Current research about the cow as a geomorphic agent: An update to Trimble and Mendel

In 1995, Stanley Trimble and Alexandra Mendel published a review article focusing in the role cattle play in influencing geomorphic processes, the cow as a geomorphic agent – a critical review. Trimble and Mendel looked at the voluminous, largely unconnected, body of literature on the various means by which cattle affect landscapes. One of the major goals of Trimble and Mendel’s work was to put forth a research agenda that they hoped would influence future studies on geomorphic impacts of cattle. During the decade and a half following, the cow as a geomorphic agent, Trimble and Mendel’s article has been cited hundreds of times by researchers from a wide variety of fields, including geomorphologists. The purpose of this paper will be to review the latest research on the geomorphic impacts of cattle. It will focus on the methodologies employed in the various studies and provide some discussion of the geomorphic concepts that have been analyzed in relation to cattle’s impacts on geomorphic processes. The grazing of animals on landscapes can have serious impacts on geomorphic processes. According to USDA statistics, the current estimated population of cattle in the United States is 970 million (USDA, 2007). In Australia, it is estimated that grazing operations utilize over 60% of the land surface (Jansen ET AL., 2001). While there has been considerable debate in recent years about the possible benefits of light to moderate grazing on landscapes, the negative impacts associated with overgrazing are undeniable. According to Trimble and Mendel, cattle can directly reshape the surface of the Earth (Trimble and Mendel, 1995). In order to establish some basic background information about the role cattle play in geomorphology, a brief summary of Trimble and Mendel’s paper will be our starting point. According to Trimble and Mendel, the most damaging effects of cattle grazing take place in two distinct areas of the landscape: upland slopes and riparian zones. On upland slopes, cattle alter the land through sheer force. As cows move upslope, they exert an incredible amount of force on the soil. One study Trimble and Mendel reviewed estimated that a cow walking on a level surface exerted upwards of 250kPa of vertical force (see Scholefied and Hall, 1986 in Trimble and Mendle, 1995). The physical manifestations of this force can result in increased soil erosion, reduced infiltration rates, and changes to the bulk density of soil. Changes to the characteristics of the soil on upland slopes have dramatic consequences in downstream areas, as increases in both sediment levels and stream power often occur. The negative effects of cattle grazing increase when the soil is wet. In fact, Trimble and Mendel identify the amount of soil moisture present as a key variable for predicting the impact of cattle on a landscape. Given this relationship between soil moisture and land degradation from cattle grazing, it makes sense that riparian areas would be especially susceptible to damage from cattle. Figure 1. An example of soil erosion on upland slopes as a result of cattle grazing. http://www.calwalkers.com/2003/42003k.jpg Cattle can cause extensive damage to riparian zones. Once again, it is the movements of cattle that often cause the damage. Stream banks are particularly susceptible to increased erosion because of cattle grazing (see Fig 2). According to Trimble and Mendel (1995), cattle prefer riparian zones to other parts of the landscape. Cattle’s preference for riparian zones is due in part to the easy access to water sources and the high level of vegetative material often available. Another factor is that the cattle favored by ranchers in the western world originated in the cool wet climates of Northern Europe. Therefore, in the much hotter and drier semi-arid zones of the western United States, cattle are forced seek out riparian zones in order to find shade and an escape from the heat. Cattle’s preference for riparian zones provides the designers of management programs with significant challenges. Fig 2. The results of cattle grazing in a riparian area. www.buffer.forestry.iastate.edu/.../erosion.htm One of Trimble and Mendel’s main goals for writing the cow as a geomorphic agent was to encourage future research on the role cattle play in geomorphic processes. In the conclusion of their article, Trimble and Mendel, provide some suggestions for researchers to consider in order to ensure that future projects focus on the geomorphic implications of cattle grazing. They encourage researchers to place their studies “within the hydroclimatological, edaphic and geomorphological dimensions of the areas being studied so that controlling variables may be more readily isolated” (Trimble and Mendel, 1995, pg. 232). Being able to identify the key variables related to the impacts of cattle grazing is crucial for the design of successful predictive modeling. Trimble and Mendel also call for more projects that look at the impacts of cattle grazing in specific regions of the world in order to contrast the ways in which environmental factors influence grazing