Individual study: Effect of ungulate grazing on plant and insect communities in Ponderosa pine Pinus ponderosa grasslands on the Mogollon Rim, Arizona, USA
Rambo J.L. & Faeth S.H. (1999) Effect of vertebrate grazing on plant and insect community structure. Conservation Biology, 13, 1047 -1054
Grazing is a commonly used management tool in the conservation of natural and semi-natural grasslands. In this study, the effects of native deer and elk and domestic cattle grazing on plant and insect communities in semi-arid, Ponderosa pine Pinus ponderosa –grasslands on the Mogollon Rim, Arizona were compared. Insect abundance and diversity in long- and short-term grazing exclosures were compared to immediately adjacent areas that were grazed consistently and intensively by the ungulates. Plant species composition and abundances relative to growth forms (i.e. forbs vs. grasses vs. shrubs and trees), life histories (i.e. annual, biennial, perennial), and native and non-native species in grazed and ungrazed treatments, were also determined.
Grazing exclosures: In the summer of 1996, plant and insect richness and abundance were sampled in three meadows (at Houston Draw, Merritt Draw and Buck Springs) bordered by Ponderosa pines on the Mogollon Rim (an escarpment defining the southwestern edge of the Colorado Plateau, c.2,200 m altitude). At each site, a 4-m high, solar-powered electrified exclosure fence (each encompassing about 40 ha) had prevented ungulate grazing for the previous 8–9 years. Vegetation inside the exclosures was dense and approximately 1 m tall, whilst that outside was <10 cm tall.
Additionally, 1-year suppression of grazing on plant diversity at two nearby sites in 10 small (5 × 5 m) exclosures constructed in 1995, was assessed. These enclosures prevented grazing by cattle but also probably by elk and deer as they avoid jumping into small exclosures to feed; none showed evidence of ungulate grazing during the duration of the study. These small exclosures occupied an area partially covered by pine canopy. Cattle are rotationally grazed on all five sites each summer for periods of 4 to 30 days, depending on forage condition. Elk and deer forage year-round, except when prevented from doing so by snow.
Sampling: In June and July 1996, plants and insects were sampled along two parallel transects, one inside and outside of each long-term exclosure. Species richness and abundance of plants was sampled in 30 grazed and 30 ungrazed 1 m² quadrats (10 outside and 10 inside) placed at 10-m intervals along transects. Insects were sampled in July 1996 by sweep-netting along the transects. Insects were identified to at least family and thereafter were designated as morphotypes.
In July 1996 at the two short-term grazing exclosure sites, plant species richness and abundances was determined within the 10 exclosures and in equal-sized areas outside them.
Plants: Plant species richness was higher in two of three grazed grasslands, as compared to that in ungrazed areas inside the three large exclosures. Similarly, plant species richness was higher in grazed areas relative to ungrazed areas at one of the two series of smaller, short-term exclosure sites. However, evenness of plant distribution was greater inside the long-term exclosures but was reduced inside ungrazed short-term exclosures relative to grazed areas. Relative abundances of forbs, grasses, trees, and shrubs, and native and introduced plants did not differ between the long- and short-term exclosures compared to adjacent grazed areas. Relative abundances of some plant species changed when grazers were excluded.
Insects: Insect species richness was the same in exclosures and grazed areas. Insect abundance however, was 4 to 10 times higher in the ungrazed exclosure vegetation.
Conclusions: The results indicate that at the study sites investigated, whilst ungulate grazing may increase plant richness it equally may decrease insect abundance. In terms of conservation, grazing regimes should be aimed at conferring benefits to desired plant and insect communities, and individual species of high conservation concern.
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