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.