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Feeding spectaculars: brown bears and salmon not just a tourist attraction, by Hakan Binek

June 15, 2011

Alaska is arguably one of the most beautiful states, containing vast mountain ranges encircled by expansive boreal forests, with numerous streams, lakes, and rivers speckled throughout. This gorgeous piece of land also happens to contain one of North Americas largest predators, the brown bear or Ursus arctos. Post cards, magazine covers, and TV shows often highlight the exciting events of bears snatching up hoards of unsuspecting salmon jumping into the air. However, this occurrence is more than just a spectacular feeding frenzy. The interaction plays a major part in sustaining plant life.

Salmon spend the majority of their lives in the ocean, only migrating to Alaska’s rivers and streams to spawn during the months of July and August. As a result their bodies contain high levels of marine-derived nutrients, or MDN, which are nutrients that originate from the ocean. Soon after spawning the salmon die and their carcasses decompose in the rivers and streams. Previous studies have determined that this results in a beneficial release of MDN into the freshwater systems of Alaska, which can increase aquatic plant growth. A recent study analyzed the effects that bear predation of salmon has on nutrient levels of the soil surrounding Alaska’s streams and rivers.

Holtgrieve and Schindler from the University of Washington, and Jewett from Stanford University, wanted to know if bears transferred MDN to the soil, and what MDN did to the nitrogen cycle. Nitrogen is the most abundant element in the atmosphere. The nitrogen cycle, simply put, is the process of how nitrogen is incorporated into the soil, used by organisms in its various forms, and released back into the air. Given that nitrogen is a limiting resource for plants, the nitrogen cycle is important because it plays a major role in plant growth and productivity.

The study took place in the Wood River drainage of southwest Alaska on four streams neighboring Lake Nerka, a popular place for the commercial harvest of salmon. When bears began to feed on salmon in the summer of 2003, the researchers identified sites of high bear activity based on trampled vegetation, salmon carcasses, and bear feces. Over the course of the salmon run, Holtgrieve, Schindler, and Jewett took soil samples to measure levels of nitrogen in its various forms. They also stuck PVC pipe into the ground to collect nitrogen that was being released into the air, as a result of decomposition in the soil. Data was collected through the summer of 2005, and the numbers were compared to reference sites in which bear activity was limited.

In the areas with high bear activity, levels of ammonia, a form of nitrogen in the soil, were triple that of the reference sites and release of N to the air (NO2 flux) was nearly 32 times higher. NO2 flux is often indicative of the nitrogen cycle rate, meaning that an increase in NO2 flux is likely correlated to an increase in the rate at which nitrogen is being cycled in the soil.  The authors concluded that presence of MDN, likely introduced into the soil by bear excrement or partially eaten salmon carcasses being brought to shore by bears, was the culprit for the increase.

Holtgrieve, Schindler, and Jewett wanted to see if the effects of MDN on the nitrogen cycle were long term or if conditions returned to normal when new MDN was not being introduced. In the summers of 2004 and 2005 they set up electrical fencing to keep the bears out of previously known hotspots. The researchers compared their measurements to reference sites and found that conditions had returned to normal. So, although MDN can induce a massive spike in the nitrogen cycle rate, its long-term effects are relatively non-existent.

An interesting twist develops with the results of this study when recent declines in salmon population are taken into account. With less salmon migrating up streams, there becomes less predation by bears. Less predation will result in less transfer of MDN from the salmon to the surrounding soils, thus a decrease in the amount of nitrogen. Less nitrogen means that plant productivity will decrease, and the ecosystem of the land surrounding the waterways could be altered. Similarly, if the brown bear populations were to decline results such as those of the bear exclusion experiment by Holtgrieve, Schindler, and Jewett would likely be observed. Without bears to prey on the salmon, carcasses and thus MDN would not be transferred to the shoreline, and nitrogen levels would be much less than if significant predation was occurring.

Ecologists would refer to both the salmon and brown bears as keystone species, in that they generate larger effects on their ecosystems than other local species. However, the effects of one cannot be observed if the other is not present in the ecosystem. Many ecologists deem this interaction as a “mobile link”. Salmon bring the nutrients from the ocean to the inland streams. Bears pick up the nutrients (salmon carcasses) and transfer them to the surrounding soils. Without the interaction of the two, the “conveyor belt” type system of MDN to terrestrial habitats would not exist.

The recent research performed by Holtgrieve, Schindler, and Jewett shows exactly how important the interactions between brown bears and salmon are in maintaining levels of nitrogen in the soil. Conserving and maintaining the populations of both salmon and bears may be key to preserving the plant and animal species along the shorelines of rivers, lakes, and streams of inland Alaska.


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