Does Long-term Grazing by Pack Stock in Subalpine Wet Meadows Result in Lasting Effects on Arthropod Assemblages?

Pack stock are often used in mountain environments and are grazed in uplands and wetlands, particularly subalpine wet meadows. Effects of pack stock on wetlandinvertebrates are unknown. Sequoia National Park, (Sierra Nevada, USA), was an ideal location for the study of lasting stock impacts on fauna, because a) there was an 18-year database of stock usage, b) there were meadows with little grazing that could be contrasted with grazed meadows,c) there is a long winter with no stock use, and d) the start of grazing for each meadow is controlled, so we could sample after greenup but just before stock arrived. Wecould thus address persistent conditions produced by many years of stock use in isolation from any potential short termimpacts. We sampled terrestrial arthropods in paired “grazed” and “ungrazed” meadows across the Park andcollected associated vegetation data. We found some negative effects of grazing on vegetation structure, but few lasting negative or positive effects of long-term stock grazing on arthropods in these wetlands. Although it appears that pack stock do not cause lasting damage to thisarthropod assemblage, the extent of impact at the height of the grazing season remains unknown

Does Long-term Grazing by Pack Stock in Subalpine Wet Meadows Result in Lasting Effects on Arthropod Assemblages?

Pack stock are often used in mountain environments and are grazed in uplands and wetlands, particularly subalpine wet meadows. Effects of pack stock on wetlandinvertebrates are unknown. Sequoia National Park, (Sierra Nevada, USA), was an ideal location for the study of lasting stock impacts on fauna, because a) there was an 18-year database of stock usage, b) there were meadows with little grazing that could be contrasted with grazed meadows,c) there is a long winter with no stock use, and d) the start of grazing for each meadow is controlled, so we could sample after greenup but just before stock arrived. Wecould thus address persistent conditions produced by many years of stock use in isolation from any potential short termimpacts. We sampled terrestrial arthropods in paired “grazed” and “ungrazed” meadows across the Park andcollected associated vegetation data. We found some negative effects of grazing on vegetation structure, but few lasting negative or positive effects of long-term stock grazing on arthropods in these wetlands. Although it appears that pack stock do not cause lasting damage to thisarthropod assemblage, the extent of impact at the height of the grazing season remains unknown

Effects of a Long-Term Disturbance on Arthropods and Vegetation in Subalpine Wetlands: Manifestations of Pack Stock Grazing in Early versus Mid-Season

<div><p>Conclusions regarding disturbance effects in high elevation or high latitude ecosystems based solely on infrequent, long-term sampling may be misleading, because the long winters may erase severe, short-term impacts at the height of the abbreviated growing season. We separated a) long-term effects of pack stock grazing, manifested in early season prior to stock arrival, from b) additional pack stock grazing effects that might become apparent during annual stock grazing, by use of paired grazed and control wet meadows that we sampled at the beginning and end of subalpine growing seasons. Control meadows had been closed to grazing for at least two decades, and meadow pairs were distributed across Sequoia National Park, California, USA. The study was thus effectively a landscape-scale, long-term manipulation of wetland grazing. We sampled arthropods at these remote sites and collected data on associated vegetation structure. Litter cover and depth, percent bare ground, and soil strength had negative responses to grazing. In contrast, fauna showed little response to grazing, and there were overall negative effects for only three arthropod families. Mid-season and long-term results were generally congruent, and the only indications of lower faunal diversity on mid-season grazed wetlands were trends of lower abundance across morphospecies and lower diversity for canopy fauna across assemblage metrics. Treatment x Season interactions almost absent. Thus impacts on vegetation structure only minimally cascaded into the arthropod assemblage and were not greatly intensified during the annual growing season. Differences between years, which were likely a response to divergent snowfall patterns, were more important than differences between early and mid-season. Reliance on either vegetation or faunal metrics exclusively would have yielded different conclusions; using both flora and fauna served to provide a more integrative view of ecosystem response.</p> </div

Means (SEs) for predators, herbivores, and more- and less-motile taxa by grazing treatment and season.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054109#pone-0054109-t002" target="_blank">Table 2</a> for test results.</p

Means (standard errors) for faunal assemblage variables as a function of Treatment (Control, Grazed), Season (Early, Mid), and Year (2010, 2011) and results of 2×2×2 blocked ANCOVAs with elevation as a covariate.

<p>All metrics were based on 50-sweep samples, with the exception of ant species richness, which was the result of one aggregate hour of bait deployment using one honey and one tuna bait.</p>a<p>Treatment.</p>b<p>Season.</p>c<p>Year.</p>d<p>Block.</p>e<p>2010.</p>f<p>2011.</p>*<p>p<0.05 before sequential Bonferroni correction.</p>**<p>p<0.05 after sequential Bonferroni correction.</p

Means (SEs) for families, morphospecies, and expected number of morphospecies by grazing treatment and season.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054109#pone-0054109-t002" target="_blank">Table 2</a> for test results.</p

Means (SEs) for percent cover components as a function of grazing treatment and season.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054109#pone-0054109-t001" target="_blank">Table 1</a> for test results.</p

Means (standard errors) for vegetation and physical parameters as a function of Treatment (Control, Grazed), Season (Early, Mid), and Year (2010, 2011) and results of 2×2×2 blocked ANCOVAs with elevation as a covariate.

a<p>Treatment.</p>b<p>Season.</p>c<p>Year.</p>d<p>Block.</p>e<p>2010.</p>f<p>2011.</p>*<p>p<0.05 before sequential Bonferroni correction.</p>**<p>p<0.05 after sequential Bonferroni correction.</p