Potential environmental effects of pack stock on meadow ecosystems of the Sierra Nevada, USA

Abstract

Pack and saddle stock, including, but not limited to domesticated horses, mules, and burros, are used to support commercial, private and administrative activities in the Sierra Nevada. The use of pack stock has become a contentious and litigious issue for land management agencies in the region inter alia due to concerns over effects on the environment. The potential environmental effects of pack stock on Sierra Nevada meadow ecosystems are reviewed and it is concluded that the use of pack stock has the potential to influence the following: (1) water nutrient dynamics, sedimentation, temperature, and microbial pathogen content; (2) soil chemistry, nutrient cycling, soil compaction and hydrology; (3) plant individuals, populations and community dynamics, non-native invasive species, and encroachment of woody species; and (4) wildlife individuals, populations and communities. It is considered from currently available information that management objectives of pack stock should include the following: minimise bare ground, maximise plant cover, maintain species composition of native plants, minimise trampling, especially on wet soils and stream banks, and minimise direct urination and defecation by pack stock into water. However, incomplete documentation of patterns of pack stock use and limited past research limits current understanding of the effects of pack stock, especially their effects on water, soils and wildlife. To improve management of pack stock in this region, research is needed on linking measurable monitoring variables (e.g. plant cover) with environmental relevancy (e.g. soil erosion processes, wildlife habitat use), and identifying specific environmental thresholds of degradation along gradients of pack stock use in Sierra Nevada meadows.

Stream Macroinvertebrates and Habitat Below and Above Two Wilderness Fords Used by Mules, Horses, and Hikers in Yosemite National Park

“…Although hikers create fewer effects on a perindividual basis, there are more hikers than stock along trails (Törn et al 2009). Humans and equines (Cole 1995, Cole et al 2004, Holmquist et al 2014, Ostoja et al 2014) have generally, though not exclusively, negative effects on wetland vegetation and fauna. Similarly, there may be negative effects on stream macrofauna immediately downstream of fords, but it is also possible that limited increases in particulate organics, periphyton, and habitat heterogeneity could enhance diversity in these oligotrophic streams without negatively affecting sensitive taxa (Braccia and Voshell 2007).…”

Environmental and managerial factors associated with pack stock distribution in high elevation meadows: Case study from Yosemite National Park

“…Stock use, which can include horses, mules, donkeys, yaks, or llamas, is one example of a visitor-related activity that can affect the ecological integrity of preserved ecosystems Byers, 2009;Cole et al, 2004;Geneletti and Dawa, 2009). Impacts attributed to stock presence in alpine and mountainous landscapes include soil compaction, altering of hydrologic patterns, increases in erosion, and trampling (Barros et al, 2014;Byers, 2009;Kuhn et al, 2015;Ostoja et al, 2014). Research demonstrates horse trampling occurs at greater intensities than that of hikers (Weaver and Dale, 1978), and, in some cases, other hooved animals (Cole and Spildie, 1998).…”

“…Overgrazing can decrease vegetation cover and meadow productivity, shift plant species composition through selective grazing of palatable species, change local soil chemistry and micro-climate conditions, increase bare ground cover, and facilitate the encroachment of invasive or woody species (Cole et al, 2004;Kuhn et al, 2015;McClaran and Cole, 1993;Moore et al, 2000;Olson-Rutz et al, 1996a, 1996bOstoja et al, 2014). Mitigating impacts from stock in PAs, while providing access for stock users, relies on understanding the interaction between stock and the environment (Moore et al, 2000).…”

“…Research on the resistance and resiliency of grass, forb, and shrub-dominated habitats indicates proactive management is essential in limiting the propagation and proliferation of negative impacts (Cole et al, 2004;McClaran and Cole, 1993;Olson-Rutz et al, 1996b). Studies of grazing impacts in PAs are often based on inventories made before and after an animal has left the area, employing plots with varying levels of grazing treatments to measure ecosystem response, or observational studies of impacts to a variety of ecosystem components (Alzerreca et al, 2006;Cole et al, 2004;Holmquist et al , 2014;Kuhn et al, 2015;Ostoja et al, 2014).…”

“…How grazing intensity and environmental variables vary, however, has been identified as a research need within recreation ecology (Monz et al, 2010). Specifically, researchers and managers highlight a need to understand patterns of stock distribution and use during potential disturbance and how it is associated with environmental (e.g., vegetation composition, access to water) and managerial (e.g., fencing, stock release locations) factors, including overnight releases when direct supervision is less feasible (McClaran, 1989;Ostoja et al, 2014). Studies of stock resource use at the site level offer an opportunity to combine advances in data collection instruments, computing capacity, and statistical modeling methods from several disciplines interested in use-environment interactions to address this research need (Brown et al, 2013;Gaylord and Sanchez, 2014;Jeltsch et al, 2013;Kays et al, 2015;Meijles et al, 2014;Rutter, 2007).…”