Keep the bees off the trees: the vulnerability of species in the periphery of mutualistic networks to shock perturbations

Abstract

We study the phenomenon of multistability in mutualistic networks of plants and pollinators, where one desired state in which all species coexist competes with multiple states in which some species are gone extinct. In this setting, we examine the relation between the endangerment of pollinator species and their position within the mutualistic network. To this end, we compare endangerment rankings which are derived from the species' probabilities of going extinct due to random shock perturbations with rankings obtained from different network theoretic centrality metrics. We find that a pollinator's endangerment is strongly linked to its degree of mutualistic specialization and its position within the core-periphery structure of its mutualistic network, with the most endangered species being specialists in the outer periphery. Since particularly well established instances of such peripheral areas are tree-shaped structures which stem from links between nodes/species in the outermost shell of the network, we summarized our findings in the admittedly ambiguous slogan 'keep the bees off the trees'. Finally, we extend the mutualistic system to a multilayer network, where a species' position in the mutualistic network layer determines its position in a competitive network layer. We find that this multilayer setup, that allows peripheral species to avoid competition, can lead to very similar endangerment rankings as the standard setup under harsh environmental conditions, but to very different rankings under favorable conditions.