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The decision to emigrate involves balancing the benefits of leaving the current habitat and the costs and uncertainties of moving to a - often-unknown - target destination. Emigration may be the best option from poor quality habitats. More generally, as any decision making, emigration probably depends on a complex combination of individual characteristics and environmental conditions.

To what extent are the rules governing these decisions generalizable? Can ecologists predict in a meaningful way when organisms are likely to disperse?

Julien Cote (EDB UMR 5174 CNRS/UPS), Delphine Legrand, Simon Blanchet, Staffan Jacob and Alexis Chaine (SETE UMR 5321 CNRS / UPS) and Emmanuel Fronhofer (ISEM UMR Montpeller University / CNRS / IRD / EPHE) have attempted to provide an answer in their recent article in Nature Ecology & Evolution.

While evidence for the impact of dispersal on populations and communities dynamics accrues, the underlying causes of dispersal are often oversimplified or ignored. Nonetheless, causes of dispersal may involve multiple factors that vary among species, environments and individuals, and that depend on the scale at which they are considered. The original approach adopted in this study was to create comparable and integrative experimental facilities in seven European laboratories in parallel, ranging from connected microcosms in protists to semi-natural connected mesocosms in lizards.

From protists to vertebrates

The authors tested the effects of resource availability and predation risk on emigration decision in 21 species, including protists, algae, arthropods, molluscs and aquatic, terrestrial and vertebrates with aerial dispersal.

The principle was the same in each experiment: the species of interest was introduced into an initial habitat patch connected to a target patch separated by an inhospitable matrix. As predicted by the authors, emigration rates were higher when the initial patch resources were low and in the presence of predators. Interestingly, the effect of resource availability was always stronger than that of predation, with responses to predation being more vriable among taxa

What are the consequences for local and regional dynamics?

Researchers also studied the potential consequences on the stability of regional metacommunities of emigration linked to predation and resources. Their food web model indicates that dispersal depending on local conditions increases population stability locally and regionally, with indirect effects on the stability of other trophic levels.

These results show the importance of non-random dispersal for maintaining community properties at larger spatial scales and suggest that dispersal may reduce the risk of extinction. The experiment and the model, built on simple hypotheses, offer interesting research avenues to increase the biological realism of their simulations. For example, until now, the authors have manipulated and modelled the emigration phase, but the immigration to a new patch can also be influential and follow complex rules based on the conditions existing in the potential target destinations. The decision to disperse will therefore depend on both local and regional conditions.

"For us, the important thing is to implement standardized protocols across taxa - from protists to vertebrates. This allows us to control the experimental conditions, improve comparison and describe general processes and differences among species. At a time when we are trying to predict the future of species using global model and often ignoring species to species interactions, determining general versus idiosyncratic rules, is particularly necessary. On this aspect, this article constitutes a call for organized collective work on the ecological issue at stake." says Julien Cote.