Do Disturbances Alter Competitive Hierarchies? Mechanisms of Change Following Gap Creation

Abstract

Many studies have documented that patterns of species composition change after gap creation, but few have addressed the processes responsible for this change. The idea of competitive reduction, that disturbance removes biomass, reducing competitive intensity and allowing competitively inferior fugitive species to colonize, has been widely accepted but rarely tested. We suggest an alternative hypothesis, competitive change, in which changes associated with disturbance may act to alter the competitive hierarchy. Under these changed conditions, species characteristic of gaps may be competitively superior to species characteristic of undisturbed matrix areas. We quantified competitive response ability for survival and relative growth for three target species (Andropogon gerardii, a matrix grass; Coreopsis tripteris, a matrix forb; and Ratibida pinnata, a forb characteristic of gaps) in undisturbed matrix vegetation and in soil mound disturbances in a northern Ohio prairie community. For survival, a weak competitive hierarchy developed following gap creation. In contrast, for relative growth, gap creation shifted a distinct competitive hierarchy to conditions of competitive equivalence. The competitive rankings for survival in gaps were opposite those for growth in matrix conditions, with Andropogon being superior in terms of survival and Coreopsis superior in terms of growth. Then, in a factorial design, we teased apart the effects of neighborhood (biomass reduction, size structure, neighbor identity) and abiotic soil changes (mound formation) associated with soil disturbance on these competitive interactions. Abiotic changes associated with mound formation, rather than neighborhood changes, were responsible for the development of the survival hierarchy and the loss of the growth hierarchy following gap creation. When hierarchies formed, matrix species were competitively superior to gap species, supporting the idea of competitive reduction. However, no one matrix species was consistently competitively superior, and under some conditions species were competitive equivalents, partially supporting the idea of competitive change. When disturbance creates conditions of physical stress, as in this case, trade‐offs between growth and survival hierarchies may be important determinants of species response.

Effect of minor water depth treatments on competitive effect and response of eight wetland plants

“…Competitive ability hierarchies, which seem to be pervasive in plant communities (Keddy and Shipley 1989;Shipley and Keddy 1994;Suding and Goldberg 2001;Emery et al 2001), were consistent with respect to CE across the five water depth treatments, thus supporting our third hypothesis. The CE hierarchy established in this study changed little at different water depths suggesting that the integrity of these hierarchies was not affected by water level, despite the wide variation between species in their CE values.…”

What limits herb biomass in grasslands: competition or herbivory?

“…The inhibiting factors when major competitors are present could be several. Our results suggest that competition for light could be an important factor in the system because besides the decrease in monocot biomass, we found a tendency towards less dead organic matter in grass-specific-herbicide plots that could be shading plants (a result also found by Suding and Goldberg 2001). In addition Van der Wal et al (2000) also found that competition for light was more important than grazing for Triglochin maritima.…”

Plant competitive ability and the transitivity of competitive hierarchies change with plant age

“…Competitive hierarchies are known to change depending on resource identity and availability (Fortner and Weltzin 2007;Fraser and Miletti 2008;Keddy et al 2000) and disturbance (Suding 2001;Suding and Goldberg 2001), which may provide mechanisms for the coexistence of multiple species (Crain et al 2004).…”