Over the last century, woody plant encroachment has reduced the amount of herbaceous habitat in central Texas. We tested whether this habitat loss has reduced local herbaceous species richness. It has been suggested that the effects of habitat loss on diversity may not be observable for several decades, particularly in plant communities. On the other hand, these plant communities are adapted to a highly variable climate and to intermittent fire, so they may respond quickly to habitat loss. We measured the strength of the relationship between species richness and habitat amount in 4 years, which span a 28 year period, to determine which year best predicts present species richness.

In 2007, we measured species richness in 249 one-meter radius plots randomly distributed in herbaceous habitat patches at three sites on the eastern Edwards Plateau. We measured the amount of herbaceous habitat in a circle around each plot, using a series of classified (woody/herbaceous) aerial photographs from 1980, 1995, 2004, and 2008. We repeated the analyses using circles of 8 different sizes (10-100 m in radius).

Results/Conclusions

In this system the effects of herbaceous habitat amount on diversity are masked by the effects of the recent invasion of the Eurasian bunchgrass King Ranch Bluestem (Bothriochloa ischaemum) and by edaphic variation. After accounting for both of these factors, there was a significant positive relationship between habitat amount and species richness. This relationship did not differ among sites despite the many other differences among sites. The relationship with present species richness was strongest with the amount of habitat three years before, suggesting that this community responds rather quickly to habitat loss. The relationship was also strongest with the amount of habitat present within 30 m or less of the plot, suggesting that diversity is primarily affected by the size of the herbaceous patch itself.