Aggregation-diffusion in heterogeneous environments

TL;DR

This paper develops an analytical framework for aggregation-diffusion equations in heterogeneous environments, revealing complex interactions between environment and collective movement that influence emergent space use patterns.

Contribution

It provides exact steady-state solutions and an energy minimisation approach for aggregation-diffusion equations in heterogeneous settings, highlighting novel counter-intuitive behaviors.

Findings

Non-monotonic relationship between clump width and aggregation width

Positive correlation between self-attraction strength and aggregation width under strong resource attraction

Analytic solutions verified by numerical simulations

Abstract

Aggregation-diffusion equations are foundational tools for modelling biological aggregations. Their principal use is to link the collective movement mechanisms of organisms to their emergent space use patterns in a concrete mathematical way. However, most existing studies do not account for the effect of the underlying environment on organism movement. In reality, the environment is often a key determinant of emergent space use patterns, albeit in combination with collective aspects of motion. This work studies aggregation-diffusion equations in a heterogeneous environment in one spatial dimension. Under certain assumptions, it is possible to find exact analytic expressions for the steady-state solutions when diffusion is quadratic. Minimising the associated energy functional across these solutions provides a rapid way of determining the likely emergent space use pattern, which can be verified via numerical simulations. This energy-minimisation procedure is applied to a simple test case, where the environment consists of a single clump of attractive resources. Here, self-attraction and resource-attraction combine to shape the emergent aggregation. Two counter-intuitive findings emerge from these analytic results: (a) a non-monotonic dependence of clump width on the aggregation width, (b) a positive correlation between self-attraction strength and aggregation width when the resource attraction is strong. These are verified through numerical simulations. Overall, the study shows rigorously how environment and collective behaviour combine to shape organism space use, sometimes in counter-intuitive ways.