The Colonization and Formation of Equilibrium Plant Species Associations on Badger Disturbances in a Tall‐Grass Prairie

Abstract

Badger disturbances in a tall—grass prairie were used to study colonization patterns and the formation of equilibrium plant species associations in a complex mainland community. Colonization processes were described from field observations over a 4—yr period. A qualitative colonization model was developed to predict noninteractive species equilibria. Predicted colonization rates were based upon relative immigration rates determined by interactions among propagule production rates, dispersal capacities, and the source—site distances of the species involved. The immigration rates both between groups with different life history characteristics (mature prairie, prairie fugitive, and ruderal species) and within groups (K—type, intermediate, and r—type fugitive species) were predicted. Manipulation of important variables enabled different conditions affecting relative immigration rates to be simulated. Species with intermediate life history characteristic (propagule production and dispersal capacity) and located at intermediate distances from the colonization site were predicted to have the highest immigration rates. Thus, prairie fugitive forbs were predicted to have higher rates than either mature prairie or ruderal species. Immigration rates of different fugitive species onto a site were predicted to depend upon the frequency and distribution of previously colonized disturbances in the vicinity of the colonization site. In general, model predictions were consistent with field observations. The model predicted the noninteractive species equilibrium among seedlings of mature prairie, fugitive, and ruderal forbs, but not the interactive specie equilibrium among these groups of forbs. Within the fugitive species, both noninteractive and interactive species equilibria were predicted. The noninteractive colonization model was constructed for prediction of colonization patterns on local disturbances in complex plant communities, but since it utilized general life history characteristics to predict immigration rates it should also be applicable to other colonization processes. Equilibrium plant species associations were studied on badger disturbances in virgin prairie and in a less complex tract of overgrazed prairie. Peak standing crop biomass was not different on and off disturbances in either community, but biomass production occurred earlier in the year on disturbances. Although the species present differed, species diversity, equitability, and the distribution of biomass among species were similar on and off disturbances in virgin prairie and on disturbances in overgrazed prairie. The dominant species comprised 20%—26% of the biomass. In contrast, off disturbances in overgrazed prairie were different. The species diversity and equitability were low, and the biomass was concentrated in the dominant species (60%). Thus, in overgrazed prairie local disturbances depressed dominance and resulted in increased complexity of the plant species association formed. In virgin prairie alternate states of equivalent complexity resulted from local disturbances, and thus spatial heterogeneity was increased. The fugitive species comprise a guild using a common resource (disturbance sites). Direct competition among the species is reduced by spatial and temporal separation of the species. Overlap of species persistent on badger disturbances is reduced because of colonization specificity, site contingencies that determine germination and growth, and suppression of later colonists. The species guild of fugitive plants thus is characterized by diffuse competitive interactions. The reduction of direct competition enhances the formation of persistent species associations on badger disturbances, enhances coexistence, and enables species packing to occur within fugitive species guild.