Introduction
Human-induced global change is transforming local communities and
ecosystems through five main drivers: climate change, pollution,
overharvesting, land and sea use change, and invasive species (Isbell et
al. 2017; IPBES 2019). Invasive species threaten biodiversity (Sala et
al. 2000) and persistence of local communities worldwide (Gurevitch &
Padilla 2004; Bellard et al. 2016). Shifting species ranges to higher
elevations and latitudes in response to climate change (Parmesan & Yohe
2003; Sunday et al. 2012), combined with increased tourism, pet trade
and commodity transport (Chan et al. 2019; McCarthy et al. 2019; Essl et
al. 2020), are expected to accelerate species invasions globally over
the next century (Seebens et al. 2021; Sentis et al. 2021). Species
invasions can exacerbate or mitigate the pressures that ongoing
environmental change exerts on local communities by altering
biodiversity and community resilience to abiotic stressors (Walther et
al. 2002; Wardle et al. 2011; Hong et al. 2022).
Warming and nutrient enrichment are two pervasive aspects of global
change that structure local communities in aquatic (Fussmann et al.
2014; Boukal et al. 2019) and terrestrial systems (Meyer et al. 2012;
Clark et al. 2017). They modulate food web dynamics (Binzer et al. 2012;
Sentis et al. 2017) and can facilitate or prevent species invasions.
However, a general consensus on how invaders influence community
structure and persistence along temperature and habitat productivity
gradients is currently lacking. In particular, the mechanisms underlying
community-level responses to species invasions in future environments
affected by global change remain incompletely understood (Sentis et al.
2021).
Exploring the nexus between invasibility, diversity and stability of
communities (Rooney & McCann 2012; Catford et al. 2019) can help us
better understand the impacts of global change on local ecosystems
(Francis et al. 2014). The effects of species invasions on the
diversity-stability relationship have been studied in different types of
animal, animal-plant and plant interaction networks (Rooney & McCann
2012; Brose et al. 2017; Tomiolo & Ward 2018). However, previous
studies considered relatively species-rich communities with many direct
and indirect effects; focusing on food web modules could allow for more
mechanistic, causal insights.
One promising avenue towards a better understanding of these mechanisms
is to disentangle the role of environmental filters and species traits
in biological invasions (Chesson 2000; Kraft et al. 2015). Environmental
filters constrain the invader per se but also structure the local
community, which is a biotic filter that restricts the invader’s
realised niche (Kraft et al. 2015). The roles of both filters are
therefore closely linked (Thompson et al. 2018a, b). This link is often
neglected in studies that estimate future shifts in species
distributions caused by climate based on the expected performance of
invading species in new habitats (Bellard et al. 2013; Buckley & Csergo
2017; Seebens et al. 2021), but ignore the accompanying impacts of
environmental change on resident communities.
The invader’s realised niche is constrained by its trophic position and
the topology of the local food web. Available niches may be occupied by
resident species that interact with the invader directly through
consumptive interactions or indirectly through competition (Dueñas et
al. 2018). Classic work on species coexistence has proposed general
rules for community assembly (Chesson 2000; Shea 2002). The ‘R* rule’
for exploitative competition states that the species with the lowest
resource requirements is competitively superior (Tilman 1985). An
analogous ‘P* rule’ for apparent competition states that the prey that
can withstand the highest predation pressure will prevail (Holt et al.
1994). Both rules can also inform when species invade and how they
affect resident communities in the context of global change.
Among species traits, individual body mass can be used to predict
invasibility because it affects individual fitness, species interactions
and energy flows (McCann & Rooney 2009; Brose et al. 2017; Dijoux &
Boukal 2021). For example, larger species tend to prey on smaller
species, especially in aquatic habitats (Ou et al. 2017) and
warming-induced metabolic meltdown is more likely for larger consumers
than smaller ones (Rall et al. 2010, 2012). Food webs may therefore be
simpler in warmer habitats, with fewer species at higher trophic
positions (Brose et al. 2012). This could create niches for future
invaders, which could subsequently attenuate or alter food web responses
to global change through cascading effects (Reynolds & Aldridge 2021).
However, little is known about how the body mass and trophic position of
the invader affect community responses to invasions under climate
change, and simple predictions are difficult to make. For example,
previous models have shown that high consumer-resource mass ratios
associated with large consumer species can either confer a higher
extinction risk for top predators under warming or buffer the effects of
eutrophication by dampening population fluctuations (Binzer et al. 2016;
Sentis et al. 2017).
Here, we investigate in detail how consumer-resource systems respond to
species invasions along temperature and habitat productivity gradients.
To this end, we develop biomass-based models (Yodzis & Innes 1992) with
mass- and temperature-dependent biological rates parameterised using
empirically estimated relationships (Binzer et al. 2012; Fussmann et al.
2014). We simulate all possible invasions in a consumer-resource system
that can lead to the four baseline three-species food web modules
(apparent and exploitative competition, food chain, and intraguild
predation). Our aim is to decouple the influence of the invaders and
abiotic drivers. We explore (i) how temperature, nutrient levels and
body mass ratios between the resident and invading species influence
invasion success and (ii) how invasion-induced changes in species
composition, diversity and stability of local communities vary across
different food web topologies and environmental gradients.
Our main expectations are: (1) all else being equal, community responses
to invasions (Box 1) follow known mechanistic processes from community
ecology (Box 2); (2) based on the R* and P* rules and the higher
susceptibility of larger species to metabolic meltdown at warmer
temperatures, smaller invaders are more successful at warmer
temperatures, especially in less productive environments, while larger
invaders are more successful in more productive environments, especially
at lower temperatures; and (3) invasions that result in larger and
smaller consumer-resource size ratios will tend to stabilise and
destabilise the community dynamics.