The logic and realism of the hypothesis of exploitation ecosystems

Hypotheses on trophic dynamics in terrestrial ecosystems fall into two majo r categories: those in which plants are assumed to be invulnerable to their consumers and those in which the build-up of plant biomass is assumed to r equire top-down control of folivores. The hypothesis of exploitation ecosys tems (EEH) belongs to the latter category and focuses particularly on the c onsequences of the high energetic costs of maintenance of endotherms. Carni vorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting s eason, at least at high latitudes. The endotherm branch of the grazing web is thus predicted to collapse From three-link trophic dynamics (carnivores --> folivores --> plants --> inorganic resources) to two-link dynamics (fol ivores --> plants --> inorganic resources) along gradients of decreasing pr imary productivity. Consequently, the vegetation of cold and unproductive a reas is predicted to be under intense winter grazing pressure, which preven ts the accumulation of aboveground plant biomass and excludes erect woody p lants. In the most extreme habitats (e.g., polar deserts and their high alp ine counterparts), even folivorous endotherms are predicted to be absent, a nd the scanty vegetation is predicted to be structured by preemptive compet ition. Within temperature-determined productivity gradients, EEH is corrobo rated by biomass patterns, by patterns in the structure and dynamics of car nivore, folivore, and plant communities, and by experimental results. The g eneral idea of top-down trophic dynamics is supported for other autotroph-b ased systems, too, but the relevance and sufficiency of the energy constrai nt in explaining patterns in trophic dynamics appears to be variable. Moreo ver, critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained. Another open q uestion is the ability of boreal and temperate browsers, evolved in product ive environments with intense predation pressure and abundance of forage, t o prevent the regeneration of the least palatable tree species. There are, thus, many open questions waiting to be answered and many exciting experime nts waiting to be conducted.