UC DAVIS (US) — The declining number of animals at the top of the food chain is creating a downward spiral throughout the world’s ecosystems, largely due to the one predator they are vulnerable to: humans.
A new study, published in the journal Science, concludes “the loss of apex consumers is arguably humankind’s most pervasive influence on the natural world.”
The “top down” ecological effects of losing these large animals have been underestimated compared to the “bottom up” effects of environmental changes, says Tom Schoener, professor of biology at the University of California, Davis.
“There are enormous implications for all aspects of ecology, from species diversity to effects on the air, water and soil, to the emergence of human diseases and the prevalence of wildfires,” Schoener says.
The decline of apex consumers has been most pronounced among the big predators, such as wolves and lions on land, killer whales and sharks in the oceans, and large fish in freshwater ecosystems. But dramatic declines have also occurred in populations of large herbivores, such as elephants and bison.
Apex consumers are difficult to study and not amenable to the laboratory experiments that have guided a lot of thinking in ecology.
But accumulating evidence from the field shows that the loss of apex consumers from an ecosystem triggers an ecological phenomenon known as a “trophic cascade,” a chain of effects moving down through lower levels of the food chain. Examples include:
- The destruction of wolves in Yellowstone National Park led to over-browsing of aspen and willows by elk, and restoration of wolves has allowed the vegetation to recover.
- The reduction of lions and leopards in parts of Africa has led to population outbreaks and changes in behavior of olive baboons, increasing their contact with people and causing higher rates of intestinal parasites in both people and baboons.
- A rinderpest virus epidemic decimated the populations of wildebeest and other ungulates (hooved animals) in the Serengeti, resulting in more woody vegetation and increased frequency and size of wildfires, before the virus was eradicated in the 1960s.
- Dramatic changes in coastal ecosystems have followed the collapse and recovery of sea otter populations; sea otters maintain coastal kelp forests by controlling populations of kelp-grazing sea urchins.
- The decimation of sharks in an estuarine ecosystem caused an outbreak of cow-nosed rays and the collapse of shellfish populations on which the rays feed.
Despite these and other well-known examples, the extent to which ecosystems are shaped by such interactions has not been widely appreciated.
“There’s been a tendency to see it as idiosyncratic and specific to particular species and ecosystems,” says lead author James Estes, professor of ecology and evolutionary biology at University of California, Santa Cruz.
For example, some ecologists have viewed trophic cascades as an issue largely for ocean systems, but the review discusses many examples from land.
Schoener has studied the impact of a small predator, although large in its own world—lizards on small islands. Removing the lizards from an island ecosystem can lead to increased damage to plants, because insects that eat the plants (and would normally be eaten by the lizards) multiply.
The review’s findings have profound implications for conservation.
“To the extent that conservation aims toward restoring functional ecosystems, the re-establishment of large animals and their ecological effects is fundamental,” Estes says.
“This has huge implications for the scale at which conservation can be done. You can’t restore large apex consumers on an acre of land. These animals roam over large areas, so it’s going to require large-scale approaches.”
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