When and how do invasions change the diversity and stability of resident communities?
Overall, our results were consistent with the classic diversity-stability hypothesis which states that more diverse ecosystems are more resilient to disturbance (Elton 1927; Tilman & Downing 1994; Rooney & McCann 2012). However, we also observed results that deviated from this relationship: invaders could either stabilise communities by increasing local diversity, or they could disrupt initial community stability (e.g. by integrating an invading predator that destabilises the local community) and cause species extinctions. Invaders affected the diversity and stability of resident communities even when their presence was only temporary. These contrasting findings highlight the ambivalent role of invasions as both contributors to and threats to local biodiversity (e.g., Henriksson et al. 2016; Tomiolo & Ward 2018).
The effect of invasion on the diversity-stability relationship depended on the outcome of the invasion in our models. Successful invasions led to variable, outcome-dependent changes in system stability. Our results suggest that one-to-one species substitutions in simple communities rarely alter system stability, while invasions that lead to increased diversity can both destabilise (outcome type: integration) and stabilise (occupancy and, to a lesser, extent integration) community dynamics. We have found that these potential changes in stability occur primarily in relatively cold, nutrient-rich environments in communities that are vulnerable to population fluctuations caused by eutrophication. These communities are sensitive to changes in vital rates, which determine the population dynamics and interactions of their constituent species (Binzer et al. 2012; Fussmann et al. 2014). For example, a long-term study of the plankton community in Lake Washington found that community stability was lowest during a period of increased nutrient loading following a successful invasion by a subsequently dominant cyanobacterium (Francis et al. 2014).
Surprisingly, our study revealed that failed invasions can still affect the diversity and stability of local communities in cold, nutrient-rich habitats prone to the paradox of enrichment. Diversity could decrease due to increased population cycles after failed invasions of smaller consumers (EC module). Diversity could also increase due to rescue by invading top predators (TC module) if their temporary presence dampened consumer-resource cycles and rescued the basal resource, but not the consumer, from the collapse caused by enrichment. Such feedbacks from transient top predators on resident species might also arise from cascading effects of an invading top predator on lower trophic levels in more complex food webs (Woodward & Hildrew 2002; Gallardo et al. 2016; Reynolds & Aldridge 2021). However, adequate evidence of the rescue effect would require long-term data, ideally from experiments with controlled introductions and subsequent removal of the invading species (Bell et al. 2003).