Bottom-up and top-down control of dispersal across major organismal groups: a coordinated distributed experiment

Abstract

Organisms rarely experience a homogeneous environment. Rather, ecological and evolutionary dynamics unfold in spatially structured and fragmented landscapes, with dispersal as the central process linking these dynamics across spatial scales. Because dispersal is a multi-causal and highly plastic life-history trait, finding general drivers that are of importance across species is challenging but highly relevant for ecological forecasting.We here tested whether two fundamental ecological forces and main determinants of local population dynamics, top-down and bottom-up control, generally explain dispersal in spatially structured communities. In a coordinated distributed experiment spanning a wide range of actively dispersing organisms, from protozoa to vertebrates, we show that bottom-up control, that is resource limitation, consistently increased dispersal. While top-down control, that is predation risk, was an equally important dispersal driver as bottom-up control, its effect depended on prey and predator space use and whether dispersal occurred on land, in water or in the air: species that routinely use more space than their predators showed increased dispersal in response to predation, specifically in aquatic environments. After establishing these general causes of dispersal, we used a metacommunity model to show that bottom-up and top-down control of dispersal has important consequences for local population fluctuations as well as cascading effects on regional metacommunity dynamics. Context-dependent dispersal reduced local population fluctuations and desynchronized dynamics between communities, two effects that increase population and community stability.Our study provides unprecedented insights into the generality of the positive resource dependency of dispersal as well as a robust experimental test of current theory predicting that predator-induced dispersal is modulated by prey and predator space use. Our experimental and theoretical work highlights the critical importance of the multi-causal nature of dispersal as well as its cascading effects on regional community dynamics, which are specifically relevant to ecological forecasting.

Anywhere but here: local conditions alone drive dispersal inDaphnia

“…In conclusion, our results add to the growing body of evidence that condition-dependent dispersal is the norm amongst taxa, and that it is moreover capable of generating substantial differences in dispersal behaviour (Legrand et al 2015;Fronhofer et al 2017b). This growing empirical consensus warns against the simplifying assumption -used in the majority of ecological and evolutionary models -that dispersal rate is constant with respect to conditions.…”

“…Our first result -that animals increase dispersal propensity when faced with local resource shortages -has been well established empirically. Studies on taxa ranging from plants to invertebrates and vertebrates either imply, or experimentally demonstrate, that resource shortage is a powerful piece of information motivating dispersal (e.g., Bowler and Benton 2005;Martorell and Martínez-López 2014;Fronhofer et al 2017b). Our study adds D. carinata to the long list of organisms that exploit this piece of intra-patch information.…”

“…It has been repeatedly demonstrated that individuals acquire information on density and act upon it (De Meester and Bonte 2010;Fellous et al 2012;Martorell and Martínez-López 2014;Fronhofer et al 2017a). In many species, this information is acquired through food limitation (Fronhofer et al 2017b): when resources are limited, particularly in juvenile stages, individuals tend to be more dispersive. Information on the relative merits of different alternative locations can also be acquired in numerous ways, including prospecting (actively moving through, and assessing, new locations, e.g., Pärt and Doligez 2003) and by observing immigrating conspecifics (Cote and Clobert 2007).…”

Dispersal syndromes can impact ecosystem functioning in spatially structured freshwater populations

“…A common method for examining the causes and consequences of dispersal is to allow organisms to disperse through linked experimental patches ranging from twopatch pairings [27,28] to larger grids or networks [29,30]. The dispersal experiments were run according to the Dispersal Network (DispNet) distributed experiment protocol, detailed in [27]. Briefly, we set up 40 replicates of a two-patch mesocosm system in order to address rates of amphipod dispersal from one to the other patch.…”

“…The experiment had a factorial design of resource availability (alder leaves vs. no food) and predator cues (fish kairomones vs. no kairmones) in the patch of origin, with each experimental context replicated 10 times. Because we found no effect of the resource or predator cue context on dispersal rates in amphipods [27], we here pooled all data from the different treatments together and only considered the effect of dispersal status (dispersed vs. resident individuals) on subsequent leaf consumption.…”