At regional and local scales, there is ample evidence of complementarity with each species using foods, habitats and space in temporally distinctive ways. These 'ghosts' may even involve habitats and competitors that have long since disappeared. On the one hand, such ecological specialization could be considered a mechanism which allows greater species packing, higher animal densities and higher overall grazing systems productivity. On the other, this apparent complementarity may reflect intense functional interactions which may make these systems fragile to human manipulation. This ambiguity has only recently been appreciated and studies have finally progressed beyond static descriptions of patterns to the analysis of dynamic interactions.
Perturbations are essential to reveal underling processes.
Only by comparing pre-and post interactive niches and determining responses in population densities as well as resource use can we learn much about forces operating within these communities. Progress is frustrated by the logistic and sometimes ethical problems of conducting field experimentation. Consequently, most manipulative experiments have been conducted on livestock-wildlife interactions. One caution is that given the inherently dynamic interaction between plants and animals, apparent populations responses of one species to the fortunes of another may be misleading unless unraveled from a very long time-series. Functional interactions within herbivore guilds.