The current biodiversity crisis is caused by different drivers that act simultaneously leading to species extinctions and eroding biodiversity. Hence, it is crucial to understand how vulnerable ecological assemblages are to multiple stress drivers. Ecological assemblages are structured as ecological interaction networks that differ in their underlying organization. As a consequence, the way ecological assemblages respond to multiple extinction pressures may also differ. Network robustness to species removal has been assessed in different ways, mostly through the number of secondary extinctions following species removal. However, robustness can also be measured in terms of how the overall community interaction patterns changes as species are being removed. Thus, a next step is to explore how different network structural patterns such as nestedness and modularity influence network robustness, and how robustness changes when we considerer single versus multiple drivers. We created 900 model networks with fixed connectance and varying values of nestedness and modularity, and exposed them to different extinction driver scenarios. In addition to the number of secondary extinctions, for each scenario we measured how metrics related to the degree of network fragmentation changed as species were removed.
Results/Conclusions
Surprisingly, the number of secondary extinctions was greater in networks with greater nestedness, rendering them less robust. In contrast, low nestedness and low modularity were positively correlated to robustness to network fragmentation, as measured by mean path length and the number of components through time. Networks with intermediate modularity presented greater robustness to the number of secondary extinctions. Furthermore, we encountered similar patters when we compare the results we get from single versus multiple extinction drivers. However, even though there are no sharp differences among these scenarios, the multiple drivers’ scenarios resulted in amplified patterns. For example, networks with greater nestedness presented an even larger variation in the mean path length in the multiple drivers’ scenario. We show that assemblages of interacting species may respond differently to extinction drivers. Extinction drivers might have additive effects that result in extinctions and changes in the pattern in which species interact, while in other cases these effects might drive species extinct per se but render specific species more susceptible to extinction.