Implications of non-native species for mutualistic network resistance and resilience

Abstract

Resilience theory aims to understand and predict ecosystem state changes resulting from disturbances. Non-native species are ubiquitous in ecological communities and integrated into many described ecological interaction networks, including mutualisms. By altering the fitness landscape and rewiring species interactions, such network invasion may carry important implications for ecosystem resistance and resilience under continued environmental change. Here, I hypothesize that the tendency of established non-native species to be generalists may make them more likely than natives to occupy central network roles and may link them to the resistance and resilience of the overall network. I use a quantitative research synthesis of 58 empirical pollination and seed dispersal networks, along with extinction simulations, to examine the roles of known non-natives in networks. I show that non-native species in networks enhance network redundancy and may thereby bolster the ecological resistance or functional persistence of ecosystems in the face of disturbance. At the same time, non-natives are unlikely to partner with specialist natives, thus failing to support the resilience of native species assemblages. Non-natives significantly exceed natives in network centrality, normalized degree, and Pollination Service Index. Networks containing non-natives exhibit lower connectance, more links on average, and higher generality and vulnerability than networks lacking non-natives. As environmental change progresses, specialists are particularly likely to be impacted, reducing species diversity in many communities and network types. This work implies that functional diversity may be retained but taxonomic diversity decline as non-native species become established in networks worldwide.

Fig 1. Conceptualization of the relationship between non-native species and network resistance and resilience.

a. When no non-natives are present, the hypothetical network contains some generalist native species (shown in red) with multiple partners, and two specialist species (shown in black) with a single partner each. When an extinction occurs, one species and one pair of species become detached from the network. b. When non-native species (delineated in gold) are integrated into the network, they are likely to be more generalist than the average native species, each interacting with multiple partners. When an extinction occurs, one species becomes detached from the network because it was a specialist and the generalist non-natives did not partner with it; this results in an irreversibly altered species assemblage. However, the rest of the network is more robust than the natives-only network, and further network deterioration is unlikely. The functional contributions of the network (e.g., pollination) will persist in their current state, since the broader network structure remains, indicating resistance. At the same time, there has been a loss of species richness, implying reduced resilience; the network contains fewer unique species and life history traits, and this reduced diversity could hinder succession and disturbance recovery in the future.

Fig 2. Mean network metrics for analyses of native and non-native species in pollination and seed dispersal networks.

a. Species-level analysis results. b. Network-level analysis results. Results were obtained via a research synthesis and extinction simulations for 58 described mutualistic networks.

Fig 3. Hypothetical mutualistic network diagrams illustrating the contrasting roles of native and non-native species.

a. The full network contains both native (red) and non-native (gold) species. b. When non-native species are removed, the network becomes simplified but no coextinctions have occurred; all natives are still present and retain at least one interaction partner. c. When an equivalent number of species is removed at random from the network, secondary extinctions of specialist native species may occur (as has occurred for plant species M, which now lacks all partners). The network has decreased in native species diversity and is thus less resilient to future disturbance.