Relationships between plant species richness and grazing intensity in a semiarid ecosystem

Plant species richness is an important property of ecosystems that is
altered by grazing. In a semiarid environment, we tested the hypotheses
that (1) small-scale herbaceous plant species richness declines linearly
with increasing grazing intensity by large ungulates, (2) precipitation
and percent sand interact with grazing intensity, and (3) response of
herbaceous plant species richness to increasing intensity of ungulate
grazing varies with patch productivity. During January to March 2012, we
randomly allocated 50, 1.5-m x 1.5-m grazing exclosures within each of six
2,500 ha study sites across South Texas, USA. We counted the number of
herbaceous plant species and harvested vegetation in 0.25-m2 plots within
exclosures (ungrazed control plots) and in the grazed area outside the
exclosures (grazed treatment plots) during October and November 2012–2019.
We estimated percent use (grazing intensity) based on the difference in
herbaceous plant standing crop between control plots and treatment plots.
We selected the negative binomial regression model that best explained the
relationship between grazing intensity and herbaceous plant species
richness using the Schwarz Bayesian Information Criterion. After
accounting for the positive effect of precipitation and percent sand on
herbaceous plant species richness, species richness/0.25 m2 increased
slightly from 0 to ~ 30% grazing intensity and then declined with
increasing grazing intensity. Linear and quadratic responses of herbaceous
plant species richness to increasing grazing intensity were greater for
the least productive patches (<15.7 g/0.25 m2) than for productive
patches (≥15.7 g/0.25 m2). Our results followed the pattern predicted by
the intermediate disturbance hypothesis model for the effect of grazing
intensity on small-scale herbaceous plant species richness.