Connectivity, Migration, and Abundance

In the past twenty years, a little more than one percent of the approximately 85,000 dams in the United States have been removed: up to half of the total number are defunct. Writing in Nature, Richard Lovett describes the effect dam removal is having on native fauna:

On south-central Wisconsin’s Baraboo River, the removal of a string of dams has allowed sturgeon to reach their former spawning grounds. And in New England, the destruction of two dams 7–9 metres high on Maine’s Kennebec River and one of its tributaries has allowed Atlantic alewives (Alosa pseudoharengus) to repopulate 100 kilometres of previously blocked-off river. In 1999, before the first dam was taken out, no alewives were recorded in the upper part of the watershed, says Serena McClain, head of river restoration for American Rivers. By 2013, the annual run had rebounded to around 3 million.

Recently, the largest dam removal in the history of the United States on the Elwha River in Washington State was completed. Originally 210 feet high, the 94-year-old Gines Canyon Dam was removed in stages over a three-year period beginning in 2011. Multiple studies were conducted as to the effects this dam removal would have on water quality, fish population levels and habitat availability before the decision to demolish the dam was made. Researchers predict that local salmon populations, which declined over 90% since the construction of the dam, “will respond to the dam removal by establishing persistent, self-sustaining populations above the dams within one to two generations.”

A number of studies have found that dam removal can even have profound effects on local populations far upstream of the site. In one study, researchers found that native eel populations at the headwaters of the Rappahannock River increased every year after the dam was removed. Another study looked at 25 separate species along a stretch of the Pine River in central Michigan before and after dam removal. The researchers found that most species increased in abundance following dam removal, and that the majority of species formerly found only below the dam were now found upstream and downstream of where the dam stood.

Why does this happen?

Connectivity is a word ecologists use to describe how easily organisms can move from one resource-rich area to another. Better connectivity allows organisms to disperse more freely throughout a landscape. Connectivity affects the ability of genes to flow from one population to another, influences the risk of local extinction, and allows new species to colonize an area following a catastrophic event like a forest fire or hurricane. Connectivity is especially vital to the survival of migratory animals that travel each year from breeding grounds to feeding grounds. Ecologists often talk about the importance of maintaining corridors between one resource-rich patch and another, particularly as humans disrupt natural landscapes with agriculture and development. Rivers can be natural corridors for many different species to move through a human-modified landscape.

But dams dramatically reduce the connectivity of riparian, or river-oriented, landscapes. They significantly reduce the ability of spawning fish to swim upstream, for example. And they have the unintended effect of reducing populations of migratory animals. While dams affect fish and amphibian populations disproportionately, they can also reduce gene flow among plants, disrupt bird populations, and reduce water quality upstream by collecting sediment.

The good news is that thus far, the evidence shows that local populations respond very quickly to repaired connectivity in their natural habitats. And we may be only beginning to see the ecological benefits that dam removal can have on riparian ecosystems. With nearly half of the dams in the United States no longer in use, a push to removing more of them could be just downstream.

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