Loss of Top Animal Predators Has Massive Ecological Effects

“Trophic Downgrading of Planet Earth,” a review paper that will be published on July 15, 2011, in the journal Science, concludes that the decline of large predators and herbivores in all regions of the world is causing substantial changes to Earth’s terrestrial, freshwater, and marine ecosystems.

The paper claims that the loss of apex consumers from ecosystems “may be humankind’s most pervasive influence on nature.” The research was funded primarily by the Institute for Ocean Conservation Science at Stony Brook University with support from The Pew Charitable Trusts. The paper is co-authored by the Institute’s executive director, Dr. Ellen K. Pikitch, and the lead author is Dr. James A. Estes, professor of ecology and evolution at the University of California at Santa Cruz.

The review, conducted by an international team of 24 scientists, illuminates the patterns and far-reaching impacts of predation and herbivory on the structure and dynamics of global ecosystems. The researchers relied on both experimental and observational evidence, which provides a strong basis for their conclusions.

“By looking at ecosystems primarily from the bottom up, scientists and resource managers have been focusing on only half of a very complex equation,” said Dr. Estes. “These findings demonstrate that top consumers in the food web are enormous influencers of the structure, function, and biodiversity of most natural ecosystems.”

Apex consumers include animals such as big cats, wolves, bison, sharks, and great whales, and are typically large, long-lived, and not amenable to laboratory experiments. As a result, the effects of removing them from ecosystems are not easy to document. The team of scientists reviewed an accumulation of theoretical and empirical evidence on how the decline of top predators and herbivores has affected Earth’s ecosystems on land, in freshwater, and in the ocean. Their findings suggest that “trophic downgrading” – the ecological consequences of losing large apex consumers from nature – causes extensive cascading effects in ecosystems worldwide, especially when exacerbated by factors such as land use practices, climate changes, habitat loss, and pollution.

“Our review of existing studies clearly shows that a top-down cascading effect in natural systems is both powerful and widespread,” said Dr. Estes. “There is an urgent need for interdisciplinary research to forecast how a continued loss of top level consumers will further harm the planet’s ecosystems.”

This paper documents some of the negative effects that the widespread loss of these animals has already had on Earth’s biosphere, climate, biodiversity, and vegetation:
• The reduction of lions and leopards from areas of sub-Saharan Africa caused the baboon population to swell. This unexpectedly increased transmission of intestinal parasites from baboons to humans as the primates were forced to forage closer to human settlements.
• As large ungulates recovered from a devastating rinderpest epidemic in the Serengeti in Africa, herbivory increased, and the frequency of wildfire declined in that region. Wildfire frequency increased following the late Pleistocene/early Holocene decline of megaherbivores in Australia and the northeastern United States.

• Industrial whaling in the 20th century resulted in the loss of large numbers of plankton-consuming great whales, which are now known to sequester carbon into the deep sea through deposition of feces. The result has been the transfer of approximately 105 million tons of carbon into the atmosphere that would have been absorbed by whales, contributing to climate change.

“We must assume going forward that significant changes to the ecosystem are occurring when large predators and herbivores are removed from the top of the food web, and, thus, that efforts to manage and conserve nature must include these animals,” said Dr. Pikitch. “An old paradigm has shifted, and those who question this theory now have the burden to prove otherwise.”

Editor’s Notes: For a copy of “Trophic Downgrading of Planet Earth,” please contact the AAAS Office of Public Programs at 202-326-6440 or scipak@aaas.org.

Photos are available by request and can be downloaded from the Internet:
Aspen recovery: http://www.flickr.com/photos/oregonstateuniversity/5905908661/in/photostream

Wolf in Yellowstone: http://www.flickr.com/photos/oregonstateuniversity/5159133703/

The Institute for Ocean Conservation Science at Stony Brook University is dedicated to advancing ocean conservation through science. The Institute transforms real-world policy while pursuing serious science, both of which are essential for ocean health. For more information, go to www.oceanconservationscience.org.