Inertia: the discrepancy between individual and common good in dispersal and prospecting behaviour

Abstract

The group selection debate of the 1960s made it clear that evolution does not necessarily increase population performance. Individuals can be selected to have traits that diminish a common good and make population persistence difficult. At the extreme, the discrepancy between levels of selection is predicted to make traits evolve towards values at which a population can no longer persist (evolutionary suicide). Dispersal and prospecting are prime examples of traits that have a strong influence on population persistence under environmental and demographic stochasticity. Theory predicts that an 'optimal' dispersal strategy from a population point of view can differ considerably from that produced by individual-level selection. Because dispersal is frequently risky or otherwise costly, individuals are often predicted to disperse less than would be ideal for population performance (persistence or size). We define this discrepancy as 'inertia' and examine current knowledge of its occurrence and effects on population dynamics in nature. We argue that inertia is potentially widespread but that a framework is currently lacking for predicting precisely the extent to which it has a real influence on population persistence. The opposite of inertia, 'hypermobility' (more dispersal by individuals than would maximize population performance) remains a possibility: it is known that highest dispersal rates do not lead to best expected population performance, and examples of such high dispersal evolving exist at least in the theoretical literature. We also show, by considering prospecting behaviour, that similar issues arise in species with advanced cognitive and learning abilities. Individual prospecting strategies and the information acquired during dispersal are known to influence the decisions and therefore the fate of individuals and, as a corollary, populations. Again, the willingness of individuals to sample environments might evolve to levels that are not optimal for populations. This conflict can take intriguing forms. For example, better cognitive abilities of individuals may not always lead to better population-level performance. Simulation studies have found that 'blind' dispersal can lead to better connected metapopulations than cognitively more advanced habitat choice rules: the latter can lead to too many individuals sticking to nearby safe habitat. The study of the mismatch between individual and population fitness should not be a mere intellectual exercise. Population managers typically need to take a population-level view of performance, which may necessitate human intervention if it differs from what is selected for. We conclude that our knowledge of inertia and hypermobility would advance faster if theoretical studies--without much additional effort--quantified the population consequences of the evolving traits and compared this with hypothetical (not selectively favoured) dispersal rules, and if empirical studies were similarly conducted with the differing levels of selection in mind.

Eco-evolutionary feedbacks, adaptive dynamics and evolutionary rescue theory

“…Then they examined empirical evidence for such discrepancy: vertebrates and insects form well-studied taxa for dispersal in relation to habitat fragmentation and local population size. In these data, Delgado et al [77] found tangential support for the predictions, but no direct evidence. .…”

“…Early examples were discussed in Ferriere et al [76] and Dieckmann & Ferriere [1]. Delgado et al [77] discussed evolutionary suicide and rescue when individual dispersal is the target of the adaptive process. They used predictions from general theory [48,49,78] and from more organism-specific models [79] to show (theoretically) that evolution tends to favour dispersal rates that are considerably lower than the rates that would maximize 'population performance' (e.g.…”

Prospecting and dispersal: their eco-evolutionary dynamics and implications for population patterns

“…Indeed, another study has demonstrated that, under certain conditions, individuals may even decide to ignore information on the proximity of nonnatal patches, thereby moving further than strictly necessary [30]. This has a positive effect on connectivity but reduces individual lifetime reproduction success, and thus these studies highlight an important potential discrepancy between different organizational levels of selection (reviewed in [20]). This discrepancy between the individual and the population good is also evident when both information acquisition and use are very costly (figure 6c).…”

“…However, more information does not always mean better population performance [24]. Given the various costs of information acquisition and use at the level of individual, the motivation of individuals to sample their environment is an individual trait that under certain condition might not evolve to produce optimal behaviour at the population level [20]. We observed that while informed dispersal strategies led to population performance below its highest possible level (figure 6a), un-and poorly informed individuals nearly optimized population performance, both in terms of density and patch occupancy (figure 6b).…”

“…Likewise, previous empirical and theoretical studies have shown that natural selection acting at the individual level may promote traits that are advantageous to individuals but which can have negative effects on the population (reviewed in [20]). A high chance to enhance individual fitness tends to favour an inclination to prospect, insofar as prospecting can be achieved without much cost [21,22].…”