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).