What drives successful invasions and when do they occur?
Environmental and biotic filters underpin invasion success in local communities (Mitchell et al. 2006; Blackburn et al. 2011; Gray et al. 2015). Species living in warm, nutrient-poor environments such as tropical and subtropical seas (Sunday 2020; Trisos et al. 2020) may be at risk of metabolic meltdown (Pörtner & Farrell 2008), while species living in relatively cold, nutrient-rich environments such as shallow lakes at higher latitudes (Janssen et al. 2014; Glibert 2017) are vulnerable to unstable dynamics and community collapse (Oksanen et al. 1981). Previous models have shown that (1) “intermediate” environmental conditions that balance the opposing effects of warming and eutrophication can prevent biodiversity loss and maintain food web structure and that (2) larger consumer-resource body mass ratios mitigate the destabilising effect of eutrophication but tend to increase the vulnerability of top predators to warming (Binzer et al. 2016; Sentis et al. 2017). Our results extend these findings in the context of species invasions. That is, large species cannot invade warm, nutrient-limited habitats because of the risk of metabolic meltdown (Pörtner & Farrell 2008), while nutrient enrichment in colder habitats limits invasions by smaller species due to the paradox of enrichment and community collapse (Oksanen et al. 1981).
Size structure of the local community plays an additional filtering role in invasions (Gray et al. 2015). We observed that invasion success was mainly determined by size differences between resident and invading competitors, while asymmetries in size structure between adjacent trophic levels determined the fate of invading predators. This can be explained by the limiting similarity hypothesis, which states that the coexistence of species sharing the same (trophic) niche requires similar traits (MacArthur & Levins 1967), while this requirement does not hold for invaders in different trophic positions. Apart from competition for resources, we did not consider self-limiting mechanisms that would lead to stronger intraspecific than interspecific competition in our models and favour species coexistence (Holt et al. 1994). In our case, the application of the R* and P* rules (Box 2; Tilman 1985; Holt et al. 1994) can explain why only smaller competitors could successfully invade. We also considered a homogeneous environment, which tends to amplify the impact of invasive species on resident communities through high levels of interspecific competition, leading to limited coexistence due to frequent species replacement or strong resistance to the invader (Melbourne et al. 2007). This is in contrast to heterogeneous environments, where competing species with different traits can coexist through niche partitioning (Ricklefs 1977).
Our results also extend previous theory by showing that successful invasions in the IGP module depend on asymmetric competition between the intraguild predator and prey (Wootton 2017). Intraguild predators have a double advantage over pure competitors (as in the EC module) or specialist predators (as in the TC module): they depend less on a particular food source and can suppress intraguild prey through high predation pressure, even if the latter is a better competitor for the shared resource (Wootton 2017). These results are corroborated by experiments on intraguild predation between poeciliid fishes along a productivity gradient (Schröder et al. 2009), where the largerPoecilia reticulata most often successfully invaded the system and drove the smaller Heterandria formosa to extinction. Here we found that intraguild prey and predator can only coexist when environmental conditions are close to the metabolic meltdown threshold of the latter species, i.e. when high temperatures are combined with nutrient limitation.
Comparing results between modules, we found that intraguild prey (regardless of body size, IGP module), larger consumer (EC module) and larger resource (AC module) species were the least likely to successfully invade. This contrasts with the frequent successful invasions of intraguild predators (IGP module). Overall, we predict that successful invasions should involve comparatively smaller species, i.e. smaller competitors at lower trophic levels and predators that are not much larger than their prey. Invaders with other traits may need specific environmental conditions to be successful: for example, larger competitors at lower trophic levels and intraguild prey may only invade relatively warm and nutrient-poor environments that are not suitable for their predators.