Introducing resource-use complementarity
While multi-trophic interactions were intrinsic to our food-web model,
incorporating resource-use complementarity required a modification of
the producer-resource interaction. We introduced it based on two simple
assumptions: First, resource-use complementarity can only occur if
species differ in their access to resources, forming different resource
compartments, for example, due to differences in chemical forms of
resources used or their spatial distribution (e.g., access to different
soil layers). Second, we assumed that resource-use complementarity is
maximized if all species take up resources from distinct resource
compartments.
We therefore introduced differences between producer species by limiting
their resource-use to certain compartments of the resource pool to
simulate resource-use complementarity (Fig. 1). Species that access the
same compartments directly compete for the resources within those
compartments. To investigate resource-use scenarios where all species
utilize resources from different compartments (i.e., no competition),
the number of resource compartments C was defined as the maximum of
producer species richness considered in our design (i.e., 16). Further,
we assumed that all compartments were quantitatively the same. By
increasing the dissimilarity between resource-use strategies of the 16
producer species within a species pool, we created a gradual change from
no complementarity (i.e., all species access all compartments) to
maximum complementarity (i.e., each species has its own resource
compartment; Fig.1). For this gradient of resource-use dissimilarity
(RUD), we ensured that (1) all producer species had access to the same
number of compartments at a given level of RUD and that (2) accessed
resource compartments were the same for both resources considered.
At maximum producer richness, species within a community where RUD
< 1 initially always compete for resources with at least two
other producer species with overlapping compartments, up to having all
species competing with each other when RUD = 0. The competitive outcome
is determined by which species can lower the resources the most
(’R*-rule’, Tilman 1982), whether resource competition can be weakened
by trophic processes (Brose 2008), or both. To capture how the
resource-use and thus productivity Y was distributed among coexisting
producer species i, we calculated Shannon diversity Hexpas Hexp = exp( - ∑i pi
ln(pi)), with pi = Yi /
∑i Yi. Hexp reflects
aspects of richness (i.e., how many species coexist) and abundance
(i.e., how much resources each species uses) alike and is maximized at
the number of coexisting species if all species use resources evenly
(Jost 2006). Lower values indicate an uneven distribution of
resource-use. In comparison to RUD, Hexp is based on
realized instead of fundamental resource niches.