Community patterns for the fixed preference case
Along the enrichment gradient (\(N_{\max}\)), the superior resource
competitor (edible phytoplankton) can establish at the lowest enrichment
level (\(N_{\max}\) > \(0.055\)), independent of the
preference value. The invasion threshold for zooplankton depends on the
available prey biomass and the preference value for edible
phytoplankton, the enrichment level for successful invasion increasing
with decreasing preference for the phytoplankton. For enrichment levels
slightly above the invasion threshold for zooplankton, the inedible
phytoplankton and finally the parasitic fungi can successfully invade.
The invasion boundaries for zooplankton, inedible phytoplankton and
fungi are very close to each other, nearly overlapping, therefore only
the coexistence boundary is indicated in Fig. 1 (red curve). Coexistence
is not possible for too low enrichment levels and too strong preferences
for fungi.
Within the coexistence area, the mean biomass of all food web
compartments along the mycoloop increase with nutrient enrichment (Fig.
1a,b,d,e), whereas it decreases for edible phytoplankton (Fig. 1c). The
community is dominated by either edible phytoplankton (at low nutrient
availability) or zooplankton (at high nutrient availability, apart from
regions with an almost exclusive preference for fungi) (see Appendix S2,
Fig. S2.1).
The response of zooplankton along the preference gradient differs for
low vs. high enrichment levels (Fig. 1a). For low enrichment levels,
zooplankton increases with increasing preference for edible
phytoplankton. For high enrichment levels, zooplankton shows a hump
shaped relationship reaching the highest biomass at\(\ p_{Z}\approx\ 0.3\). With increasing preference for edible
phytoplankton,\(\ \)the phytoplankton decreases due to stronger top-down
pressure through zooplankton (Fig. 1c), while the biomass of fungi
increases (Fig. 1b). The abundance of the fungal host (inedible
phytoplankton) shows a hump shaped relationship with respect to the
preference, peaking at \(p_{Z}\approx\ 0.3\), where its parasite is
the preferred prey (Fig.1d). For high enrichment levels, the area with
highest inedible phytoplankton biomass overlaps with the region of
maximum biomass of zooplankton (Fig. 1a,d), indicating a strong top down
control on both prey species, releasing the phytoplankton host from
infection through fungi and from nutrient competition with edible
phytoplankton.
At preference values close to one (i.e. strong preference for edible
phytoplankton), the system exhibits oscillatory dynamics. This region
extends towards lower preference levels with nutrient enrichment (area
to the right of the black dashed line in Fig. 1). The oscillatory
dynamics are characterized by small amplitude cycles for low enrichment
and a pronounced increase of cycle amplitudes at high enrichment levels
(Fig. 1f).
In the area with stable point equilibria (area to the left of the black
dashed line in Fig.1), all compartments reach their maximum biomass at
the highest investigated enrichment level (Fig. 1a-e). Only in the
absence of zooplankton (edible phytoplankton-only state), for preference
values close to zero, the edible phytoplankton increases with enrichment
(area to the left of the red line in Fig. 1c). The maximum fungal
biomass and freely available nutrient levels are observed at high
preference values for edible phytoplankton (\(p_{Z}\approx\) 0.8) (Fig.
1b,e).
In comparison to the assumption of linear food uptake rates (Miki et al.
2011), there is no qualitative change in the biomass response pattern
along the nutrient gradient under the assumption of nonlinear food
uptake rates, as illustrated for \(p_{Z}=0.5\) (Fig. 2a,b). However,
while the phytoplankton host is predicted to reach a higher biomass
compared to its parasite throughout the nutrient gradient for the linear
case, under the assumption of saturating food uptake terms the
phytoplankton host only dominates at the highest nutrient levels (see
Appendix S2, Fig. S2.2). Furthermore, zooplankton can invade at lower
prey abundance (lower \(N_{\max}\)) compared to the linear case, so
edible phytoplankton cannot reach as high biomass levels and decreases
more steeply along the nutrient gradient compared to the linear case
(Fig. 2a,b).