Mutualistic strategies minimize coextinction in plant-disperser networks

Abstract

The global decline of mutualists such as pollinators and seed dispersers may cause negative direct and indirect impacts on biodiversity. Mutualistic network models used to understand the stability of mutualistic systems indicate that species with low partner diversity are most vulnerable to coextinction following mutualism disruption. However, existing models have not considered how species vary in their dependence on mutualistic interactions for reproduction or survival, overlooking the potential influence of this variation on species' coextinction vulnerability and on network stability. Using global databases and field experiments focused on the seed dispersal mutualism, we found that plants and animals that depend heavily on mutualistic interactions have higher partner diversity. Under simulated network disruption, this empirical relationship strongly reduced coextinction because the species most likely to lose mutualists depend least on their mutualists. The pattern also reduced the importance of network structure for stability; nested network structure had little effect on coextinction after simulations incorporated the empirically derived relationship between partner diversity and mutualistic dependence. Our results highlight a previously unknown source of stability in mutualistic networks and suggest that differences among species in their mutualistic strategy, rather than network structure, primarily accounts for stability in mutualistic communities.

Keywords: defaunation; ecological networks; global change; mutualism; plant–animal interactions; seed dispersal.

Figure 1.

A positive relationship between partner diversity and mutualistic dependence among animals in 11 empirical quantitative seed dispersal networks. Thin lines represent model fits for each network and the thick line represents the mean model fit.

Figure 2.

Fruit–frugivore interactions and the benefits of seed dispersal for plants in the Mariana Islands. (a) Seed dispersal network, with columns representing animal species and rows representing plant species, referred to by genus. (b) The relationship between species strength and the dispersal benefit ratio for plants (model output with 95% confidence intervals; detailed results in electronic supplementary material, figure S1). (c) Relationship between the pulp-to-seed ratio and species strength.

Figure 3.

The positive relationship between partner diversity and mutualistic dependence reduces coextinction and alters the influence of network structure and partner diversity on coextinction. (a) The portion of species experiencing coextinction in simulations within 11 quantitative networks when assuming all species are obligate mutualists (triangles; horizontal line in inset panel) or using the observed relationship (circles; positive relationship in inset panel). Compare between randomized and empirical structure to assess the decrease in coextinction due to empirical network structure when assuming all species are obligate mutualists (compare triangles between randomized and empirical structure) or when using the observed relationship (compare circles). Simulated within the 11 empirical networks, the relationship between partner diversity and vulnerability to coextinction when assuming that all species are obligate mutualists (b) or when using the observed relationship (c).