Support for and nuances underlying the stress-gradient hypothesis
and the role of species interactions
We found strong support for our hypothesis that biotic interactions
would become increasingly competitive in more productive environments
(Table S4; Fig. 3). Although this pattern was observed for all four
species (i.e., greater difference between blue and red lines in
productive areas in Fig. 1A-D), there were also interesting nuances. For
one species, growth rates increased with productivity when neighbors
were present (Festuca : 𝛽1 = 0.167 ± 0.12; Table S2, red data in
Fig. 1D), whereas for two, population growth rates decreased
(Plantago : 𝛽1 = -0.10 ± 0.12, Bromus : 𝛽1 = -1.00 ± 0.18;
Table S2, red data in Fig. 1B,C). As a result, each species’ fundamental
and realized niches differed (see ‘Neighbor treatments’ term for each
species in Table S3). Competition appeared to be absent for all species
at the unproductive end of the gradient, with weak evidence of
facilitation (i.e., λ without neighbors > λwith neighbors) only in a few specific cases. Plantago andFestuca patterns showed evidence of facilitation for seed
production and early germinate survival (i.e., zero-inflated component
of the model; Fig. S6B, D), while Bromus showed weak evidence of
facilitation in the fecundity (i.e., conditional) component of λ ;
Fig. S6 F).
Taken together, at a community level, productivity had opposing effects
on the number of species that could persist depending on the presence or
absence of neighbors: in the most productive locations (i.e., vegetation
index of 2), persistence without neighbors was 1.58 times greater than
persistence with neighbors (P < 0.001; Fig. 3; Table
S5). Notably, these relationships were linear at the community level
despite a diversity of non-linear relationships observed for individual
species (Table S1; Fig. 2A-D).