The Productivity Paradox

How did apparently sparse vegetation support such a wide variety of large grazing mammals? This question is at the heart of the debate about the steppe-tundra. There are several possible explanations, as follows:

(1) The steppe-tundra was a richer environment than was previously thought. Some researchers who study fossil pollen argue that the steppetundra vegetation was short, sparse, and gener ally insufficient to support large herds of grazing mammals. They believe that the fossil bones found in Alaska must represent accumulations of fossils over great lengths of time, and that large grazing populations were never large at any given time. However, vertebrate fossil studies indicate that a wide variety and abundance of animals were living on the land at any given time. Vertebrate paleontologists argue that the grassy vegetation of the steppe-tundra was rich in nutrients and sufficiently abundant to support large populations of grazers.

(2) The large grazers helped create and maintain the steppe-tundra habitat. This theory argues that trampling and grazing by large animals in tundra causes a shift in dominance from mosses to grasses. Grasses reduce soil moisture more effectively than mosses, because of their higher rates of evapotranspiration (moisture lost from plants through evaporation to the air and through the giving off of water vapor as part of the plant's respiration). When most of the large grazers became extinct at the end of the Pleistocene, the lowland tundra of Beringia shifted to a moss-dominated vegetation.

(3) Beringian environments were more varied than was previously thought. In the 1970s and 1980s, many researchers thought that the steppe-tundra ecosystem dominated nearly all the lowland regions of Beringia. More recent studies have shown that most of southwestern Alaska and parts of the Bering Land Bridge were dominated by mesic shrub tundra, where grasses were far less dominant and birch and willow shrubs were more important. Many researchers have come to accept the idea that Beringia was not clothed uniformly in steppetundra vegetation. Rather, it supported a mixture of vegetation communities, as dictated by a variety of controlling environmental factors.

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