Ranking the ecological causes of dispersal in a butterfly

Experimental evidence of the major role played by the degradation of environmental conditions in the initiation of dispersal

by Delphine Legrand and Michel Baguette

Dispersal is a key process in biology, from population and community regulation, through adaptation, to speciation. Among major advances in the field, it is widely accepted that 1) dispersal is a multi-causal process triggered by a range of social and environmental factors, and 2) different phenotypes with contrasted dispersal abilities coexist within species. The current challenge is to elucidate how social and environmental factors will influence the dispersal decision of individuals with distinct phenotypes.

In this study, our aim was to determine the hierarchy of environmental, social and phenotypic factors influencing the decision to emigrate. This is particularly challenging because it is the interaction between the individual phenotype and the particular social and environmental context that will influence why, when and where individuals disperse, which means that dispersal is inherently an idiosyncratic process that will vary across organisms, space and time.

Using the unique experimental platform of the Metatron, we tested how individuals with different phenotypes reacted to the environmental context (weather conditions and habitat quality) and to social factors (variation in population density and sex-ratio). By ranking the different dispersal causes, we showed in a butterfly model species that among 10 tested potential drivers of dispersal, 1) environmental conditions prevail over social factors and individual phenotypes, and 2) weather conditions and habitat quality were the main drivers of dispersal. This last result is obviously crucial in the current era of global changes.

(a) Aerial photograph of the Metatron. The basic units of the Metatron are cages of 10 × 10 m, 2 m height fenced by tarps near the ground and a solid entomological net above, with herbaceous vegetation mowed twice a year. Cages are connected by 19 m, S-shaped narrow corridors. The present experiment was performed on the left hand cages of the photograph. (b) Photographs of a butterfly high-quality (dense vegetation cover) and a low-quality (sparse vegetation cover) cage. (c) 3D representation of the experimental design. We released 120 butterflies into eight departure cages (butterfly pictures on the scheme), each connected to an initially empty arrival cage.

Our results corroborate previous observational and correlation data documenting the importance of weather conditions and habitat quality on the emigration decision in butterflies, a group for which the dispersal process is well-documented. But more importantly, our procedure provided a quantitative ranking of the dispersal causes and is applicable to a wide range of terrestrial and semi-terrestrial organisms.

Full paper here: http://onlinelibrary.wiley.com/doi/10.1111/ecog.01283/abstract

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