Phase out summer grazing, say researchers

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The Sierra Nevada Mountain range serves as an important source of drinking water for the State of California. However, summer cattle grazing on federal lands affects the overall water quality yield from this essential watershed. The authors of an article in the Journal of Water and Health argue that high-altitude summer grazing must be phased out.

The greatest economic value of the Sierra Nevada Mountains is derived from the provision of 50% of California's fresh water for domestic use. Since 1900, California's population has increased from 1.5 million to over 36 million persons. Maintaining water quantity and quality is challenging due to the unique geographic features of the Sierra Nevada. Melting snow must pass through a fragile ecosystem prior to runoff into lowland reservoirs. Much of the watershed consists of surface or near-surface granite or metamorphic bedrock with little topsoil and therefore little buffering capacity. As a result, small amounts of environmental pollution may have a significant impact on aquatic life.

Cattle grazing has been a part of this landscape since the 1850s. However, the detrimental effect of cattle on alpine water quality was noted as far back as the 1880s, and cited as one of the reasons to establish Yosemite National Park in 1890. The authors of an article published in the Journal of Water and Health argue that long-term costs to society are high, in terms of both ecology and public health.

Over the past 150 years, deposition of rate-limiting substances such as phosphorus (P) and nitrogen (N) compounds has resulted in eutrophication of much of the Sierra Nevada, with increases in phytoplankton species and biomass. Cattle manure contains high amounts of both N and P compounds, and 100 head of cattle will collectively deposit 50kg of N and 25kg of P each day on the range (based on a mean animal weight of 400kg). Thus, fecal matter from cattle with N and P as well as other nutriments contributes to the eutrophication process. In addition, this has promoted conditions which increase bacteria, other microorganisms and the frequency of algal blooms.

In watersheds where cattle have grazed, 96% of surface water samples contained significant indicator levels of E. coli of 100 CFU/100 ml or more, placing these waters at high risk of harbouring the large variety of harmful microorganisms. In contrast, the California water board does not allow more than 2.2 CFU/100 ml of E. coli in water used to irrigate vegetable crops. Several studies from other areas of the US have demonstrated a high prevalence of coliforms in watersheds grazed by cattle. In addition, predicted increases in temperatures from climatic change will warm streams, creating more favourable conditions for growth of toxic algae and pathogenic microorganisms.

The ecological costs of grazing on public lands can be dramatic and include loss of diversity, lowering of population densities for a variety of taxa, disruption of nutrient recycling and succession and changes in the characteristics of terrestrial and aquatic habitats. However, despite discussions on the impact of cattle grazing in the Sierra Nevada, some ranchers have recently pressured the USDA Forest Service to expand cattle grazing tracts.

The authors instead propose limiting summer-time cattle grazing in the Sierra Nevada Mountains on public lands to lower elevations. Their proposal is based on collective research and observations on watershed geology, climate, precipitation, snowmelt, flora and fauna of the alpine regions of these mountains. By the end of a five-year phase-out period, summer cattle grazing should be restricted to areas below 1,500m elevation in the Central and Northern Sierra and 2,000m elevation in the Southern Sierra. Phase-out proposals should be adopted as soon as possible to ensure long-term protection for this crucial source of water for California.

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