VERTEBRATE HERBIVORES AND NORTHERN PLANT-COMMUNITIES - RECIPROCAL INFLUENCES AND RESPONSES

Vertebrate herbivores in northern latitudes are generalists in their selection of forage species. Their foraging behaviour is influenced by the availability of vegetation of high nutritional quality, abiotic disturbance events, the effect of plant species on nutrient cycling, and the degree to which herbivores can forage on a diversity of plant species. In resource-limited, early successional environments their foraging activities may accelerate or delay plant succession, depending on the type of plant-animal interaction. They may increase the turnover rate of nutrients, particularly nitrogen, thereby sustaining net primary production of preferred forage plants. Alternatively, the movement of animals from locality to locality at the landscape level allows them to exploit available nutrient pulses in space and time in environments where high turnover rates cannot be sustained. The long-term effects of herbivory on the vegetation in the latter case appear to be minimal. There is also a group of plants which are unsuitable as forage because of the presence of secondary compounds. The low digestion efficiency when these plants are eaten correlates well with the recalcitrant nature of the tissues to microbial decomposition, the slow turnover of nutrients in the soil and the slow growth rates of the plants. Overall, herbivores do not appear to influence plant species assemblages in the different communities in the long term of alter the trajectory of plant succession at the landscape level, but they may have considerable short-term effects on vegetation, particularly at fine spatial scales (1-20 m). Exceptions to these generalizations are primarly at the local level and involve the top-down influence of human agencies in altering herbivore numbers, thereby creating trophic cascades and leading to the destruction of plant communities.