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.
Table 1.
Network analysis metrics relevant to the centrality and importance of non-native species within networks and included in the analyses performed here.
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.
Table 2.
Network-scale metric comparisons of full empirical networks with each of the following: (1) reduced networks from which non-natives were removed; (2) reduced networks from which taxa equivalent in number to the non-natives were removed at random; (3) simple null model reduced 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.