Introduction

Grazing by livestock, or any other herbivore, has the potential to impact biodiversity. Biodiversity is simply defined as the great variety of life-forms present. The term can be scale-dependent, referring to a backyard, a landscape, a continent, or the planet. Scientists generally recognize two scales, community (alpha diversity) and landscape (beta diversity). There are three types of biodiversity: habitat diversity, genetic diversity, and species diversity. Livestock grazing has the potential to affect all three (either positively or negatively), but can have the most direct effect on habitat diversity. All herbivores consume selected plants and plant parts from their environment. This selective defoliation through the removal of photosynthetic or reproductive tissues impedes the grazed plant's ability to compete with other ungrazed plants in the community. Grazing animals are important agents of environmental change, acting to create spatial heterogeneity, accelerate successional processes, and control switching between alternative states. Chronic herbivory can change composition, structure, and production of plant communities (habitat). With a decline in habitat diversity a concomitant decline in species diversity can be expected; some species simply have no place to live. If populations of individual species fail to persist because of declining habitat diversity, then genetic diversity is reduced. However, one must consider that some changes in plant structure, composition, and production may in fact improve or have no effect on species diversity because new habitats are created. Some species may decline, but others will increase. Such variables as season of use, intensity of defoliation, frequency of defoliation, and forage selectivity separately and together impact the grazed plant community. Knowledge of these variables can be of use to managers designing grazing systems to minimize the impacts on biodiversity.

occur in the aftermath of the disturbance. Typically, if the disturbance is severe enough to destroy most plants in the community, the first recolonizing plant communities are simplistic and over time increase in complexity. This progression of plant communities is termed succession. Disturbance does not usually destroy or uniformly modify a given landscape. Therefore, at any one time there are varying communities that exist at different time intervals from the last disturbance event. These different stages of succession provide habitat diversity across the landscape and provide the potential for species diversity. As species occupy various habitats that may be somewhat dissimilar, over time their genes are altered through selection to adapt to the different habitats, providing genetic diversity.

Humans have drastically altered landscapes through their management. Livestock grazing certainly has been a major force of change, both in the past and today, particularly in parts of the world where education in land management has not occurred. Livestock grazing in the western United States in the late nineteenth and early twentieth centuries devastated ecosystems. Damages still persist. Unmanaged livestock grazing can lead to a dominance of unpalatable, chemically defended plants, some of which may be invasive. Other management practices have also greatly influenced landscapes; fire suppression in forests and rangelands, alteration of stream channels for irrigation, and logging forests are a few examples. To assess the influence of livestock grazing on biodiversity, one must also take into consideration human actions on the landscape. Wild herbivores provide yet another confounding influence. In the western United States deer and elk are significant forces of influence on plant community structure and composition; in the East, it is primarily white-tailed deer. These other confounding influences may, in some cases, have greater impacts than livestock grazing.

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