Understanding the mechanisms that underlie species interactions is essential for predicting community responses to ongoing change. Climate variability can enhance species coexistence if species have different, nonlinear responses to climate conditions (via the storage effect). Enhanced species diversity may mitigate a direct, destabilizing effect of climate variability on community properties (e.g., biomass) if some species compensate for climate-driven declines in others. Importantly, the strengths of these relationships may shift with novel drivers. Global circulation models predict an increase in rainfall variability, which may facilitate coexistence via a storage effect. Ongoing species introductions may further enhance this effect (via an increase in functional diversity) or dampen it (via a reduction in co-evolved species).
We experimentally tested the relationship between climate variability, species coexistence and community stability in a California grassland. We used rainout shelters with removable roofs to create four rainfall conditions: fall drought, spring drought, continuous drought and control. Within each rainfall treatment we manipulated the composition of a dominant, non-native grass (Avena) and forb (Erodium) by seeding them in different ratios (10:0, 9:1, 5:5, 1:9, 0:10) and densities (320, 3,200, 32,000 seeds/m2). We measured seed production and biomass at peak production.
Results/Conclusions
Overall, we found that climate variability enhanced species coexistence in a California grassland, but this effect did not mitigate a destabilizing effect on community biomass. Under maximum rainfall (i.e., the control), Avena exhibited high growth rates when rare and was strongly self-limited when common. Avena showed no difference in per capita growth rate when rare vs. common under the three drought conditions. In contrast, Erodium per capita growth rates increased when rare under drought conditions but not in the control. Erodium additionally showed an interaction in which this effect was strongest at low densities, but dampened at high (when a low seeding ratio of Avena still resulted in a high total number of Avena). Together, these results indicate that climate variability enhanced coexistence among two competing non-native species. Although the two species showed strongly differential responses to rainfall, species tradeoffs did little to stabilize biomass across rainfall conditions (no seeding ratio × rainfall interaction). Biomass was sensitive to seeding ratios – biomass increased with the ratio of Avena – suggesting that species effects, not rainfall responses, matter most for predicting the stability of communities under climate change.