Balancing act

When the concept of conservation biology was first introduced in the 1970s, it applied to the species that were disappearing due to deforestation in the tropics. Biologists began to realize that species were going extinct as a direct result of human activity. As conservation science evolved over the decades it has become a multidisciplinary melding of population biology and ecology, economics, and sociology. Quite often the decisions about which species and/or habitats should be conserved are based on human exploitation of some resource. Conservation biology, like every other form of science, costs money, and often funding agencies have an implicit or explicit expectation of economic gain from conservation efforts.

There may also be direct conflicts between conservation activities within a habitat or ecosystem. Take, for example, the beaver and the kokanee salmon, two iconic animals of Taylor Creek. I was up at Lake Tahoe this past weekend, finally able to visit Taylor Creek during the spawning season for the salmon.

As I wrote about earlier, the kokanee is a land-locked sockeye salmon (Onchorhynchus nerka) that migrates from Lake Tahoe into Taylor Creek to spawn; it was introduced as a game fish to the Tahoe basin in the 1940s. It has since become a favorite denizen of Taylor Creek and has spawned a festival all of its own.

The kokanee, like other Pacific salmonids, requires cold, clear water to reproduce successfully. This brings it into direct conflict with Taylor Creek’s other iconic animal, the beaver (Castor canadensis). The beaver’s range historically extended into the Sierra Nevada; however, from the late 19th century into the first decades of the 20th century beavers were viewed as pests and systematically exterminated. As biologists began to understand how beavers affect overall riparian ecosystem health, state and federal agencies re-introduced beavers to the Tahoe basin in the 1930s and 1940s. Whether or not you consider beavers to be native to Taylor Creek, there is no disputing that they are there now.

Beavers, of course, are known for the logging and damming activities. They fell trees, strip off the branches, and use the logs to build dams across rivers. This forms a pond of still water above the dam, where the access to the beavers’ lodge is located. Beavers are herbivores, eating the bark and wood of trees in addition to some aquatic plants. They are nocturnal, but although we returned to Taylor Creek at dusk we did not see any. Evidence of their activities was all around. The phrase “busy as a beaver” is very apt; the dam in the photo above is about twice as tall as it was when I was here in August.

The conflict between the kokanee salmon and the beavers arises because these animals live in the same place but have different requirements for water flow. As I mentioned above, the salmon need cold, clear water. Their eggs will suffocate and die if water temperature is too high, because warm water holds less dissolved oxygen than cold water. Flowing water also helps guide the returning adults to their spawning grounds. Beavers, on the other hand, take active measures to stop or severely restrict flow in the creek. The pond that forms above a beaver dam is very calm and the bottom becomes silty or muddy, the exact opposite of what the salmon need.

Balancing the conservation needs of these popular animals has been a challenge at Taylor Creek. Do you promote the non-native salmon by destroying beaver dams? Or let the beavers do their thing, at the probable expense of the salmon? How much of the decision is due to the fact that beavers are probably native to the Tahoe basin, while the kokanee are undeniably not? And what do Tahoe’s human residents and visitors want more, salmon or beavers?

This year, the strategy has been to leave the dams, but install pipes running through them so that water continues to flow. However, you can see from the photo above that the dam is still holding back about half a vertical meter of water. Plus, as of now no salmon have made it up past the dam; rangers have been seining adult salmon from the creek below the dam and putting them into the stream profile chamber so visitors can see them. Perhaps the salmon are able to spawn in the creek below the dam.

The ecosystem of the Tahoe watershed has been severely affected by the introduction of non-native species. Lake trout, brook trout, rainbow trout, largemouth bass, bluegills, and even goldfish have been released (deliberately or inadvertently) into the lake, and have extirpated the Lahontan cutthroat trout (Oncorhynchus clarki henshawi), the only native salmonid in either Lake Tahoe or Fallen Leaf Lake. The kokanee salmon also falls into this category, and likely competes with the Lahontan cutthroats for food. Recent attempts to re-introduce the Lahontan cutthroat trout have had mixed success. Very interestingly, it appears that the Lahontan cutthroat can move back and forth across beaver dams while the kokanee cannot. Co-evolution, anyone? It seems clear to me that the Lahontan cutthroat trout, which after all shares a long ecological relationship with beavers, is the salmonid that is best adapted for the Taylor Creek ecosystem. As charismatic as the kokanee salmon is, from a biological perspective it really doesn’t belong in Taylor Creek. Perhaps one easy way to restore this ecosystem to a more natural state is to stop removing and damaging beaver dams, and let the kokanee go extinct.

Remember how I said that economics plays a part in conservation? There are several charter fishing companies at Lake Tahoe, all of which have an economic interest in the maintenance of several introduced species in the lake. So in addition to balancing the ecological needs of kokanee and beavers in Taylor Creek, conservation efforts must also address the economic needs of local businesses. These are challenges that we will continue to face all over the planet if we want to live more harmoniously with the natural world.