Collective Navigation

Abstract

Throughout the natural world, organisms face the challenge of locating the resources necessary for their survival. Often this is achieved through the use of localized information such as chemical cues or environmental gradients. It is thought that collective behavior plays a fundamental role in this process for many organisms, ranging across multiple scales from cells to ungulates. Theoretical studies have shown that grouping behavior can greatly improve animals' ability to climb gradients. However, although these studies have demonstrated the theoretical benefit of collective sensing, empirical tests of these predictions are extremely rare, and implications for ecosystem consequences are entirely absent. In this thesis we show cooperative signaling can lead to emergent group-level gradient tracking in numeric simulations, and that, though costly, this cooperation can be maintained through evolution in stochastic environments. Using a novel experimental setup we demonstrate that collective sensing of complex environmental gradients emerges in schools of real fish and we reveal a simple mechanism behind this effect: individual speed modulation according to local conditions plus social interactions produces group-level taxis. Next, by performing a meta-analysis of empirical data on anadromous salmon migration we find a consistent trend of increased homing accuracy in years of greater population density, and investigate the potential for collective navigation to be driving this widespread pattern. We then explore the population-level impacts of migratory groups using collective navigation during critical migrations and show that while collective navigation can boost a population size it can induce Allee effects and lead to a collapse of the migration, population size and genetic structure if survival pressure is too high. Finally we step back and examine the evolutionary trade-offs between dispersal and local adaptation within a general metapopulation model suitable for migratory populations. Our results indicate that the joint evolution of the two traits produces discontinuities and hysteresis in the evolutionary stable strategies around a critical level of environmental heterogeneity.