From individuals to ecosystems: Integrating the effects of competition and disturbance on coexistence in annual plant communities

Background/Question/Methods

Biodiversity loss and its inherent consequences are becoming more critical to the economically minded and scientific communities alike. Therefore, disentangling the basic mechanisms by which species coexistence is promoted and maintained becomes paramount. Competition is accepted as one of the primary drivers in sustaining coexistence. It allow for stable coexistence in environmentally homogeneous landscapes through a tradeoff between colonization and competitive ability. Models of natural systems must incorporate environmental heterogeneity through means like niche differences in resource acquisition and stochastic processes. Our experimental model system of annual plant species grown in 80 plots of full seven-species mixtures and monocultures allows us to examine inter- and intraspecific plant interactions. For the last three springs, prior to seed set, the plots were subjected to one of five intensity levels of disturbance applied to randomly selected patches within the plot landscape matrix. An interspecific tradeoff between colonization and competitive ability has been recognized in hierarchically structured assemblages, and here disturbance introduces environmental variation as well. Small seeded species are expected to thrive in highly disturbed plots as they can exploit their colonization ability and large-seeded species will persist in low disturbance plots despite their lower numbers of offspring and because of their competitive ability.

Results/Conclusions

Upon comparing mixture and monoculture plots, the mixture plots had 30% higher biomass yields than the average mixture, revealing a significant positive effect (F_1,76 = 22.2, P < 0.001). However, as of 2011, there was no interaction with the disturbance treatment. As the highly disturbed plots remove approximately 88% of the possible contributing individuals before they reproduce, an effect of disturbance on biomass was anticipated but not found. The lack of an effect of disturbance suggests that the species are able to produce enough seeds to compensate for the individuals lost from disturbance in this system. Within mixtures there was also no effect of disturbance on diversity, however, a highly negative correlation was found between diversity and the number of adult plants (r = -0.542, df = 22, P < 0.01). Thus far it appears that the initial ideas of this study about competition in heterogeneous disturbed environments are incorrect, but that competition may drive diversity loss over longer periods of time.