Conversations with James Estes: Keystone species interactions and The Serengeti Rules

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There’s a delicate balance that separates order from chaos. Things that seem permanent and unshakable prove to be transient and fragile. In nature, ecosystems are held in a state of precarious equilibrium called keystone species, or keystones. These organisms hold entire food webs together, very often as top predators but not always. Scientists have learned that the removal of these keystones from a given ecosystem causes vibrant landscapes to transform from fertile to barren. This understanding has provided conservationists with a strategy to reverse the degradation of ecosystems, whether it be natural or man-made.

On October 9th, a documentary will be premiering on PBS Nature called The Serengeti Rules. The film features several scientists whose discoveries have contributed to the understanding of keystone species and have hypothesized rules for maintaining a sustainable habitat.

James Estes, one of the researchers involved with the project, set aside time to discuss the role of keystones in an ecosystem.

SCIENTIFIC INQUIRER: Just for some background, can you define what an ecosystem is and the features a well-balanced ecosystem possesses?

JAMES ESTES: An ecosystem, as conceived by Sir Arthur Tansley, is an assembly of interacting species and the physical elements of their environment. Wikipedia defines an ecosystem as “An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system.”

SI: The notion of a keystone species is a central concept in The Serengeti Rules. What is a keystone species and what role do they play?

JE: Keystone species are strong interactors (meaning that they have strong influences on the distribution and abundance of other species) and are also comparatively rare in nature.

SI: Was there surprise when it became evident that a keystone species doesn’t have to be a top predator or a carnivore?

JE: I wouldn’t say it was a surprise to discover that a keystone needn’t be a carnivore. But most keystones are in fact high trophic level predators.

SI: Can something as small as a microbe be a keystone or do organisms at the microscopic level have their own ecosystem isolated from the macro?

JE: Microbes can. Be keystones – species that cause disease, for example. Normally, to do this they must become abundant enough to make a large number of other animals or plants sick but in the beginning they are commonly rare. Microbes are fundamentally important elements of all ecosystems.

SI: What happens to an ecosystem experiencing downgrading?

JE: A downgraded ecosystem is one that has lost it’s high level keystone carnivores. There is no exact definition of what happens, other than that the distribution of may if not most of the associated species change substantially.

SI: What happens when an invasive species is introduced into a system?

JE: If they are strong interactors (not all invasive species are), they strongly influence the distribution and abundance of other species. There is no exact prescription for how this occurs. It depends on the nature of their interactions with other species. So an invasive might cause the natural top predator to declines, thus leading to increases in that predator’s prey and decreases in that they are prey’s prey. There are many other pathways by which such influences may occur.

SI: Are humans a keystone species? Or are we an invasive species?

JE: Humans have been referred to as hyper-keystones. Like all wide ranging species, humans are invasive in the sense that they have spread from a smaller center of origin (Africa, depending on how far back you want to look at the evolution of life.

SI: The Serengeti Rules closes with efforts to counteract downgrading. How effective have efforts at upgrading been?

JE: There have been some successes, such as the recovery of sea otters and the repatriation of wolves to the United States. But the world in general remains highly downgraded.

SI: Can an artificial keystone be created or mimicked?

JE: Probably. In general human efforts to do this directly (by culling deer to restore the influences of wolves or by harvesting sea urchins to mimic the influence of sea otters) have not been successful. But there have also been proposals for what has been referred to as Pleistocene rewinding — the use of ecological surrogates to repatriate the functional roles of extinct species. For example, there was a North American lion, now extinct that was similar to but somewhat larger than the modern African lion. African lions might thus be used to replace the lost function of North American lions.

SI: What is the biggest question when it comes research into keystone species?

JE: There are lots of details. But in my opinion the biggest outstanding question is what kind of future we humans with to live in.

The Serengeti Rules airs on October 9th on PBS.


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