Node-by-node disassembly of a mutualistic interaction web driven by species introductions

Abstract

Interaction webs summarize the diverse interactions among species in communities. The addition or loss of particular species and the alteration of key interactions can lead to the disassembly of the entire interaction web, although the nontrophic effects of species loss on interaction webs are poorly understood. We took advantage of ongoing invasions by a suite of exotic species to examine their impact in terms of the disassembly of an interaction web in Patagonia, Argentina. We found that the reduction of one species (a host of a keystone mistletoe species) resulted in diverse indirect effects that led to the disassembly of an interaction web through the loss of the mistletoe, two key seed-dispersers (a marsupial and a bird), and a pollinator (hummingbird). Our results demonstrate that the gains and losses of species are both consequences and drivers of global change that can lead to underappreciated cascading coextinctions through the disruption of mutualisms.nteractions among species are the ties that bind communities together. Thus, understanding the mechanisms and consequences of both direct and indirect interactions is fundamental to elucidating the assembly and disassembly of ecological communities in a changing world (1-3). Interaction webs are a way to summarize the diversity of interactions among species in a community (4, 5). The properties of interaction webs include the number of nodes (i.e., species richness), links between the nodes (i.e., species interactions), nature of the links (i.e., antagonistic or mutualistic), and the strengths of those interactions (4, 6). The addition of novel species to an interaction web, particularly of invasive species, can have important implications for stability, depending on how the novel species alters the properties of interaction webs (7), especially if these species affect critical nodes of the web, such as keystone species (3,8,9) or particular interactions. Species losses, especially those involved in keystone mutualisms, could disassemble an interaction web by triggering a cascade of linked coextinctions (3, 10-12). The idea that connectance promotes stability and that the loss of keystone mutualisms leads to web disassembly relies on the notion that mutualisms and the species engaged in them are fundamental to maintaining the structure and diversity of ecological communities (4, 13-15). Here we show how the addition of novel species from disparate taxa and the ensuing loss of a keystone species leads to the node-by-node disassembly of an interaction web in Patagonia, Argentina. Taken together, these results demonstrate that simultaneous gains and losses of species are both consequences and drivers of global change that can lead to underappreciated cascading effects.In particular, we take advantage of an ongoing natural experiment triggered by the invasion of a suite of exotic ungulates and wasps to examine whether the disruption of mutualisms and the loss of keystone mutualists leads to the disassembly of an interaction web (Fig. 1). The pivotal nodes of ...

Correlated loss of ecosystem services in coupled mutualistic networks

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“…Simulations suggest that extinction of plants can cause correlated co-extinctions in different interaction networks when these are coupled through shared plant resources 7 . This implies that plants are critical nodes of coupled interaction networks 7,9 , and that plants may mediate a correlated response of such coupled networks to habitat degradation.…”

Impacts of invasive biota in forest ecosystems in an aboveground–belowground context

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