Marjorie Haond - Université Côte dAzur, INRA, CNRS, ISA, France - Keywords: Population colonisation, Population dynamics, Individual based models, microcosme experiments

Marjorie Haond - Université Côte dAzur, INRA, CNRS, ISA, France - Keywords: Population colonisation, Population dynamics, Individual based models, microcosme experiments

Contribution title: Frozen in space: an experimental demonstration of range pinning

Understanding how and why some populations expand their range while others reverse or stagnate is a major issue in ecology. Spatial dynamics of expanding populations depend on extrinsic factors like habitat quality and quantity, or intrinsic factors related to population demography. In particular, low density on the expansion front may trigger what is known as an Allee effect. An Allee effect describes a decrease in population growth rate at low density, which makes colonisation harder and negatively impacts expansion dynamics. In some cases, an Allee effect may even cause a “pinning” phenomenon, in which a population is unable to recolonize its suitable habitat after a local extinction.
Despite the high number of references to range pinning in the literature since its theoretical definition in 2001, this phenomenon has never been confirmed empirically and its generality remains to establish. We hypothesized that other demographic factors than the Allee effect correlated with colonisation failures at small density could also create range pinning, and that this phenomenon might be more common than what was previously thought. In order to confirm our hypothesis, we chose a joint approach of modelling and laboratory experiments.

We used individual based models to investigate the possibilities for range pinning in presence of an Allee effect or of positive density-dependent dispersal, which induces a correlation between population density on the expansion front and colonisation success. Then, we monitored recolonisation dynamics following local extinctions in laboratory microcosms using Trichogramma hymenopteran wasps, a biological model that displays positive density-dependent dispersal.
We evidenced range pinning in presence of positive density dependent dispersal in both simulations and experimental microcosms, provided that the carrying capacity of the habitat is low. This is consistent with previous results documenting the impact of carrying capacity on propagation speed in presence of an Allee effect or of positive density-dependent dispersal. This suggests that positive density dependent dispersal has similar properties to the Allee effect as regard to expansion dynamics, and that range pinning is a shared property of pushed expansion dynamics.