Critical transitions and the collapse of mutualistic networks

Background/Question/Methods

In the last decades, increasingly strong concerns have been raised about declining pollinator abundances and its potential effects on the maintenance of biodiversity and agricultural productivity. A gradual decline in pollinator abundance may not necessarily remain gradual. Pollinator abundances may go through critical transitions from one state to another, when the conditions that cause the decline worsen. Returning to the former state after such a transition may require a large change in conditions.

The networks of interactions between plants and pollinators are known to display a high degree of nestedness, i.e., the more specialist species interact only with proper subsets of those species interacting with the more generalists. Here we use a dynamic model describing two mutualistically interacting species groups (e.g. plants and pollinators) to investigate the interrelationship between nestedness and the occurence of critical transitions.

Results/Conclusions

We show that network structure highly influences the scale at which critical transitions occur. In randomly structured plant-pollinator communities, multiple extinction events precede the final collapse of the community. On the other hand, nested communities exhibit only one point of community-wide collapse.

The fraction of species that have net positive effects on each other increases strongly as the network becomes more nested. This leads to an increase in overall facilitation, but also increases the dependency of species on each other. Such interdependencies may provide an explanation for the large-scale shifts observed in communities with a nested network structure.