Ecological integration of alien species into a tree-parasitic fungus network

The diversification of species and their interactions during the course of evolution has produced ecological networks with a complex topology. This topology influences the current functioning of ecosystems. It is therefore important to investigate whether the species introduced recently by human activities have merged seamlessly into recipient ecological networks by developing interactions quantitatively and qualitatively similar to those of native species, or whether their establishment has altered the topology of the networks. We tackled this issue in the case of a well resolved interaction network between 51 forest tree taxa and 154 pathogenic fungal species. We found that alien and native species with similar phylogenetic histories and life-history strategies had similar types and numbers of interactions. Our results also suggest that the clustered architecture of the network has not been altered by the integration of alien species. It therefore seems that a few centuries have been sufficient for the network to assimilate the newly introduced species. This rapid integration was unexpected for a plant-pathogen network, because selection acts continually on plants, favouring the emergence of defences against new pathogens and impeding the development of new interactions. However, it was recently shown that perturbation of the structure of ecological networks might be overlooked if species interactions are not quantified. The tree-parasitic fungus network considered in this study is binary. We might therefore end up with different results by using quantitative data.