beetles survived better than beetles marked for classic capture-mark-recapture analysis (Le Gouar, Dubois, Vignon, Brustel, & Vernon, 2015).…”

Habitat loss and fragmentation increase realized predator–prey body size ratios

“…By constraining reproduction, herbivore population growth is strongly reduced, thereby promoting predator mobility even when P is high. This mechanism is confirmed by the observation that lowering resource growth speed within the resource‐herbivore model also resulted in selection of larger herbivores (Hillaert, Hovestadt, et al, ). Under low P and low herbivore reproductive values, the largest predators can no longer persist due to food limitation and selection turns towards smaller average predator sizes.…”

“…Theoretical studies have demonstrated that large individuals within species’ populations or large species within communities can be selected with increasing levels of isolation and habitat fragmentation due to their high ability to cross unsuitable matrix, that is their gap‐crossing ability (Etienne & Olff, ; Hillaert, Hovestadt, Vandegehuchte & Bonte, ). These dynamics are especially prominent in systems where resources are static and when consumers move in a fully informed manner (Hillaert, Vandegehuchte, Hovestadt & Bonte, ).…”

The physiology of movement

“…As these physiological changes are anticipated to be correlated with demographic traits and behaviours, hence forming behavioural syndromes [26,81,170] they can eventually impact equilibrium population sizes and their fluctuations [171], as mediated by costs during movement and changes in local growth rates (e.g., [172,173]). Such feedbacks can even be lagged if physiological responses are mediated through maternal effects, as for instance the case by induced hormonal effects [33].…”

“…The physiological control of movement should therefore be treated as a reaction norm, and as for models including feedbacks between evolution and ecology [197], we expect a realistic but simplified consideration of feedbacks between environmental cues, resources and physiological processes to improve the predictive power of the available models. The integration of simple allometric and metabolic rules offers in this respect promising avenues [171,[198][199][200][201], as do dynamic energy budget models [202][203][204]. It is less obvious to which degree any hormonal feedbacks need to be integrated.…”