Community response to invasion: the role of species body mass ratios
We examine the role of body mass ratios in community response to invasion from two perspectives: competition in the AC, EC and IGPC module and predation in the TC and IGPP module. To this end, we investigate the role of body mass ratios α and β between competitors and the role of size structure asymmetry between multiple trophic levels given byδ . For brevity, we summarise here only the general patterns of invasion outcomes (Fig. 2) and effects on community regime state (Fig. 3); Text S3 and Table S9 provide further details.
Resistance to invasion and species substitution dominate the results for the three modules with invading competitors (Fig. 2a-c, S2ab and S3a-c), with predominantly neutral effects on the community regime (ΔS = 0; Fig. S2cd, dotted lines in Figs. 3a-c and S3f-j). Invasion-induced increase in stability is less frequent but occurs in all modules except EC (Figs. 2b and S3g). Integration of the invader and rescue of the resident species are rare and limited to TC and IGP modules (Figs. 2a-c and S2ab). Invasion-induced vulnerability occurs only for smaller (β < 1) or, very rarely, much larger (β \(\gg\)1) invading consumers in the EC module (Fig. 2b) and promotes system instability (Figs. 2b and S3g).
The predominant outcomes in the AC, EC and IGPC modules, i.e. resistance to invasion or substitution of the resident competitor, correspond to predictions based on the P* and R* rules (Box 2) and depend strongly on the resident: invader body size ratio (Fig. 2a-c, Text S4). In the AC module, a smaller competitor can sustain a higher equilibrium predator biomass and exclude a larger competitor (P* rule, Figs. 2a, S4a-l and S5a-l). In the EC module, a smaller consumer has lower resource requirements at equilibrium and therefore excludes a larger competitor (R* rule, Figs. 2b, S4s-x and S5s-x). Invading smaller resource in the AC module can stabilise the dynamics and prevent collapse (ΔS > 0 for α < 1; Figs. 3a and S3f), while invasion of a smaller consumer in the EC module can destabilise the dynamics, leading to population cycles or collapse (ΔS < 0 for β < 1; Figs. 3b and S3g). In the IGP module, a smaller intraguild prey is competitively superior to the intraguild predator (Fig. S6g-l) but cannot withstand its predation pressure (Figs. 2c and S6m-o). Intraguild prey therefore collapses immediately after its introduction (Fig. S6m-o) or is displaced by an invading intraguild predator (Fig. S6a-f) as soon as the biomass of the latter becomes too high, with a stabilising effect similar to that in the AC module (for β > 1; Fig. 3c).
Community responses to an invading top predator in the TC and IGP modules vary predictably with the size-structure asymmetry between trophic levels characterised by δ (Figs. 2de and 3de). Successful invasion of the top predator in the TC module requires the presence of an intermediate consumer, which is more common with large α (Fig. S1) and thus smaller δ values. That is, an invading top predator is more likely to integrate or to rescue a resident species than to fail if it is more similar in size to the resident consumer (for δ< 1, Fig. 2d). In this way, the top predator triggers oscillations more frequently, but prevents community collapse through the rescue effect (Fig. 3d). Changes in community composition and stability decrease when the resident consumer and the resource become more similar in size (δ > 1 in Figs. 2d and 3d). In this case, the invasion of a comparatively large top predator usually fails and the resident system collapses due to the paradox of enrichment driven by the resident consumer-resource interaction (cf. Fig. S1b). On the other hand, the intraguild predator in the IGP module feeds on two prey populations, which explains the independence of community resistance from δ (Fig. 2e). Given the influence of the intraguild prey-resource mass ratio on the dynamics of the resident system (Fig. S3), species substitution occurs more frequently the more similar the size of the intraguild predator and intraguild prey (δ < 1), while niche occupancy occurs more often the more similar the size of the intraguild prey and shared resource (δ > 1, Fig. 2e). Invading intraguild predator stabilises the resident community and prevents its collapse more often as it gets more dissimilar in size to the intraguild prey (δ> 1; Fig. 3e).