Along the San Pedro River, beaver are the most important ecosystem engineer.

Keystone wildlife species, also known as ecosystem engineers, are animals whose impacts on the environment are disproportionate to their abundance. To understand keystone wildlife species one first has to know that a keystone is the wedge-shaped stone at the crown of an arch that locks the other pieces in place. Without a keystone, an arch would be just a pile of rubble.

University of Washington zoologist, Robert Pain, introduced the ecological principle called keystone species in 1969. Pain described keystone species as “drivers” ⎯ species vital to maintaining the ecosystem in which they live⎯and called all other species “passengers”: animals just along for ride.

It’s been a while since 1969 so to put it in context, other events that occurred in that year included: Apollo 11, the first human spaceflight was launched, Concorde broke the sound barrier for the first time, and a photographer named Iain Macmillan took a photo of a zebra crossing in London that became the cover of the Beetles album “Abbey Road.” Which brings us back to the wildlife connection: the zebra! It is a “passenger” on the plains of Africa, dependent upon the “driver” ⎯the elephant⎯because elephants knock down trees on the savanna and thus maintain open grassland habitat critical to zebras.

Along the San Pedro River in southeastern Arizona, beaver are the most important ecosystem engineer. They modify river and pond environments to benefit not only themselves, but also other wildlife and even the riparian vegetation itself by resetting ecological succession in streamside habitat. Beaver set back succession by cutting down mature cottonwoods and willows for food and to build dams and houses. This action opens up the old tree cover so new plants: grasses, forbs, cottonwoods, and willows can get a start. Like planting seeds, new growth ensures the future of the ecosystem as long as there is water. Without water, succession will take the land back to desert. So, in a way, the river itself is the real “driver” of the riparian ecosystem along the San Pedro.

Of the eighty-four wild mammals and over one hundred bird species that inhabit the San Pedro Riparian National Conservation Area year-round, most are “passengers” but a few besides beavers are “drivers.”

A “driver” less charismatic than beaver is the lowly pocket gopher, sometimes referred to as those “!#%-ing things” that root up my lawn and garden and leave behind dirt mounds that mess up my lawn dart course and clog up my lawnmower.

From an ecological standpoint, pocket gophers are very important in a balanced ecosystem because they mix and aerate the soil when they create mounds by bringing sub-surface soil above ground. The effects of light, water and wind break down the soil minerals and make them available to plants, thereby increasing surface soil fertility. Digging by gophers also increases plant diversity since different species of plants are given the chance to establish on fresh soil mounds. Another benefit: the mounds themselves reduce sheet soil erosion on steep slopes during heavy rains.

Another important keystone species, the black-tailed prairie dog, once occurred along the San Pedro but is now absent. Prairie dogs helped shape the grassland ecosystems of the Great Plains and southwestern desert grasslands because like gophers they bring subsoil to the surface to fertilize the land and stimulate nutrient rich grasses to grow. The elimination of prairie dogs over large areas has resulted in less available habitat for about 150 associated wildlife species that are highly dependent on the grassland environment maintained by prairie dogs. For instance, several animals such as burrowing owls and badgers that live in burrows, and mountain plover that prefer short grass prairie are more common in prairie dog colonies than on the landscape in general.

Another digger, the banner-tailed kangaroo rat, is a “driver” for much the same reason as the prairie dog and pocket gopher, but is distinctive in that this keystone species found in grassland, mesquite and creosote bush habitats, builds large soil mounds up to 12 inches or more high and 10 feet in diameter. K-rats are seedeaters and often live where grassland and shrublands biomes meet. K-rats assist ranchers by preventing transition of grasslands to shrublands because they consume shrub seeds like creosote bush and mesquite before the seeds can grow and produce bushes that crowd out desirable grasses.

Humans can do a great deal to conserve both “drivers” and “passengers” in native ecosystems. We just need to pay attention to what nature is telling us and help out where we are needed, especially with the “driver” species. Protect them and many, many other species will benefit.

Wildlife Biologist, Writer, and Outdoor Photographer R.J. (Bob) Luce has lived near and photographed the San Pedro River and its environs in southern Arizona for 17 years. His publications include: River of Life, Four Seasons along Arizona’s Rio San Pedro and two mystery books, all of which are available at the San Pedro House bookstore. Submitted by Friends of the San Pedro River.

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