Abstract
Ecological theory uses Brownian motion as a default template for describing ecological movement, despite limited mechanistic underpinning. The generality of Brownian motion has recently been challenged by empirical studies that highlight alternative movement patterns of animals, especially when foraging in resource-poor environments. Yet, empirical studies reveal animals moving in a Brownian fashion when resources are abundant. We demonstrate that Einstein's original theory of collision-induced Brownian motion in physics provides a parsimonious, mechanistic explanation for these observations. Here, Brownian motion results from frequent encounters between organisms in dense environments. In density-controlled experiments, movement patterns of mussels shifted from Lévy towards Brownian motion with increasing density. When the analysis was restricted to moves not truncated by encounters, this shift did not occur. Using a theoretical argument, we explain that any movement pattern approximates Brownian motion at high-resource densities, provided that movement is interrupted upon encounters. Hence, the observed shift to Brownian motion does not indicate a density-dependent change in movement strategy but rather results from frequent collisions. Our results emphasize the need for a more mechanistic use of Brownian motion in ecology, highlighting that especially in rich environments, Brownian motion emerges from ecological interactions, rather than being a default movement pattern.
Highlights
Ecologists apply Brownian motion and diffusive dispersal as default models for animal movement [1,2], both at individual and at population levels [3,4,5]
Despite obvious differences between movement in particles and organisms, our study shows that in analogy to physics, encounters between organisms result in Brownian motion, in particular when found in encounter-rich environments
By separating truncated from non-truncated steps, we were able to show that this change in movement pattern is entirely the consequence of increased encounter rate, as we did not observe a shift in intrinsic movement strategy
Summary
Ecologists apply Brownian motion and diffusive dispersal as default models for animal movement [1,2], both at individual and at population levels [3,4,5]. We provide evidence that, as in physics, Brownian walks in animal movements can be caused by frequent encounters, rather than being the result of adaptation to high-density conditions. In density-controlled experiments with young mussels (Mytilus edulis), we were able to distinguish between intrinsic movement strategy and the effects of resource density by separating the movement steps that were truncated by encounters from those that were terminated spontaneously. As the movement of individual mussels can be experimentally studied in considerable detail, this system provides a unique opportunity to investigate how animal movement patterns are affected by truncation of moves owing to encounters. We create an individual-based model of selforganized pattern formation in mussel beds to examine whether mussel density could cause a change in the efficiency of Brownian and Levy walks, explaining a possible active shift in mussel movement strategy. We use a general argument to demonstrate that the interplay between any intrinsic movement strategy and frequent ecological encounters will often result in Brownian motion
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