Grazing by Large Herbivores

Grazing is an important process in shaping a landscape. Grazers have a quite complicated multiscalar approach according to the different positions of landscape, selecting plant communities, feeding stations (patches), plants, and plant parts. Grazing modifies the structure of grasslands. Sparse sward yields larger bites and leads to a more rapid depletion than short dense cover (Laca et al. 1994).

For instance, bison interact with the patch structure of grasslands at several spatial and temporal scales. Grazing reduces the standing biomass while vegetation diversity is increased as a consequence of the intermediate disturbance hypothesis.

Patchiness is often the result of different combinations of disturbances; for instance, fire+grazing+urine deposition. It is well documented that bison, like many other large grazers, have a preference for patches created by urine deposition. The urine deposition is a disturbance of small size («0.25 m2), but Steinauer and Collins (2001), using experimental control plots sprayed with bison urine, have found an increase in size and severity of grazing for patches with urine deposition. In conclusion, grazing is initiated in a urine patch that is very attractive to bison due to an increase in biomass productivity and quality (through N addition to soil nutrients) and this has consequences for the total grassland mosaic.

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