Results
Plant diversity-productivity
relationships
To investigate the potential drivers behind plant diversity-productivity
relationships, we compare the effects of food-web and resource-use
scenarios (see Fig. 1) on productivity at both ends of the plant
diversity gradient. In monocultures without animals, we find that a
spatial overlap in plant resource access (‘spatial resource overlap’)
has no effect on productivity (Fig. 2, plant species richness of one;
Fig. S1A; green points). Instead, differences occur across the different
food-web scenarios. Specifically, monocultures without animals are the
most productive, closely followed by those embedded in spatially
non-nested food webs (dark blue points). In spatially nested food webs,
plant productivity of monocultures is the lowest on average but shows
the largest variation with a weakly positive response to an increased
spatial resource overlap (light blue points). We rarely found unviable
monocultures. The few examples we recorded were spread across all
resource-use scenarios and more common in spatially nested (93/1600)
than in spatially non-nested food webs (30/1600), and never occurred in
communities without animals. When focusing on monoculture productivity,
an interaction with neighbouring plants through a spatial resource
overlap therefore emerges as having little effect, rendering differences
in food web architecture as the main driver.
Our analyses reveal some striking effects of having a spatial resource
overlap on the diversity-productivity relationships in plant communities
without animals (Fig. 2, green lines). Without a spatial resource
overlap we find neutral relationships between productivity and species
richness (Fig. 2A). However, as soon as plants are able to access
resources of the neighbouring patches (i.e. with spatial resource
overlap), we find positive effects of plant diversity on productivity
that are similar across resource-use scenarios (Fig. 2B-E; Fig. S1B).
Taken together, these results suggest that a positive response of
productivity to species richness in plant communities without animals
requires a spatial resource overlap, but already small amounts of
spatial resource overlap (i.e. Fig. 2B) suffice to saturate these
relationships.
In spatially non-nested food webs, plant communities show a strong
decrease in productivity with increasing richness in most resource-use
scenarios (Fig. 2A-D; Fig. S1B; dark blue lines). Diversity-productivity
relationships are most negative when spatial resource overlap is
smallest (Fig. 2B). Across the gradient of resource-use scenarios, plant
monoculture productivity is constant (Fig. S1A), while it increases
considerably at higher plant species richness (Fig. 2B-E; Fig. S1B).
This culminates in neutral diversity-productivity relationships when
spatial resource overlap is maximized (Fig. 2E). Thus, in communities
with spatially non-nested food webs, a strong spatial resource overlap
with neighbouring plants has a positive effect on plant
diversity-productivity relationships.
In contrast, plant communities in spatially nested food webs display
positive diversity-productivity relationships in the majority of cases
(Fig. 2B-E, light blue line). We only find negative effects of plant
diversity on productivity when there is no spatial resource overlap
(Fig. 2A). However, productivity at both ends of the diversity gradient
displays large amounts of variation. As soon as plants have access to
resources of neighbouring patches (i.e. with spatial resource overlap),
productivity increases with diversity, reaching values with little
variation that are similar to those in plant communities without animals
(Fig. 2B-E). Together with having the lowest average productivity in
plant monocultures compared to all other food web scenarios (Fig. S1A),
this makes plant communities in spatially nested food webs exhibit the
most positive diversity-productivity relationships (Fig. 2B-E).
Plant community
composition
Our prior results show that differences in the plant
diversity-productivity relationships are mainly driven by varying
productivity at the highest plant diversity levels (Fig. 2). To better
understand these differences between food-web and resource-use
scenarios, we investigated how plant community composition differs
between scenarios at the highest plant diversity level of 16 species.
Without a spatial resource overlap (i.e. spatial resource overlap at 0),
realized species richness, realized plant density, and Shannon diversity
display the highest values within each food web scenario considered
(Fig. 3). In communities without animals, the values are at their
absolute maximum (Fig. 3, green line). In spatially non-nested food
webs, the plant communities show a tendency towards lower values of
realized richness and density, and Shannon diversity is clearly lower,
indicating an increased heterogeneity in the plant community (Fig. 3,
light blue line). For spatially nested food webs, plant communities
display a slightly reduced plant species richness and density and have
the lowest Shannon diversity (Fig. 3, dark blue line). Thus, spatially
nested food webs support the least diverse plant communities when there
is no spatial resource overlap between neighbouring plants.
The compositional response of plant communities without animals to
increasing the spatial resource overlap between neighbouring plants
stands out as it displays a delayed but harsh drop for all three
compositional variables (Fig. 3, green lines). This leads to plant
communities that lose almost half of their plant individuals when
spatial resource overlap is highest (Fig. 3B), and includes the
extinction of slightly more than three species on average (Fig. 3A).
Since Shannon diversity decreases more than species richness (Fig.
3A&C), an increased spatial resource overlap increases the
heterogeneity in the plant communities without animals. Taken together,
the effects of increasing the spatial resource overlap are most severe
for plant communities without animals.
Plant communities in spatially non-nested food webs follow very similar
patterns compared to communities without animals (Fig.3, dark blue
lines). However, the negative effects of increasing the spatial resource
overlap are less pronounced for plant richness and density, culminating
in about a quarter of plants and only about two species lost when the
spatial resource overlap was highest (Fig.3A&B, dark blue lines).
Shannon diversity was generally lower than in communities without
animals, reaching the lowest values at maximum spatial resource overlap
compared to all other scenarios (Fig. 3C, dark blue line). When spatial
resource overlap is high, spatially non-nested food webs are therefore
enhancing differences between plant species more than any other
scenario.
Compared to the other food web scenarios, plant community composition in
spatially nested food webs show the weakest response to changes in
spatial resource overlap. Especially realized plant species richness,
which displays an average loss of only one species, was independent from
spatial resource overlap (Fig.3A, light blue line). Similar to spatially
non-nested food webs, only about a quarter of plants are lost when
spatial resource overlap is highest (Fig.3B, light blue line). Shannon
diversity again decreases with increasing spatial resource overlap but
ends up stabilizing over the last two steps of the spatial resource
overlap gradient (Fig.3C, light blue line). Overall, these findings
suggest that spatial resource overlap between neighbouring plants
matters the least in spatially nested food webs.