a story about the elk, the beaver, the willow and the wolf

Gray wolf. Credit: G. Kramer; Source: US Fish & Wildlife Service National Digital Library.

The complex, interdependent relationships between ecosystem components can be an immensely fascinating topic to some people (including the administrator of this website). However, an understanding of how these relationships interconnect is an essential piece of knowledge for developing sound landscape management practices. Scientific research can lead us to this knowledge.

This is not to say that local, observational or anecdotal knowledge is of no value. It is certainly better than no information at all, and often aligns well with scientific understanding.  However, there are so many intricate factors and complex processes at work in a functioning ecosystem – soil structure, soil organisms, soil erosion, hydrology, plant roots, plant litter, herbivory, canopy micro-climate, predation…just to name a few. If we want to find reliable and effective management practices that restore and sustain riparian ecological communities, we must take into account these many interdependent factors. To better understand how we, as humans, directly or indirectly affect riparian area health, we must be aware that the effects of our actions, both intended and unintended, often proceed in unpredictable ways.

Bull elk. Credit: R. Hagerty; Source: US Fish & Wildlife Service National Digital Library.

Take, for example, this study by Kristin Marshall, N. Thompson Hobbs and David Cooper out of Colorado State University (2013) entitled Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction. They look at the relationships between wolf and elk populations and the health and sustainability of riparian willow communities in Yellowstone National Park. The elk populations have soared there because wolves had been eradicated from the park a long time ago. Wolves eat elk which helped to keep the elk populations balanced. Too many elk have led to over-browsing of the willows, which reduces their ability to grow, survive and reproduce and which also impairs their ability to perform important riparian functions. The health of riparian zones in the park has declined.

One might expect that simply reintroducing the wolves into the park will reduce elk numbers and take some pressure off of those poor willows. However, the researchers suspected that it wouldn’t be that easy.

Beaver with willow branch. Credit: S. Hillebrand; Source: US Fish & Wildlife Service National Digital Library.

You see, there also used to be beavers in the historical ecological picture. However, when the elk over-browsed the willows, this  meant that there was not enough willow material for beavers to do their business, which drove them away. The researchers wanted to find out if and where beavers fit into this ecological equation.

The researchers tested whether reintroduction of wolves alone would be enough for the riparian willows to recover (more wolves → fewer elk → less browse → more willow). For willow communities to recover and become sustainable, plants need to be about 2 m high so the elk can’t reach them. The researchers also simulated beaver activity by flooding some study areas. They did this to compare the effects of 1) reducing browse alone, 2) beaver flooding alone, and 3) the combination of flooding and reducing browse by elk on the recovery of the willows.

The researchers found that, indeed, the combination of both reducing browse and elevating ground water (beavers!) was necessary for the riparian willow community to fully recover. It turns out that the beavers and the willow had an understanding. Beavers would build dams using the willows, which floods the area and elevates local water tables. These higher water tables mean that water in the soil is more accessible to the plants, which enables the willows to grow. The beavers use these willow to build dams which floods the areas and…well, you can see how this cycle continues. (It is worth it to note that there may have been some additional factors that led to the beavers closing shop in this area.)

So we can see from this story that, when it comes to ecosystem processes, nothing acts in isolation. They are called a systems or a reason! The health of riparian areas rely on a gigantic number of interrelated features and processes to be in balance. Many of these we do not understand nor can we predict. However, research can help us to see them a little more clearly.

This study article is published in the journal Proceedings of the Royal Society B: Biological Sciences. A new release regarding this study can be found here.