Jefferies et al. state that there is little evidence that across northern ecosystems herbivores do influence plant species assemblages in the different communities in the long-term, or alter the trajectory of plant succession. However, they may have considerable influence on vegetational composition of communities in the short-term. Herbivores may slow the rate of successional processes, increase the rate of plant succession, or in some cases, create a vegetational mosaic. The exceptions to these generalizations are primarily at the local, rather than at the landscape level, and invariably involve the top-down influence of human agencies that have altered herbivore numbers. Thus, creating trophic cascades and instability in both plant communities and soil conditions. At larger scales of space and time other types of disturbances, such as fire, wind storms, snow, and ice action, have a marked influence on both rate and trajectory of plant succession, largely as a result of direct and indirect effects on the frequency and intensity of nutritional pulses. These disturbances are necessary triggers for subsequent successional changes in vegetation, which, in turn, attract intense foraging from herbivores seeking high nutritional forage quality. Jefferies et al. conclude that overall, herbivores do not appear to influence plant species assemblages in the different communities in the long term or alter the trajectory of plant succession at the landscape level, but they may have considerable short-term effects on vegetation, particularly at the fine spatial scales.
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