The Logic and Realism of the Hypothesis of Exploitation Ecosystems

Abstract

Hypotheses on trophic dynamics in terrestrial ecosystems fall into two major 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 require top-down control of folivores. The hypothesis of exploitation ecosystems (EEH) belongs to the latter category and focuses particularly on the consequences of the high energetic costs of maintenance of endotherms. Carnivorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting season, 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 (folivores → plants → inorganic resources) along gradients of decreasing primary productivity. Consequently, the vegetation of cold and unproductive areas is predicted to be under intense winter grazing pressure, which prevents the accumulation of aboveground plant biomass and excludes erect woody plants. In the most extreme habitats (e.g., polar deserts and their high alpine counterparts), even folivorous endotherms are predicted to be absent, and the scanty vegetation is predicted to be structured by preemptive competition. Within temperature-determined productivity gradients, EEH is corroborated by biomass patterns, by patterns in the structure and dynamics of carnivore, folivore, and plant communities, and by experimental results. The general idea of top-down trophic dynamics is supported for other autotroph-based systems, too, but the relevance and sufficiency of the energy constraint in explaining patterns in trophic dynamics appears to be variable. Moreover, critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained. Another open question is the ability of boreal and temperate browsers, evolved in productive environments with intense predation pressure and abundance of forage, to prevent the regeneration of the least palatable tree species. There are, thus, many open questions waiting to be answered and many exciting experiments waiting to be conducted.

Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

“…The results of this study demonstrate that the biomass of organisms occupying lower trophic levels depends both on the number of trophic levels and the production of the system, which are in line with the predictions from prey-dependent models of both long-term (EEH) (Oksanen et al 1981;Oksanen and Oksanen 2000) and short-term population dynamics (Nisbet et al 1997). Consumer-controlled compartments of the algal community, i.e., the number of algal recruits, increased with algal production when three trophic levels were present in the food web, while gastropod grazer biomass increased with algal production only when the food web was reduced to two trophic levels with the removal of the larger predators.…”

“…However, the top-down control of food web dynamics depends on complex interactions between environmental conditions, resource supply, individual species traits, and food web complexity (Polis and Strong 1996;Polis 1999;Menge 2000;Oksanen and Oksanen 2000;Shurin et al 2002;Hopcraft et al 2010). These interactions indicate that predator effects on food web configuration are highly context dependent and that ecosystem-specific properties determine responses to exploitation of higher trophic levels (Strong 1992;Shurin et al 2002;Hopcraft et al 2010).…”

“…The hypothesis of exploitation ecosystems (EEH) predicts that community level trophic cascades interact strongly with the productivity of the ecosystem (Fretwell 1977;Oksanen et al 1981;Oksanen and Oksanen 2000). According to the EEH (Oksanen et al 1981), top-down control of herbivores and bottom-up control of primary producers should dominate in high-productivity areas, generating a positive relationship between productivity, primary producer biomass, and carnivores.…”

Exploring the diet of arctic wolves (Canislupusarctos) at their northern range limit

“…Since cold areas may be more affected by climate change than temperate regions (Gitay et al 2002), global warming is expected to cause significant alterations to Arctic environments (Chapin III and Körner 1996). Terrestrial Arctic ecosystems are, to a large extent, regulated by bottom-up processes linked to plant-herbivore interactions (Oksanen and Oksanen 2000). However, recent food-web modeling suggests that global warming could lead to a productivity induced shift in the body-size distribution of large vertebrate prey, which could result in an increased importance of top-down regulation of Arctic ecosystems through predation processes (Legagneux et al 2014).…”

Management of introduced mammals in New Zealand

“…However, few New Zealand pest control operations have been conducted and monitored in such a way as to inform (or be informed by) this theory (Choquenot & Parkes 2001). This lack is changing as managers recognise the scale of the problems they face and the need to be efficient , and as they start to consider how to manage multiple threats at different trophic levels (e.g., Oksanen & Oksanen 2000), or by using food web approaches (e.g., Pimm 1982), or to measure benefits at the ecosystem level (e.g., Wardleetal. 2001).…”