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).