Wolf reintroduction in Yellowstone shows large predators help forests rejuvenate – and may increase biological diversity

wolf Yellowstone forest ecosystem
In 1995-1996 the gray wolf was reintroduced to Yellowstone National Park – after it had been hunted to extinction some 70 years earlier. Now, 15 years later, a new ecological balance is gradually forming between a stable number of wolves and a less dominant elk population – of which many plants and animals profit.

A special report (PDF) was released today by researchers of Oregon State University who have compared ecological changes between the late nineties and now for the Lamar River basin, which lies entirely in the national park

Most notable is the survival rate of sprouts of aspen trees. In 1998 still 100 percent of young aspen trees in the study area were being eaten by elk. Now that has reduced to 20 percent.

Apart from aspen trees, also cottonwood (another tree of the poplar family) and willow have recovered. Willow recovery has also led to a noticeable increase in songbird populations (such as that of the common yellowthroat, warbling vireo and song sparrow) as it provides nesting habitat.

Recovery of young deciduous trees has led to an increase of beaver populations around the Lamar River system from 1 in 1996 to 12 in 2009, which in turn has led to greater fish abundance. [Other research shows beaver ponds lead to a 75 times higher presence of duck species in Wyoming streams.]

The elk population has decreased from >15,000 in the early nineties to 6,100 in 2010 ‘and remaining elk now have different patterns of movement, vigilance, and other traits.’

Paradoxically many other predator species have benefited as well. This is because with the absence of wolves coyotes had grown to dominance in Yellowstone. The coyote numbers have again declined since the wolf’s return, which means there is now more small prey left for species like the red fox, ravens and bald eagles.

After its reintroduction the wolf population has slowly increased to a peak of almost 100 individuals in 2003 and has since showed some variation, gradually declining to just over 40 wolves.

Yellowstone wolves speciesDevelopment of various species numbers after wolf reintroduction in Yellowstone National Park.

However “these are still the early stages of recovery, and some of this may still take decades,” says William Ripple, Oregon professor in the Department of Forest Ecosystems and Society. “But the signs are very encouraging.”

Meanwhile an increase in the number of bisons in the Lamar Valley is impacting tree growth in that area – a development to which the wolf population may need to reply. “It may be necessary for wolves not only to be present but to have an ecologically effective density,” says OSU co-author Robert Beschta, a professor emeritus of forestry.

He also notes it’s not clear whether wolves can play their ecological role outside confined borders of national parks. “Mechanisms to deal with human and wolf conflicts also need to be improved.”

A publication in Science earlier this year warned of worldwide ecosystem disturbances following large predator declines.

Wolf Yellowstone ParkA resting gray wolf in snow-covered Yellowstone Park, judging the nutritional value of a photographer from afar. [Photos by Oregon State University.]

Meanwhile for many forest systems, ranging from tropical rainforests to temperate and boreal forests, it’s not just overgrazing by dominant herbivores, there are other threats too – that wolves and other large predators can do little about, like fire. In Yellowstone’s case this may still lead to an inevitable ecosystem shift halfway through the century, a special PNAS study stated last July.

There’s only so much conservation measures can do to abate an environmental stress that is imposed from above…

© Rolf Schuttenhelm | www.bitsofscience.org

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