Print PageThe effect of rainfall patterns - in particular interannual rainfall variability - on species coexistence is only beginning to be understood. While environmental variability is often thought to destabilize coexistence, two known theoretical mechanisms may produce the opposite effect: The storage effect (SE) and relative nonlinearity of competition (RN). The latter (RN) has not fully been tested in any natural ecosystem, despite its potential importance for coexistence. RN requires that species differ in (a) their effects on resource variability, and (b) the way in which resource variability influences average long-term per capita population growth due to the degree of nonlinearity in the functional response to resource availability. When these conditions are met, coexistence on a single, fluctuating resource is possible under certain resource input regimes. Here, I ask whether RN is stabilizing the coexistence of two competing, water-limited annual grass species at Sedgwick Natural Reserve, California - both under current and future rainfall patterns. During the 2009-2010 growing season, I experimentally imposed a gradient of rainfall treatments, ranging from very low to very high, onto monocultures of two annual grass species, Bromus diandrus and Avena barbata. The resulting data on biomass and seed production, soil moisture availability and consumption, as well as seed germination rates, can be used to fully parameterize a novel consumer-resource competition model that describes both within-season biomass production and between-season population dynamics in this system. This model can then be analyzed to test whether the two species are predicted to coexist via RN under both current and future rainfall scenarios.
Results/Conclusions
In both species, within-season biomass and seed production were a positive function of soil moisture availability. Preliminary analyses suggest that Avena responded to soil moisture in a slightly concave fashion, while Bromus showed a slightly convex response. Pending germination fractions (to be measured in early 2011), these results suggest that the long-term per capita population growth rate of Avena is reduced by rainfall variability (due to averaging over a concave function) while that of Bromus is increased (due to averaging over a convex function). Since soil moisture consumption was proportional to biomass production in both species, Avena should leave a greater range of soil moisture unconsumed, while Bromus should strongly reduce fluctuations in unconsumed soil moisture. Hence, each species may, when dominant, create conditions that favor its competitor. The full analyses and final results will be available at the time of the presentation.
