Photo Credit: Tim Sheehan
September 28, 2012

The Value of Disturbance

In 1984 I was lucky enough to get the best summer job a geology graduate student could have: map the geology of northwestern Montana. This summer, I returned to the area for the first time since then. In the intervening years, global climate change has moved from a little-known hypothesis to a well-established and documented phenomenon. Also in those years, I have studied to become an ecologist. As I traveled in and around the Glacier National Park, looking at the rocks, rivers, and forests, I couldn’t help but think how the recent geological history of the area relates to its current ecological state and what might happen there in the coming decades.

Geologists view time differently from most people. When I refer to the “recent” geological history of this area, I’m talking about the last two million years. During this time, and until around ten thousand years ago, ice sheets have advanced and retreated several times across much of the northern half of North America. During these times, northwestern Montana would have looked much like the ice fields of Alaska.

This last retreat has left behind a spectacular landscape of mountains and valleys carved by glaciers. The mountains are composed of banded rock ranging in color from dark brown to light tan, and mostly covered by conifer forests. Some of the valleys are also forested, while others are scrublands and agricultural fields. At higher elevations in and around Glacier National Park, the plants and animals of the area reflect the gradual process of biological succession that takes place as glaciers retreat. Newly exposed rocky areas are colonized by lichens and early successional plants. Some animals, such as pikas and ground squirrels build nests and store food deep in the crevasses of scree fields. As soil builds up from these early colonizers, grasses take root, shrubs grow, and eventually conditions become favorable for trees, predominately conifers, to take root. What was at first bare rock has become a forest, while other areas that have become exposed give the early colonizers a place to thrive. Larger herbivores and omnivores such as mountain goats, bighorn sheep, elk, and bears directly shape plant succession as they eat and trample vegetation. Omnivores and carnivores such as bald eagles and cougars contribute indirectly by keeping herbivores in check.

In ecological terms, the ecosystem that evolved in this area is resilient, meaning it can quickly recover from disturbances such as extreme weather events and wildfires. Disturbances vary in scale and severity. A single wildfire, for example, commonly burns more intensely in some areas – killing all the trees – and less severely in others – killing only a few, or only smaller trees. Over time, repeated disturbances result in a landscape that is a patchwork of areas in various successional stages supporting plants and animals that can quickly colonize an area that suffers a disturbance.

However, an ecosystem under conditions outside its normal disturbance regime (the frequency, severity, and types of disturbances) may not be able to recover. One example of this is the area to the southwest of Flathead Lake in northwestern Montana. As the last ice age ended, it was exposed to a series of cataclysmic floods which stripped away soil, leaving behind a barren area which to this day supports much less vegetation than the surrounding area.

Unfortunately, we may be witnessing a broader example of a loss of resilience in this region, and this time the cause is us. Wildfire suppression not only builds up fuels within forests, but it also stresses species that colonize burned areas. Global climate change has brought warmer summers and more frequent droughts in the western United States. Not only do these conditions make forests more susceptible to fire, they stress trees making them more vulnerable to disease and insect infestation. They also stress many of the animal species within the ecosystem.

At lower elevations in northwestern Montana, frequent, low intensity fires have produced forests in which large, fire-tolerant ponderosa pines flourish in sparse stands. As I observed on several hikes, young conifers such as Douglas-fir and cedar crowd mature Ponderosa pine trees. This crowding is the result of fire suppression. Add a hot, dry summer to this crowding and the stage is set for a “super fire” – a large fire that burns so fast and so hot that it cannot be contained and that it kills everything in its path.

So what happens when you have an ecosystem that has lost its resilience and falls victim to a catastrophic fire? Does something else take its place? If so, what? Is there any way to make current ecosystems more resilient in the face of climate change? These are questions that ecologists everywhere are grappling with. Scientists are using climate models, vegetation models, and species distribution models to explore which ecological communities and ecosystems might thrive under projected future conditions. By researching these questions, we can hopefully find ways to help ecosystems become more resilient and avoid a world that will take thousands of years to recover from our mistakes.

About the author:
Tim Sheehan, Ph.D.
Ecological Modeler, Team Lead- Decision Support & GIS
Tim is an Ecological Modeler at CBI working on a wide range of computational models to analyze alternative futures for ecosystems. In addition to his recently completed master's degree in Biology from the University of Oregon, he also has master's degrees in Geology and Computer Science.
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