Fitness differences override variation-dependent coexistence mechanisms in California grasslands

While most studies of species coexistence focus on the mechanisms that
maintain coexistence, it is equally important to understand the mechanisms
that structure failed coexistence. For example, California annual
grasslands are heavily invaded ecosystems, where non-native species have
largely dominated and replaced native communities. These systems are also
highly variable, with a high degree of rainfall seasonality and
interannual rainfall variability – a quality implicated in the coexistence
of functionally distinct species. Yet, despite the apparent strength of
this variation, coexistence between native and non-native annuals in this
system has faltered. It is therefore uncertain to what degree rainfall
variation can offset average fitness differences between native and
non-native annual plants in California grasslands to promote coexistence,
nor what coexistence mechanisms are most relevant. To test these dynamics,
we implemented a competition experiment between five annual species of
native forbs and non-native grasses. We grew individuals from each species
under varying densities of all other species as competitors, under either
wetter or drier early-season rainfall treatments. Using subsequent seed
production, we parameterized competition models, assessed the potential
for coexistence among species pairs, and quantified the relative influence
of variation-dependent coexistence mechanisms. Overall, we found little
potential for coexistence. Competition was dominated by the non-native
grass Avena fatua, while native forbs were unable to invade non-native
grasses. Mutual competitive exclusion was common across almost all species
and often contingent on rainfall, suggesting rainfall-mediated priority
effects. Among variation-dependent mechanisms, the temporal storage effect
had a moderate stabilizing effect for most species, while relative
nonlinearity in competition was largely destabilizing, except for the most
conservative non-native grass, which benefited from a competitive release
under dry conditions. Our findings suggest that rainfall variability does
little to mitigate the fitness differences that underlie widespread annual
grass invasion in California, but that it influences coexistence dynamics
amongst the now-dominant non-native grasses.