Trophic cascades

How predators structure ecosystems

Research at Oregon State University led by Bill Ripple and Robert Beschta has provided probably the most famous example of how terrestrial predators can change whole ecosystems. Ripple, Beschta, and colleagues studied what happened when wolves returned to Yellowstone National Park after an absence of 70 years. Wolves were reintroduced in the mid-1990s, and the population recovered to 14 packs – with almost 100 animals – by 2009. The recovering wolf population not only reduced numbers of their main winter prey, elk, but also changed their behavior.

Wolves can change ecosystems (Photo: Sandra Peterson)

Wolves can change ecosystems (Photo: Sandra Peterson)

The researchers found that in places where elk were at greatest risk from attack by wolves, such as in narrow stream valleys, aspens and willows – which previously had been browsed down by elk – were growing taller. Ripple and Beschta suggested that this was evidence for a “trophic cascade”: the presence of wolves was changing the behavior of the elk, and shifting their distribution within the landscape. In turn, the change in elk behavior was having an impact on the growth of trees, which themselves provided habitat for a wide range of other species.

The return of wolves may even change rivers. Ripple and Beschta found that after the extirpation of wolves in Yellowstone, riverbanks had eroded more rapidly, and their courses had become straighter and less meandering. With the return of wolves and the recovery of riverbank vegetation, the rivers in Yellowstone are returning to their meandering form, with benefits for a wide range of species, from willow flycatchers to beavers. Of course, the return of wolves is not the only factor: according to Ripple, both “top-down” (predator-driven) and “bottom-up” (plant-driven) processes shape Yellowstone’s complex and changing ecosystem. But the impact of wolves on willows is a timely reminder that everything in ecological systems is connected.

Since this early work on trophic cascades in Yellowstone, it has become increasingly apparent that predators – from sharks to tigers – have similar “keystone” influences in other ecosystems. Ripple and coauthors have recently called on world leaders to prioritize global conservation of predators.

 

Aspens_Ripple.jpg

Key paper:

Ripple, W.J. & Beschta, R.L. (2004) Wolves and the ecology of fear: can predation risk structure ecosystems? BioScience 54: 755–766.

 

Image: Aspen regrowth in Yellowstone - William Ripple/Oregon State University (Creative Commons license)

Dead wood

from hazard to habitat

Oregon State University researchers working at the H.J. Andrews Experimental Forest transformed the way we think about dead wood in forests. Up to the 1970s, snags, logs, branches and dead roots were viewed as an inconvenient, untidy hazard. “Coarse woody debris” in streams was thought to block fish migration, and was routinely removed.

One-fifth of forest species rely on dead wood (Photo: Mark Harmon)

One-fifth of forest species rely on dead wood (Photo: Mark Harmon)

However, the OSU researchers realized that dead wood plays a hugely important role in forest ecology. Their research, summarized in a seminal paper led by Mark Harmon in 1986, described the many ways in which healthy living forests depend on dead wood. Snags (standing dead trees) provide homes for cavity-nesting birds and bats. Downed logs allow tree seedlings to gain a foothold, while sheltering salamanders, beetles and small mammals. It is now estimated that up to 20% of the total species diversity in mature forests is directly associated with dead wood!

Downed wood in streams and rivers traps sediment and organic matter, slows down the flow of water, and provides habitat for fishes and other aquatic organisms. Dead wood of all kinds stores carbon, provides a substrate for important bacteria and fungi, and breaks down to enrich forest soils. These findings, based on years of painstaking research at the H.J. Andrews Experimental Forest, were used to inform management plans and guidelines in the Pacific Northwest, and their influence continues to be felt globally. Forest management policy in many countries (e.g., Canada, Australia, Sweden) now requires conservation of dead wood.

Not only did this scientific endeavor help to maintain the ecological values of forests, it also saved money. Ending the practice of removing dead wood from national forests in the Pacific Northwest saved tens of millions of dollars each year. Stimulated by the work at Oregon State University, researchers around the world have uncovered the importance of dead wood in all kinds of forests, from the tropics to the taiga. Living forests need dead wood.

Red belt fungus on dead wood (Photo: Mark Harmon)

Red belt fungus on dead wood (Photo: Mark Harmon)

Key paper:

Harmon, M.E., Franklin, J.F., Swanson, F.J., Sollins, P., Gregory, S.V., Lattin, J.D., Anderson, N.H., Cline, S.P., Aumen, N.G., Sedell, J.R., Lienkaemper, G.W., Cromack Jr., K. & Cummins, K.W. (1986) Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research 15: 133–302.

Banner image: Leon Ingulsrud (Creative Commons license)