Document: KAT-3-38-13

Fugitive species are not always competitively inferior: Evidence for a shift in competitive hierarchy following soil disturbance in a grassland plant community.

NASH SUDING, K.* and D.E.GOLDBERG

Univeristy of Michigan, Ann Arbor, MI 48109-1048 ¹

Abstract:
Although it has been well documented that species composition changes following a disturbance, the processes responsible for these changes are less well understood. The often-assumed idea of competitive reduction, that disturbance, by removing neighbor biomass, allows species that are inferior competitors but superior colonists to persist, has rarely been tested. We suggest an alternative: By changing neighborhood and abiotic conditions, a disturbance can shift competitive rankings of species. We tested these hypotheses by assessing the competitive response abilities of three prairie species representative of varying disturbance regimes in both gap (simulated soil mound disturbances) and matrix (undisturbed prairie) conditions. We found that gap creation has little effect on survival competitive hierarchies, and these rankings are similar to species' relative abundances in matrix conditions. However, for growth, a well-defined hierarchy was broken down to conditions of competitive equivalence following a disturbance. These results indicate that the assumption of competitive reduction is not entirely justified. We also teased apart the effects of several aspects of a disturbance to assess whether it is neighbor biomass or other changes associated with gap creation that are responsible for the change in competitive rankings. In a factorial design, we measured how changes in neighborhood conditions (neighbor biomass, neighbor identity, size-structure) and changes in abiotic soil conditions (mound formation) affected competitive hierarchies. Although the reduction of neighbor biomass associated with a disturbance decreased competitive intensity, it did so similarly for all species. No change in any neighborhood characteristic affected the growth competitive hierarchy. In contrast, changes in abiotic soil conditions (decreased compaction and soil moisture associated with mound formation, regardless of neighborhood) shifted the competitive hierarchy to competitive equivalence. In addition to neighbor biomass removal acting to decrease the intensity of competitive interactions (as predicted by competitive reduction), changes in abiotic conditions may act to change the nature of competitive interactions following a disturbance.

Keywords: Competitive hierarchy, soil disturbance, tallgrass prairie, fugitive species

This abstract is being presented at: 10:45 AM in session:
Oral Session #72: Plant Competition.