Top-down and bottom-up forces in mammalian herbivore - vegetation systems: an essay review

For almost 50 years ecologists have debated why herbivores generally don't increase in numbers to such levels as to deplete or devastate vegetation. One hypothesis is that herbivore populations are regulated at low densities by predators, and a second hypotheses is that plants are fundamentally poor food for herbivores. This has lead to two main hypotheses about the role of herbivores in structuring vegetation: the bottom-up'' and top-down'' hypotheses. Here I survey the literature, with a focus on field experiments designed to investigate the soil resource-vegetation-mammalian herbivore system, specifically asking five questions about how each trophic level responds to (i) resource addition, (ii) vegetation removal, (iii) herbivore removal or reduction, (iv) herbivore addition, and (v) the interaction of resource levels and herbivory? I use these to develop 12 testable predictions. I document the major areas of research as they relate to these 12 predictions, and use these to evaluate weaknesses and limitations in field methods. There are surprisingly few terrestrial studies that conduct factorial manipulations of multiple nutrients or herbivores, even though it is clear that these are essential. Specifically, I argue that a manipulative experimental approach is the most valuable way to advance our theory and understanding, and I advocate the continued use of long-term factorial field experiments that simultaneously manipulate soil resources levels and herbivory (preferably at multiple levels), repeated in a range of environments in which individual species or functional groups are monitored.