Network theory predicts ecosystem robustness across environmental conditions

Theory posits that a network's link-species relationship predicts how changes in species richness S lead to changes in the number of links L between species. This relationship quantifies resistance to secondary extinctions and therefore gives pivotal information on ecosystem robustness. However, changes in S often coincide with environmental shifts, which can lead to unexpected impacts on L. In this paper, we constructed link-species relationships from 18 ecosystems using 1081 observations collected across different environmental conditions. We found that environmental noise (unspecified spatio-temporal variation) and environmental gradients (directional change) profoundly affected ecological network size (S and L), community composition, and induced network rewiring, with a Novotny's rewiring index up to 0.47. Yet, we found the log(L)~log(S) relationship to be remarkably constant across environmental conditions. Only in two of the 18 ecosystems did we find changes in environmental conditions to shift the log(L)~log(S) relationship down, implying an overall drop in L but not in how species loss affects L. Our results show that network theory predicts ecosystem robustness across environmental gradients, which is encouraging for conservation.