It's all about connections: Hubs and invasions in habitat networks

Background/Question/Methods

Understanding the factors underlying invasive processes, such as establishment and spread, is critical to predict the occurrence and magnitude of biological invasions. In general, those factors do not only depend on the invader itself, but also on the characteristics of the invaded area. In the metapopulation theoretical framework, landscapes are approximated as networks of discrete habitat patches. These patches are nodes of a network connected by edges that represent dispersal pathways between them. Many properties can be quantified to accurately characterize spatial heterogeneity in the connectivity between patches in a network. Node centrality for instance, measures the contribution of an individual habitat patch to landscape connectivity; highly central nodes are called “hubs”. By facilitating individual dispersal, those hubs can promote spread, but also decrease local establishment because of mechanisms like demographic stochasticity and Allee effects. By combining modeling and experimental approaches, we investigated how invasion dynamics are affected by the presence of hubs in a landscape. We first simulated invasions using a stochastic discrete-time metapopulation model. Then we challenged model predictions by performing experimental introductions of minute parasitoid wasps in artificial laboratory landscapes with patches characterized by different levels of centrality.

Results/Conclusions

Simulations evidenced faster invasive spread of metapopulations over landscapes containing patches of very high centrality. This relationship was stronger when founding individuals were introduced in the hub itself, and decreased with the distance between the introduction patch and the hub. Establishment rates decreased overall in the presence of Allee effects, but the impact was greater for introductions performed directly in the hub. Experimental results on the parasitoid wasps confirmed the positive impact on spread of introducing individuals in a hub. However, introductions performed further from the most central patches failed to spread and suffered from local competition. The results of this study highlight the importance of spatial heterogeneity in connectivity within an invaded landscape as a factor affecting establishment success and subsequent population spread. Yet, they also indicate that this effect is greatly conditioned by the location of the introduction itself.