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.
Highlights
The current rates of anthropogenic extinction are unprecedented [1], but the resulting extinction of ecological interactions may cause far more pervasive impacts [2], including widespread coextinction of dependent species such as mutualists [3]
Using global databases and detailed field experiments from 30 seed dispersal networks, we found consistent support for a positive relationship between partner diversity and mutualistic dependence for both animals and plants in the seed dispersal mutualism
This empirical pattern caused an order-ofmagnitude reduction in coextinction relative to predictions made using the typical assumption of network models that all species are strongly dependent on mutualistic interactions [7,10,11,13,24,25,26,27,35]
Summary
The current rates of anthropogenic extinction are unprecedented [1], but the resulting extinction of ecological interactions may cause far more pervasive impacts [2], including widespread coextinction of dependent species such as mutualists [3]. Empirical studies that categorize animals in seed dispersal networks as obligate, partial, or opportunistic frugivores and report their plant partner diversity [19,20,22] suggest that mutualistic dependence is positively related to partner diversity. The assumption that all species depend heavily on mutualistic interactions underlies the putative links between partner diversity and coextinction risk and between network structure and stability. We use global interaction network and diet databases to assess the relationship between the diversity of animals’ plant partners and their dietary dependence on fruit in 29 globally distributed seed dispersal networks. We use a stochastic simulation model [14] that allows inclusion of data on mutualistic dependence and quantitative interaction data Under existing assumptions, this model yields qualitatively equivalent conclusions regarding the importance of partner diversity and network structure for extinction as the other topological By simulating extinction in the presence or absence of empirical network structure and the empirical relationship between partner diversity and mutualistic dependence, we compare the relative importance of each of these factors for minimizing coextinction in mutualistic networks
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