Spatial memory shapes density dependence in population dynamics.

Louise Riotte-Lambert,Simon Benhamou,Christophe Bonenfant,Simon Chamaillé-Jammes

doi:10.1098/rspb.2017.1411

Abstract

Most population dynamics studies assume that individuals use space uniformly, and thus mix well spatially. In numerous species, however, individuals do not move randomly, but use spatial memory to visit renewable resource patches repeatedly. To understand the extent to which memory-based foraging movement may affect density-dependent population dynamics through its impact on competition, we developed a spatially explicit, individual-based movement model where reproduction and death are functions of foraging efficiency. We compared the dynamics of populations of with- and without-memory individuals. We showed that memory-based movement leads to a higher population size at equilibrium, to a higher depletion of the environment, to a marked discrepancy between the global (i.e. measured at the population level) and local (i.e. measured at the individual level) intensities of competition, and to a nonlinear density dependence. These results call for a deeper investigation of the impact of individual movement strategies and cognitive abilities on population dynamics.

Highlights

Density dependence is a major feature of population dynamics and its pervasiveness in wild populations has been demonstrated repeatedly
We developed an individual-based model integrating simple movement rules and their consequences on life histories to compare the population dynamics of populations of with- and without-memory individuals separately, to answer three key questions: (1) As memory use improves the foraging efficiency of individuals, does it result in a larger carrying capacity of the environment and/or in stronger resource depletion for a given population size?
(2) As memory use leads to restricted space use and to some degree of spatial segregation, does it lead to a discrepancy between the actual intensity of competition experienced 2 by a given individual and the one expected when using the overall population density as an index of competition? (3) As memory use enables individuals to dynamically adapt their exploitation of resources to local conditions and to segregate, but probably only up to some population density [33], can it lead to a nonlinear density dependence of the population growth rate?

Summary

Introduction

Density dependence is a major feature of population dynamics and its pervasiveness in wild populations has been demonstrated repeatedly (metaanalysis in [1]). (3) As memory use enables individuals to dynamically adapt their exploitation of resources to local conditions and to segregate, but probably only up to some population density [33], can it lead to a nonlinear density dependence of the population growth rate?. As the growth rates of populations of with-memory individuals appeared to decrease nonlinearly with population size (see the Results section), we investigated nonlinearity by fitting the Beverton– Holt model [6,38], the theta-logistic model, where the shape parameter u was estimated from the data, and a second-order polynomial regression. We ran additional simulations (100 for each parameter setting) to investigate the sensitivity of the carrying capacity and of the occurrence, location, and abruptness of the break point in the density-dependence curves of populations of with-memory individuals to variations in the values of key parameters. All simulations and analyses were performed using Julia v. 0.3.9 [41] and R [42], respectively

Results

Findings

Discussion