Network Topology and Species Loss in Food Webs: Robustness Increases with Connectance

Food-web structure and complexity can mediate effects of species loss, such as cascading extinctions. We simulated species loss in 16 food webs from a variety of ecosystems. The food webs experienced much greater secondary extinctions when the most trophically connected species were removed compared to random species removals. These patterns appear related to skewed degree distributions in food webs, which generally display exponential or uniform distributions. Our analyses generalize prior research that found similar patterns of node loss in biological and nonbiological networks with power-law distributions. Food web robustness (the level of primary removals required to induce 50% total species loss) to random and most-connected species loss does not relate to species richness or omnivory, but increases significantly with greater connectance (links/species2). We also found strong evidence for the existence of thresholds where food webs display greatly increased sensitivity to removal of most-connected species. Higher connectance delays the onset of this threshold. Least-connected species removal often has little effect, but in several food webs results in dramatic secondary extinctions. We relate these findings to the diversity-stability debate, effects of species richness on ecosystems, keystone species, and extinction rates.