The effects of habitat fragmentation on herbaceous plant diversity in Central Texas

Central Texas vegetation is often a mosaic of patches of woody plants and patches of herbaceous vegetation; the latter consists almost entirely of species not found in woody patches. We take advantage of this mosaic to investigate the effect of fragmentation on the herbaceous plant community. Species richness was recorded in randomly-located 1m radius circular plots in herbaceous patches. Aerial photographs were converted to binary (woody versus herbaceous) maps. These maps were used to calculate a set of fragmentation measures for the herbaceous habitat within circles 3m to 56m in radius around the center of each plot. Species richness was negatively related to fragmentation, perhaps because increasing fragmentation reduced effective population sizes, in accordance with classic island biogeography theory. This relationship was strongest when fragmentation was measured within ~10 m of the center of plot; the relationship was weaker at larger and smaller scales. This suggests that properties of the ‘patch’ of herbaceous vegetation surrounding each plot have the greatest effect on richness, rather than, for example, the number and size of nearby patches. We also show that common measures of fragmentation are not independent of the proportion of the landscape that is habitat. We used some simple mathematical approaches to estimate the effects of fragmentation per se, that is, the effect of fragmentation above and beyond the effect of the amount of habitat in the landscape, for several measures of fragmentation.