THE EFFECTS OF HERBIVORY AND GRANIVORY ON TERRESTRIAL PLANT SUCCESSION

Based on resource availability models of plant defense investment, several authors have predicted and documented that herbivores tend to accelerate the pace of plant community succession. A literature survey of experimental and observational tests of the effects of above-ground, native herbivores on terrestrial plant succession reveals a more complex pattern than anticipated from earlier reviews on this subject. Although herbivory tends to hasten succession from grasses, shrubs, and pioneer trees to persistent trees (often including conifers), it typically retards succession from earlier seres, if such seres are present (e.g., as in secondary succession of old fields). The dichotomy in herbivore effects early and late in old field succession reflects differentially high herbivory on intermediate seral species (again grasses, shrubs and pioneer trees), whose principal defense against herbivores is rapid, compensatory growth in comparatively favorable resource environments. In plant communities ranging from boreal forests to African savannas, both herbivore preferences, and their effects on community succession, may be predictable from the underlying plant resource regimes and plant defenses evolved in response to these regimes. Individually or as a ''functional group'', palatable species might be regarded as ''keystone resources'', whose fluctuating abundances are likely to influence communities of both producers and consumer species on a local and landscape scale. In contrast to herbivory, granivory is predicted and found to fall differentially on larger-seeded, later successional species, and to inhibit succession in a diversity of plant communities. In this way, granivores often behave as ''keystone predators''. At present, there are too few data to place confidence in the generality of this result. An understanding of patterns in the effects of consumers on plant community development may be useful for both managing natural ecosystems and restoring disturbed habitats.