Results
In the absence of animals, simple producer communities show positive
diversity-productivity relationships across all levels of resource-use
dissimilarity (RUD; Fig. 2a, overall average relationship shown by black
line). The shape of the diversity-productivity relationship depends on
the level of RUD and can be exponential (high RUD), sigmoidal
(intermediate RUD), or saturating (low RUD) on a
log2-scale of horizontal diversity (Fig. 2a, colored
lines). Only for the special case in which all producers exploit the
same resource compartments (i.e., RUD = 0, Fig. 2a), the relationship is
neutral
At maximum producer species richness, all resources were accessible in
all RUD scenarios, effectively maximizing yields regardless of RUD (Fig.
2a&b). On the contrary, the access to resources in monocultures
directly depends on RUD: without RUD, all monocultures have access to
all resource compartments, whereas, under the highest RUD, each
monoculture can only utilize one-sixteenth of the total resource pool
(i.e., one resource compartment). Consequently, monoculture yields (Fig.
2a, log2 yields at producer richness of zero) and thus
expected yields YE (Fig. 2b, red dots) decreased
linearly with increasing RUD. As a result, net diversity effects at
maximum producer richness increased linearly with RUD, starting at zero
net diversity effects when RUD = 0 (Fig. 2c). Since net diversity
effects almost exclusively partition into complementarity effects with
selection effects only playing a minor role (Fig. S1-3), RUD exhibits
the behavior expected from resource-use complementarity. In comparison,
the realized complementarity of the resources used by the producer
community (i.e., Hexp) did not change linearly along the
RUD gradient (Fig. 4b, Fig. S4).
Communities initialized with intermediate levels of producer richness
(i.e., 2, 4, or 8 producer species) all failed to maximize yields at
high levels of RUD (Fig. 2a&b), which led to reduced net diversity
effects (Fig. 2c). For example, at maximum RUD, where species are never
functionally redundant as they all access species-specific resource
compartments, the loss of species directly lowers the resource
availability and thus the primary production of the producer community.
With decreasing horizontal diversity, net diversity effects became lower
over more extensive parts of the RUD gradient. As the number of species
necessary to fully utilize all resource compartments increases with RUD,
losing species has the most severe effects at higher levels of RUD. Even
though the resource availability might decrease, more species could
coexist with increasing RUD (Fig. 4a, Fig. S5). The value of RUD at
which net diversity effects were maximized shifted from its maximum in
16-species mixtures towards intermediate values in 2-species mixtures.
Consequently, the power of RUD to explain the strength of net diversity
effects depends on the completeness of the species pool. Regardless, as
long as species differed in their access to resource compartments (i.e.,
RUD > 0), net diversity effects were consistently positive
(Fig. 2c).
Introducing multi-trophic interactions and increasing animal richness
increased net diversity effects on primary productivity (Fig. 3a, Fig.
S1). For multi-trophic interactions to increase horizontal diversity
effects, reductions in primary production due to animals feeding on
producer species had to be larger in monocultures than in mixtures. A
decrease in productivity of some producer species, which was apparent in
monocultures and thus expected yields (Fig. 3e, Fig. S6), could be
compensated in mixtures by competing producer species that shared
resource compartments. Independent from animal richness, this maximized
productivity in mixtures in most cases (Fig. 3d, Fig. S6) while also
allowing more producer species to coexist (Fig. 4a, Fig. S5). Because
the degree to which multi-trophic interactions could reduce monoculture
productivity and thus expected yields does depend on how many resource
compartments were accessible, increasing RUD reduced the potential of
animals to alter horizontal diversity effects. This bottom-up control
led to weaker effects of increasing animal richness and reduced
variability of net diversity effects when increasing RUD (Fig. 3a, Fig.
S1). When resource-use was not overlapping (i.e., RUD = 1), all producer
species had access to independent resource compartments. Without
animals, this led to maximum net diversity effects (Fig. 2c). In
scenarios with animals, primary productivity losses due to consumption
could not be compensated by other producer species. As a result, we
found weak negative effects of multi-trophic interactions compared to
no-animal scenarios in that specific case (Fig. S1). A loss of producer
richness expanded the range of RUD in which producers could be limited
to use distinct resource compartments, which makes multi-trophic
interactions more likely to affect net diversity effects negatively.
With increasing animal richness, we found that complementarity effects
increased from low levels in scenarios without animals to values that
could be several times higher. The positive impact of animal richness on
complementarity effects was exceptionally high in scenarios with low RUD
(Fig. 3b, Fig. S2). A reduced producer richness can weaken these
positive effects. Similar to scenarios without animals, the loss of
producer species shifts the level of RUD at which net diversity effects
maximize from high to medium values (Fig. S1). Further, the introduction
of animals created both positive and negative selection effects. Neutral
selection effects occurred in producer communities without animals. With
increasing animal richness, we found that selection effects first
increased (positive selection effects at low animal richness) and then
decreased (negative selection effects at high animal richness). When RUD
was high, selection effects were mostly neutral or negative (Fig. 3c,
Fig. S3).
Complementarity and selection effects describe a gradual switch between
two ways by which animals increase diversity effects: First, the
positive selection effects found predominantly at lower animal richness
indicate that more productive species in monoculture, i.e., species that
experienced a lower feeding pressure, were on average also more
productive in mixtures (Fig. 3c, Fig. S3). Second, negative selection
effects found at high animal richness indicate that less productive
monoculture species benefit more when growing in mixtures (Fig. 3c, Fig.
S3). Because complementarity effects increase more than selection
effects decrease, we found an overall increase of net diversity effects
with increasing animal richness (Fig. 3a, Fig. S1). Without animals,
selection and complementarity effects were entirely determined by RUD.
Only when adding multi-trophic interactions, selection and
complementarity effects directly responded to producer coexistence
rather than RUD (Fig. S7-8). This effect was less apparent for net
diversity effects (Fig. S9). Thus, increasing animal richness caused
higher realized producer species richness (Fig. 4a, Fig. S5) and
consequently higher realized complementarity in resource-use (i.e.,
Hexp; Fig. 4b, Fig. S4). In multi-trophic communities,
producer species of low body-mass were less likely to survive as their
productivity was reduced more due to herbivorous feeding. In simple
producer communities, producer species survival and productivity were
mostly independent of body-mass (Fig. S10, Fig. S11). Hence, the
patterns in selection effects with increasing animal richness (Fig. 3c,
Fig. S3) can be partially attributed to systematic shifts in the
producer communities’ body-mass structure. Interestingly, we found that
the survival of animal species was roughly constant at 80% across
gradients of animal and producer richness (Fig. S12).