Animal Studies Bibliography

Estes, James A., et al. 2011. Trophic Downgrading of Planet Earth. Science 333, 301-306.

(Summarized by Cameron T. Whitley, Animal Studies Program, Michigan State University)

It is well understood that the history of Earth has been marked by several mass extinctions, which have altered the presence and course of life. Currently, “our planet is in the early to middle stages of a sixth mass extinction” caused largely by Homo sapiens (p. 301). The current (6th) mass extinction has been characterized by the loss of apex consumers. “Recent research suggests that the disappearance of these animals reverberates further than previously anticipated with far-reaching effects on processes as diverse as the dynamics of disease; fire; carbon sequestration; invasive species; and biogeochemical exchanges among Earth’s soil, water, and atmosphere.

There is a long history in ecological theory that suggests that apex consumers are essential to the functioning of ecosystems. For instance, the idea of “trophic cascades,” suggests that apex consumers shape the ecosystem through impacts that filter down through the ecosystem much like a ‘cascade.’ Additionally, the idea of “alternative stable states” suggests that ecosystems are often nonlinear, especially when concerning tipping points or thresholds. The third key concept is the idea of “connectivity, ” which “holds that ecosystems are built around interaction webs within which every species potentially can influence many other species” (p. 301). The authors draw on these three key concepts to lay the foundation for trophic downgrading. Trophic downgrading occurs when the loss of apex consumers “reduces the food chain length, thus altering the intensity of herbivory and the abundance and composition of plants in largely predictable ways” (p. 302).

Because of the stochastic nature of the environment, the impact of apex consumer loss may take decades to recognize, and when it is recognized will most likely be irreversible. What is well established is that trophic cascades have been documented in all of the world’s major biomes. Given that top-down forcing and trophic cascades have a documented influence on the abundance of autotrophs and species it is not surprising that the reduction of apex consumers would also have devastating impacts that may lead to alternative ecosystem states. The relative impact of apex consumers on autotrophs is less well understood, as there is extreme variability in the environment. As the authors state, “in some cases, the influence of apex consumers is to suppress herbivory and to increase the abundance and production of autotrophs… Apex consumers in other systems reduce the abundance and production of autotrophs” (p. 303). The authors continue the discussion by addressing the known and suspected indirect effects of apex consumer loss.

For instance, wildfires are often attributed to warming and drying, however, “the global distribution and biomass of vegetation are poorly predicted by temperature and rainfall, and recent analyses suggest that interdependencies among predation (including disease), herbivory, plant communities, and fire may better explain the dynamics of vegetation” (p. 304). Second, disease is often “attributed to climate change, eutrophication, and habitat deterioration” (p. 304), but numerous studies have shown an interplay between predation and disease. Third, the authors refer to physical and chemical influences where, for example, the loss of apex consumers changes phytoplankton density, which can affect “the uptake of CO2 and the direction of carbon flux between lakes and the atmosphere” (p. 304). The forth effect of apex consumer loss, mentioned by the authors is the presence of invasive species. As is commonly known, invasive species thrive when they are removed from a top-down environment. As a result, “the loss of native predators leaves ecosystems more vulnerable to invasion by nonnative species” (p. 304). The final effect of apex consumer loss noted by the authors is in the loss of biodiversity. This effect has been intensified because confinement of species to managed or protected locations has not been as successful as intended.

The authors note that, “many practicing ecologists still view large animals in general, and apex consumers in particular, as ecological passengers riding atop the trophic pyramid but having little impact on the structure below” (p. 306). A standard practice in research has been to document the influences of apex consumers, a practice that requires all individual species to be carefully studied. Routinely, these studies lead to the same conclusion, that apex consumers matter. Instead of attempting to prove the necessity of each apex consumer species to the wellbeing of his or her ecosystem, the burden of proof should be inverted, so that researches assume that apex consumers (as a collective) have far reaching influences and that on rare occasions, if any exist, that “consumers do (or did) not exert strong cascading effects” (p. 306).


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