Wild Neighbors: Force of Nature: How Beavers Build the World

By Joe Eaton
Thursday July 02, 2009 - 09:58:00 AM
 One of several dams built by the beaver family in residence in Martinez.
Ron Sullivan
One of several dams built by the beaver family in residence in Martinez.

Although you may not think of beavers as Bay Area wildlife, they’re common along the Contra Costa shoreline and even more abundant in the Delta, a major source for the 19th-century fur trade. The beaver family that moved into lower Alhambra Creek in Martinez three years ago is still going strong. The founding pair had their third litter of kits this year, around the time the surviving 2-year-olds struck out on their own. The yearlings are helping care for the new arrivals. 

There’s some ambivalence about having beavers downtown, and some merchants in particular see the rodents more as a flood hazard than a tourist draw. For details, including this summer’s Aug. 1 Beaver Festival, see the website ( maintained by beaver advocate and Worth a Dam founder Heidi Perryman. 

One of the striking things about the Martinez beaver story is the way other critters have moved in to the habitat created by the damming of the creek. Observers have seen many more fish-eating birds: cormorants, black-crowned night herons, belted kingfishers. Muskrats have shown up—a mixed blessing, as they’re more likely than the beavers to tunnel into banks—as have river otters. A mink, probably interested in the muskrats as prey, was photographed this year. Piece by piece, a new biological community is assembling. 

The notion of beavers as ecosystem engineers is nothing new. We’ve long been fascinated by the way these rodents alter the environment with their dams, lodges, and canals. Beaver works can raise water tables, change stream temperatures, keep intermittent streams from drying out during the hot months. They transform landscapes from aspen forest to pond, and eventually to meadow. They create habitat for a host of other species, from clams and midges on up. 

Richard Dawkins, Darwin’s pit bull, used beavers and their constructions to illustrate his concept of the “extended phenotype” in his book of that name—phenotype being the physical traits that the genes code for. To Dawkins, the beaver’s phenotype included more than the flat tail, the chisel teeth, and the special mite-removal claw. It also incorporated the lodge, the dam, and the pond—all equally products of the genetic template. 

More recently, a group of British and American scientists—John Odling-Smee, Kevin Laland, and Marc Feldman—have given this an interesting twist in what they call “niche construction theory” ( “Niche,” one of the few terms that have migrated from ecology to economics, is a matter of what a species does for a living and where it does it. The niche-constructionists’ work bristles with math, but the basics are straightforward. 

Odling-Smee and his colleagues question the idea that evolution is just a matter of organisms being passively shaped by their environments. The whole natural selection thing is about fortuitous adaptation to changing external conditions, as when a few lucky elephants with more body hair were born into a cooling world. Organisms that pass on more of their genes than their less-well-adapted kin win the selection game. 

What if adaptation is not just a one-way street, though? What if an animal (or plant, or fungus) changes its environment enough to create a whole new set of selective pressures? That, it’s argued, is what happened over time with beavers. They changed their world, and changed their own evolution in the process. 

That’s fine for beavers, but how typical are they? One species may seem like a shaky foundation for a theory. (Or two, if you include that other world-changer Homo sapiens.) Some niches are pretty narrow. How could a limpet, whose niche consists of sitting on a submerged rock and grazing on algae, affect its own evolution?  

But the niche-construction folks offer a long list of species that alter their surroundings enough to feed back into the selection process. Consider the climate-controlled megacities of the termites, the subterranean fungus farms of the leaf-cutter ants, the tons of soil shifted by earthworms. Consider the chaparral plants whose resinous leaves kindle the fires that help their seeds germinate. Every bird that builds a nest is physically modifying its niche, in however small a way. Everything that breathes, eats, or excretes changes its environment. 

And the ripple effects can go far beyond the constructing species. When they dam a creek, beavers are altering the evolutionary trajectories of caddisflies, trout, and wood ducks. This, I think, is the part that bothers Dawkins so much. But think about it. We know that human-modified environments affect the destinies of other species. What’s so different about beavers—or earthworms? 

That aspect of niche construction theory makes for messy flow charts. Dawkins dislikes the whole concept and is said to have called it “pernicious.” It makes you think, though, and its authors claim it can generate testable propositions. I’ll be interested to see where it all leads.