Community response to invasion: the role of environmental conditions and food web topology
Temperature, nutrient levels and size structure of the resident community influence its composition, stability (Text S2 and Fig. S1) and response to invasion. We first focus on responses for fixed body mass ratios α = β = 10 describing invasions by a 10-fold larger resource species in the AC module (Fig. 1a), a 10-fold smaller consumer species in the EC module (Fig. 1b), a large top predator in the TC module (Fig. 1c), and a medium-sized intraguild prey (Fig. 1d) or a large intraguild predator (Fig. 1e) in the IGP module.
The impact of an invader on the resident community varies with temperature, nutrient levels and the invader’s trophic position (Fig. 1f-p). The community resists invasion in the following cases: (1) the invading consumer or predator suffers from metabolic meltdown at combinations of relatively high temperatures and low nutrient levels (Fig. 1g-1j, blue area top left), (2) the invading resource (AC module) or consumer (IGP module) is competitively inferior to resident resource or intraguild predator, respectively, at a wide range of intermediate temperatures and nutrient levels (Fig. 1f and 1i, blue area away from top left and bottom right) and (3) the consumer-resource system collapses due to the paradox of enrichment at combinations of relatively low temperatures and high nutrient levels (Fig. 1f-1j, blue area bottom right).
Successful invasion and occupancy of a vacant niche occur at combinations of relatively high temperatures and low nutrient levels that are above the extinction limit (caused by metabolic meltdown) of the local consumer or predator, but also below the extinction limit of the invader for invading larger resource species (AC module), smaller consumers (EC module) and intraguild prey (IGP module; Fig. 1f, 1g and 1i, yellow areas). Invading intraguild predator occupies a vacant niche when the intraguild prey goes extinct due to the paradox of enrichment at sufficiently low temperatures and high nutrient levels (Fig. 1j, yellow area).
Furthermore, a larger resource outcompetes and substitutes the resident resource in the AC module under environmental conditions just below the extinction limit of the resident consumer (Fig. 1f, green area), because a larger resource provides more energy to the consumer due to lower consumer: resource body mass ratio. An invading smaller consumer (EC module) and intraguild predator (IGP module) substitute the competitively inferior resident consumer over a much wider range of intermediate temperatures and nutrient levels; in the latter case, the environmental conditions must be sufficiently below the extinction threshold of the invading predator (Fig. 1g and 1j, green area).
Only invading top predator (TC module) can integrate into the community across a broad range of environmental conditions (Fig. 1h, light brown area). Intraguild prey integrates into the community when environmental conditions are just below the metabolic meltdown threshold of the resident intraguild predator, making the latter a poor competitor for the shared prey (Fig. 1i, light brown area). Intraguild predator integrates into the community when conditions are just below its own extinction threshold (Fig. 1j, light brown area; note that the extinction threshold is higher than in Fig. 1i due to the additional intraguild prey).
Vulnerability to invasion occurs for a smaller consumer in the EC module at low temperatures and sufficiently high nutrient levels (Fig. 1g, black area), where the resulting lower consumer-resource mass ratio triggers population oscillations. Finally, at sufficiently low temperatures and high nutrient levels, an invading top predator (TC module) rescues the resident resource by dampening population oscillations during its temporary presence in the system, so that only the top predator and resident consumer die out (Fig. 1h, ochre area).
These module-specific outcomes of invasions are reflected in different effects on community stability. Invading basal resource (AC module), intraguild prey and intraguild predator (IGP module) does not alter system stability except the invading intraguild predator, which can stabilise the dynamics over a narrow range of combinations of nutrient levels and (low to moderately high) temperatures (Fig. 1l, 1o and 1p). Successful invasion at low temperatures and high nutrient levels in the EC module always destabilises the community towards cycles or complete collapse due to the paradox of enrichment (Fig. 1m). Finally, successfully invading top predator may or may not change system stability depending on temperature and nutrient levels (TC module, Fig. 1n).