Using food web dominator trees to catch secondary extinctions in action

Abstract

In ecosystems, a single extinction event can give rise to multiple 'secondary' extinctions. Conservation effort would benefit from tools that help forecast the consequences of species removal. One such tool is the dominator tree, a graph-theoretic algorithm that when applied to food webs unfolds their complex architecture, yielding a simpler topology made of linear pathways that are essential for energy delivery. Each species along these chains is responsible for passing energy to the taxa that follow it and, as such, it is indispensable for their survival. To assess the predictive potential of the dominator tree, we compare its predictions with the effects that followed the collapse of the capelin (Mallotus villosus) in the Barents Sea ecosystem. To this end, we first compiled a food web for this ecosystem, then we built the corresponding dominator tree and, finally, we observed whether model predictions matched the empirical observations. This analysis shows the potential and the drawbacks of the dominator trees as a tool for understanding the causes and consequences of extinctions in food webs.

Figure 1

(a) Simple hypothetical food web and (b) its corresponding dominator tree. The removal of node a would cause the disconnection of nodes c, d, e, while the removal of any other node different from the root would cause no secondary extinction.

Figure 2

The food web of the Barents Sea ecoregion. The root (external environment) is labelled as node 1. Capelin is labelled as node 75. Electronic supplementary material B provides the correspondence between the other species and numbers in the food-web graph.

Figure 3

Dominator tree for the Barents Sea ecosystem. The capelin (node 75) dominates over Rissa tridactyla (102), U. aalge (103) and Uria lomvia (104).