Keystone and Strongly Interactive Species, and Trophic Cascades

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Two primary goals of the conservation movement are to protect ecologically important lands and to protect threatened and endangered species. However, while the loss of species and of habitat are the driving forces behind the biodiversity crisis, the loss of certain types of species interactions is a serious problem for those concerned with maintaining ecological processes, resilience and diversity.

In this article we will summarize a few of the most important types of species interactions. It will help the reader understand why Wildlands Network’s conservation strategy is built on a three-legged stool: protection of core habitats, corridors between them, and protection of carnivores (and other animals that fulfill roles outlined below.)

The simplest way to describe ecology is that everything in the natural world is connected to everyone else, with varying degrees of strength of connection. Species – both plant and animal – are connected through the food web, and each species can affect the composition of the landscape that others species live in.

The presence or absence of one or several key species often determines the distribution and abundance of many other species. Trying to conserve an ecosystem with only some of its native species is like trying to maintain a car with only some of the parts. If the missing parts are non-essential, like the stereo and air conditioner, the car will still work. If you remove the tires, it won’t get far. Ecology is more complex than auto-repair; therefore it is generally difficult for scientists to determine a species is “non-essential.” However, based on past experience with a variety of ecosystems, we can be rather certain about which species are the equivalents of the car’s tires, and are thus vital to retain in the ecosystem.

Strongly Interactive Species

A species is considered “strongly interactive” when its absence leads to significant changes in some feature of its ecosystem(s). Such changes include structural or compositional modifications, alterations in the import or export of nutrients, loss of resilience to disturbance, and decreases in native species diversity.  Some authors emphasize the type of interactions; among these are mutualists such as pollinators, spore and seed dispersers; consumers (such as large predators), and ecosystem engineers such as beaver.

Ecosystem Engineers Species

Ecosystem Engineers Species whose activities affect and enhance physical or biological habitat structure have been referred to as “ecosystem engineers.” These engineers significantly modify their habitat in ways that often increase local species diversity. Beavers, for instance, create wetlands by building dams in streams. Elephants can convert a woodland into scrubland or grassland. Other examples of ecological engineering include mound building by termites, burrowing and grazing by prairie dogs, and habitat conversion by bison.

Keystone Species

Paine (1969) first used the term “keystone species" for particularly strong interactors: species whose activities maintain species and habitat diversity and whose effects are disproportionate to their abundance. Where the density of a keystone species falls below some threshold, the species diversity in the area may decrease, triggering ecological chain reactions, ending with degraded or simplified ecosystems. Among such are large predators.Keystone species can be thought of as having the highest per capita interaction strengths.

Foundation Species

Many strongly interactive species are too abundant to be classified as keystone species, and are considered “foundation species.” Examples include many deciduous and mast-producing trees (trees that produce edible nuts, seeds, etc), such as fig trees in the tropics and oaks and chestnuts in temperate areas.) Bison, in their former numbers, are another example.

Trophic Cascades

The elimination of predators destabilizes ecosystems, setting off chain reactions that eventually cascade down the trophic ladder (food web pyramid) to the lowest rung, often reducing habitat complexity and species diversity.* In 1980, Paine coined the term “trophic cascade” to describe this process such multi-trophic level interactions. The altered state is less biodiverse and more simple.

The “Paine Effect” describes this loss of keystone predators and resultant degradation and species loss. Robert Paine famously removed a species of starfish from study plots on Washington’s coast and noted an unpredictably intense set of changes occur in his plots, which were quickly overtaken by mussels usually kept in check by the starfish.

A widely known example is provided by gray wolves in Yellowstone, where extirpation (local extinction) of this large carnivore led to an irruption in the numbers of elk, causing changes in vegetation structure, species composition, and diversity. Without wolves keeping these ungulates in check, elk achieved much higher densities and shifted their behavior to a more concentrated feeding pattern, leading to the virtual disappearance of major vegetation types such as aspen and willow-beaver wetlands in some areas. Once wolves were restored to the park, over-browsed vegetation was given a chance to recover and the park’s natural vegetative pattern began to return.

Another cascading pathway in Yellowstone is the wolves-coyotes-pronghorn antelope connection. In the absence of wolves, coyote numbers increased. Because coyotes are major predators on young antelope, the antelope population was reduced in the absence of wolves. When wolves returned, coyote numbers dropped by about 50 percent, followed by an increase in antelope numbers by about fifty percent.  Large carnivores are not the only type of keystone species (remember the starfish example). Smaller examples include prairie dogs, flying squirrels, and sapsuckers.

Successful conservation of North America’s natural heritage requires us to preserve, and where necessary, reintroduce populations of keystone and other strongly interactive species. These species must be conserved at ecologically effective population numbers; i.e.,with enough abundance for the species to play the roles described above, versus a scattered handful of individuals that are largely symbolic. Protecting habitat is insufficient on its own. The habitats we conserve will unravel if they are not regulated by the full complement of native keystone and other strongly interactive species.

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*Keystone species and trophic cascades are related to food web architecture and dynamics. Basically a keystone controls a competitively superior prey and thus facilitates indirectly multispecies coexistence at that level. Some examples have led authors to conclude that keystone species are necessary ingredients of strong trophic cascades (Polis et al 2000). This is not the case. Keystone species are notable because they concentrate much of the interaction strength of an entire trophic level in a single species, but across nature more generally, keystone species possessing such concentrated interaction strength are probably the exception rather than the rule. There can be trophic cascades between species all of which are predators. Also, not all ecosystems have trophic cascades or keystone species.

Strongly Interacting Species: Conservation Policy, Management, and Ethics
Michael E. Soulé, James A. Estes, Brian Miller, and Douglas L. Honnold
BioScience • February 2005 / Vol. 55 No. 2

Ecological Effectiveness: Conservation Goals for Interactive SpeciesMichael E. Soulé, James A. Estes, Joel Berger, and Carlos Martinez Del Rio
Conservation Biology • October 2003/ Volume 17, No. 5
Trophic Cascades – Predators, Prey, and the Changing Dynamics of Nature
Edited by John Terborgh and James Estes
Island Press, 2010
For further reading, we suggest:

The Wolf’s Tooth – Keystone Predators, Trophic Cascades, and Biodiversity
Cristina Eisenberg
Island Press, 2010

 
Photos: Courtesy of Larry Master, masterimages.org