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.competitive interactions | species coexistence | species extinction | species richness

Environmental stress, facilitation, competition, and coexistence

“…The large negative effect of the harsh environment on Watersipora's density-independent growth rate (k w ; Table 1) means that Watersipora has less ability to maintain positive population growth in the harsh environment and thus is less tolerant of interspecific competition even though the strength of competition (a wb ) is unchanged. Such a result is consistent with theoretical work that challenges the logic of traditional conceptual models for the effect of stress on competition, on the basis that these models rarely incorporate the direct negative effect of harsh environments on species' abilities to tolerate competition (Chesson and Huntly 1997; see also Violle et al 2010, Fox 2012. Our results do not question the veracity of conceptual stress gradient models such as the SGH (some of our data support the SGH), but instead demonstrate the importance of placing predicted changes in the strength or frequency of interactions in a population-dynamic context Huntly 1997, Freckleton et al 2009).…”

“…Many classic (e.g., Connell 1978, Grime 1979, Huston 1979 and modern (e.g., derivatives of the Menge and Sutherland [1987] stress gradient hypothesis; Callaway 2007) theories of community dynamics continue to emphasize a diminished role for competition in harsh environments, but modern species coexistence theory does not (Chesson and Huntly 1997, Chase et al 2002, Violle et al 2010, Fox 2012. Understanding how these theories can mutually inform each other is therefore an important goal.…”

Stochastic competitive exclusion leads to a cascade of species extinctions

“…In principle, a plausible way to conduct the experiment would involve simple protist microcosms where species compete for a shared resource (see, for example, Violle et al (2010) and references therein). Lowering the amount of resource could be associated to a decrease in the carrying capacity, and we have shown that the extinction cascade mechanism is expected to arise as long as the carrying capacity (resource availability) is reduced, see Fig.…”

Predictable bacterial composition and hydrocarbon degradation in Arctic soils following diesel and nutrient disturbance

“…Microbial communities can also be disrupted by nitrogen inputs, and reductions in respiration, biomass and diversity have been observed following nitrogen addition (Allison et al, 2007;Ramirez et al, 2012). Experiments with protists have shown that competition can intensify at high levels of disturbance, when multiple surviving organisms are at the limit of their tolerance (Violle et al, 2010), so a competitive advantage in a specific environment may have more of a role in taxonomic dominance than the ability to quickly metabolize the petroleum carbon source.…”