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.