Interactions between top-down and bottom-up factors in arctic tundra plant communities

Background/Question/Methods

The limited growing season and low temperatures of the arctic summer create a short, stressful window in which plants must grow and reproduce. Not surprisingly, much of the terrestrial ecological research in the Arctic has focused on these and other bottom-up controls on ecosystem function. In the past several decades, environmental controls on microbial activity and soil nutrient transformations have been examined with a more recent emphasis on the role of plant species composition in regulating soil nutrient availability. Missing from most of these studies has been the role of herbivores and predators. This lack of knowledge is particularly problematic now as the Arctic is undergoing unprecedented alterations of mean annual temperature, timing of snowmelt, and growing season length. Such climate changes affect soil nutrient availability, thus altering plant productivity and community structure. Presumably climate change also affects herbivores and predators directly (e.g., more amenable air temperatures may allow migratory animals to remain on the tundra for longer periods of time) as well as indirectly (e.g., greater plant productivity increases food availability). Here I describe the state of knowledge of how top-down and bottom-up factors interact in arctic tundra, illustrated with relevant case studies.

Results/Conclusions

Experiments in North American and Scandinavian tundra have shown complex results in terms of the interaction of top-down and bottom-up factors. In one study in Norwegian tundra, for example, predators protected dwarf shrubs from mammalian herbivory in a classical trophic cascade, but effects differed across communities and species. In arctic Alaska, my colleagues and I have found that the effects of mammalian herbivores differ in communities with different levels of primary productivity, ranging from detrimental to stimulatory effects. Somewhat similarly, a recent study in Greenland documented that mammalian herbivores can offset vegetation changes caused by artificial warming. A better understanding of top-down controls in arctic tundra is inhibited by several factors including the lack of knowledge of predation rates (with the exception of a few well studied Scandinavian sites) and the dearth of studies of invertebrate herbivory. Research that incorporates both bottom-up and top-down factors in arctic tundra is still infrequent, hampering our ability to make predictions of how terrestrial arctic ecosystems are changing with climate warming.