Experimental demonstration of the importance of competition under disturbance

Abstract

Ecologists have long recognized the roles of competition and disturbance in shaping ecological communities, and the combinatorial effects of these two factors have been the subject of substantial ecological research. Nevertheless, it is still unclear whether competition remains as an important structuring force in habitats strongly influenced by disturbance. The conventional belief remains that the importance of competition decreases with increasing disturbance, but limited theory suggests otherwise. Using protist communities established in laboratory microcosms, we demonstrate that disturbance does not diminish the importance of competition. Interspecific competition significantly increased rates of species extinction over a broad disturbance gradient, and increasing disturbance intensities increased, rather than decreased, the tempo of competitive exclusion. This community-level pattern is linked to the species-level pattern that interspecific competition led to most frequent extinctions of each species at the highest level of disturbance that the species can tolerate. Consequently, despite a strong tradeoff between competitive ability and disturbance tolerance across the competing species, species diversity generally declined with disturbance. The consistent structuring role of competition throughout the disturbance gradient underscores the need to understand competitive interactions and their consequences even in highly disturbed habitats.

Fig. 1.

Tradeoff between competitive ability and disturbance tolerance exhibited by the studied species. Rankings of species disturbance-tolerance ability (best tolerator ranked as the first) and competitive ability (best competitor ranked as the first) were obtained based on the results of the single-species and bispecies experiments, respectively (Materials and Methods). Capital letters correspond to the names of the ciliated protist species used in the experiments. C, Colpoda sp.; CK, Colpidium kleini; CS, Colpidium striatum; GS, Glaucoma scintillans; H, Halteria grandinella, L, Loxocephalus sp.; PA, Paramecium aurelia; PB, Paramecium bursaria; PC, Paramecium caudatum; S, Spirostomum teres; TP, Tetrahymena pyriformis. Linear regression line is shown along with the data.

Fig. 2.

Temporal patterns of species richness and extinction along the disturbance gradient. (A–E) Observed species richness in the multispecies experiment was plotted against disturbance intensity at different sampling dates; data are means ± SE. (F–J) Number of cumulative species extinctions, with and without interspecific competition, was plotted against disturbance intensity at each sampling date. As all populations reached equilibrium states in the single-species experiment by day 30, the expected cumulative species extinctions at t = 6 mo were the same as those on day 30.

Fig. 3.

The relationship between the highest disturbance level that the species can tolerate in the absence of interspecific competition as recorded in the single species experiment (Disturbance level_{limit of tolerance}) and the level of disturbance at which interspecific competition led to the most frequent extinctions of each species in the multispecies experiment (Disturbance level_{most competitive exclusion}). Solid and broken lines are the linear regression line and 1:1 line, respectively.