DOI: 10.5061/dryad.2bvq83bww
title: Relationships between plant species richness and grazing intensity in a semiarid ecosystem

description: 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 (&lt;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.
description: We sampled vegetation in the study sites during October and
 November (autumn) 2012-2019 inside and outside 1.5 m x 1.5 m grazing
 exclosures. Fifty grazing exclosures were randomly allocated with each of
 six 2,500 ha study sites during January - March 2012. Grazing exclosures
 were rerandomized each year after vegetation sampling. Vegetation sampling
 consisted of counting the number of herbaceous plant species with a 0.25
 m<sup>2</sup> sampling frame in the exclosure and in the
 grazed area outside the exclosure and then harvesting herbaceous
 vegetation inside and outside the exclosures. Harvested plant material was
 dried at 45 C to a constant mass and weighed. Data were entered in an
 excel spread sheet and statistical analyses were performed using
 Statistical Analysis Systems (SAS) software.
description: # Relationships between plant species richness and grazing intensity in a
 semiarid ecosystem
 [https://doi.org/10.5061/dryad.2bvq83bww](https://doi.org/10.5061/dryad.2bvq83bww) Description of data: Column 1 in the data set identifies the geographic location (site) where data were collected and the exclosure number. SAV is the San Antonio Viejo Ranch. There were three 2,500 ha sites on the ranch designated SAV1, SAV2, and SAV3. BV is Buena Vista Ranch, SR is Santa Rosa Ranch, and EELS is East El Sauz Ranch. Column 2 provides a unique identifier number for each pair of an exclosure and grazed plot. Column 3 is the year of data collection (data were collected in October/November of the designated year). Column 4 signifies that data were collected in autumn of the designated year. Column 5 is the standing crop of herbaceous vegetation in the exclosure in grams per quarter meter squared and, Column 6 is the standing crop of herbaceous vegetation in the grazed plot in the same units of measure. Column 7 is the number of herbaceous plant species per quarter meter squared in the exclosure and, Column 8 is the number of herbaceous plant species per quarter meter squared in the grazed plot. Columns 9 and 10 provide the rainfall at the location of the exclosure in August and September, respectively, before vegetation sampling was conducted. Column 11 is the percentage of sand in the soil at each exclosure based on USDA National Resource Conservation Service data.