The balance between positive and negative interactions in a savanna

by Batchelor, Margaret Elizabeth

Facilitation (the positive effects of one species on another), in theory, can lead to positive spatial associations between species. Competition (the negative effects of one species on another) can lead to negative spatial associations. Therefore the magnitude and the direction of the spatial association between Quercus fusiformis and Juniperus ashei were measured in multiple savanna sites on the Edwards Plateau in central Texas. Stratified random sampling was used to determine the number of Juniperus ashei individuals in each of three size classes found in each of three types of habitat (under Quercus fusiformis canopy, under Juniperus ashei canopy, and in open grassy areas). This observed spatial distribution was compared with the distribution generated by a null model that assumed no interaction between J. ashei individuals and habitat type. J. ashei individuals were found in significantly higher than expected frequencies under woody canopies and in significantly lower than expected frequencies in the open grassland. The positive association with Quercus fusiformis and Juniperus ashei canopies was significant for all size classes of target J. ashei individuals, but the strength of the pattern decreased with size. Mechanisms other than facilitation could explain the spatial distribution of J. ashei on the Edwards Plateau (e.g. seed dispersal). This study suggests that if facilitation exists in this system, it probably occurs between woody canopy and J. ashei during the early stages of its life history. 5