Wild Rice Population Dynamics and Nutrient Cycles

Populations of plants and animals typically undergo regular oscillations or boom-and-bust cycles in nature. Ecologists have long been interested in the causes of these oscillations. Ecological theory predicts that any delay in a population will cause it to oscillate. Most of the processes that have been proposed as causes of delays are purely population processes, such as delays in birth rate. But if the recycling of limiting nutrients between decaying plant material (litter) and live plants is delayed, this could also cause a population to oscillate.

Wild rice is an interesting wetland plant species whose populations have long been known to oscillate. Wild rice is an annual plant and so the population must meet all its nutrients requirements for growth from the release of nutrients in decaying litter from previous years. Almost all the nutrients wild rice needs for growth are taken up in a brief window in spring-early summer of the year. But litter from the previous year decomposes so slowly that it does not even begin to release nutrients until at least the end of summer or not even until the second year of decay. We hypothesize that this delay in the recycling of nutrients through decaying litter causes wild rice populations to oscillate.

Besides being ecologically interesting, wild rice is an important staple food for members of Minnesota's Ojibway (Chippewa) tribe. Wild rice harvesting is a defining feature of Ojibway society. Ojibway culture includes legends of wild rice spiritual ceremonies conducted during harvest time. Wild rice approaches sacred food status and band members identify wild rice as a symbol of what it means to be Ojibway. The Ojibway tribe manages over a dozen wild rice lakes in northeastern Minnesota for wild rice production in as natural a manner as possible. Therefore, what causes oscillations in wild rice production is of interest, culturally and scientifically, to tribal members as well as to a wider scientific community. This research will involve close collaboration with members of the Fond du Lac band of Ojibway.

The effect of delays in nutrient release on wild rice population dynamics will be tested by the following three-pronged approach: (1) making long-term observations of wild rice productivity, seed production, and nutrient availability in sediments in a series of wild rice lakes; (2) manipulating litter, nutrient inputs, and seed vs. seedling establishment in large mesocosms (stock tanks) and measuring total productivity and nutrient uptake, mean plant weight and nutrient uptake, seedling emergence, and nutrient availability; and (3) constructing and testing a computer model of wild rice nutrient uptake, productivity, litter decay, and seed production and emergence.

Students will be recruited from the tribal community college to assist in this work. This project will assist tribal biologists in addressing problems of natural resource management of significance to Ojibway values and show tribal students the potentials of a career in the environmental sciences. The tribe is anticipating that the results of this study will provide critical information for its development of nutrient criteria specifically for wild rice waters, and for improving management decisions from a better understanding of nutrient cycling in wild rice ecosystems.