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).