We simulate the loss of nodes and links with four different scenarios: (1) uniformly at random ( Albert et al.
If a node loses all of its links, it becomes extinct. In this work, we only consider the existence of an interaction but not its weight.Īt each time step, an extinction event is modeled by removing a pollinator for node removal scenarios or an interaction for the interaction removal scenario. The interactions between resources and consumers are represented by the incidence matrix A( n× m), whose entries A ij are equal to 1 if there is a mutualistic relation between nodes i and j, and A ij=0 otherwise. These are composed of two different kinds of nodes: resources, here the n plants, and consumers, the m insect pollinators. The interactions found in general in mutualistic ecological communities are naturally represented with bipartite networks. 2018) modularity and nestedness remain basically unchanged, in contrast to our results when extinction mechanisms are at play in a real bipartite network. We further emphasize our results by showing how in a model of mutualistic network growth ( Valverde et al. The study of the changes in community structure under different extinction scenario sheds light on the fragility of ecological communities to species extinctions. We present a detailed analysis of the community structure in response to the loss of pollinator species, using an empirical mutualistic network. Our study focuses on consumer removals, as pollinators have a higher immediate extinction risk than plants and also loss of a pollinator species may cause the co-extinction of plants that depend on them ( Memmott et al. Here, we analyze how species extinction affects the structure of mutualistic networks. The emergence of modularity is crucial for community ecology because such a compartmentalized structure can greatly influence dynamics, as the compartments buffer the spread of perturbation across the network ( Gardner and Ashby 1970 Gilarranz et al. In the context of ecology, loss of biodiversity as a consequence of environmental perturbations disrupts ecosystems and their functioning significantly. Biological systems, and in general any complex system, are expected to withstand the loss of elements, either by random failure or driven by a directed perturbation (e.g., environmental change or a targeted attack) ( Burgos et al. 2005) and stability, which can be measured as the largest eigenvalue of the appropriate matrix ( May 1972).
2018), where specialists interact with proper subsets of the species that generalists interact with modular organization, that captures the block structure ( Bascompte 2010 Vázquez et al. Empirical mutualistic networks exhibit a number of macro-scale structural features such as nestedness ( Bascompte et al. Mutualistic interactions between species are often represented as bipartite networks, where the interactions occur between two groups of species (generically resources and consumers), but not within the groups. The increase of the effective modularity observed in some scenarios could provide some protection to the remaining ecosystem. Our results show that perturbations and disruptive events affect the connectivity pattern of mutualistic networks at the mesoscale level. The modularity reaches a stationary value and nodes remain in the same community after joining the network. Second, we compare extinctions in a real mutualistic network with the growth of a bipartite network model. The bipartite network reorganizes showing an increase of the effective modularity and a fast decrease of the persistence of the species in the original communities with increasing number of extinction events. For node removal, we consider random, directed and sequential extinctions for interaction removal, we consider random extinctions. We model extinctions as node or interaction removals. First, we consider a real mutualistic network and introduce extinction events according to several scenarios. We study how the community structure of bipartite mutualistic networks changes in a dynamic context.
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