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Effects of Migratory Geese on Plant Communities and Nitrogen-Cycling Processes Within a Coastal Salt-Marsh Ecosystem

by Amy Zacheis, Department of Biology and Wildlife and Institute of Arctic Biology

 Arctic-nesting geese often depend on feeding in spring staging grounds to acquire protein and fat reserves necessary for successful migration and reproduction. Their use affects plant communities in staging areas, and geese may destroy wetland habitat, particularly if goose populations are large or if they feed on below-ground plant parts.

The purpose of this research is to determine whether bird use has a positive, negative, or neutral effect on goose habitat in a spring staging area in Upper Cook Inlet (UCI). Information from this study can be used as a baseline to assess possible future perturbations to the system. The salt marshes of UCI are vulnerable to development and recreational use because they are close to Anchorage. They are also susceptible to the long-term effects of global warming, particularly a sea level rise. To determine whether these changes could affect migratory habitat, information on goose-ecosystem interactions is important.

This work was conducted in the Susitna Flats of UCI, west of Anchorage. Susitna Flats is an important staging area for Wrangel Island lesser snow geese and for several subspecies of Canada geese. Geese feed by grazing on new growth of the plants Triglochin maritimum, Plantago maritima, and Carex spp. In addition, snow geese grub for below-ground plant parts, particularly the roots of P. maritima.

The objectives of this project are to assess the overall impacts of use by geese on the salt-marsh plant communities and to examine the effects of trampling and defecation by geese on nitrogen-cycling processes. To address our objectives, we set up 88 pairs of matched 1-m2 plots, evenly distributed along 11 transects throughout Susitna Flats. In April of 1995, 1996, and 1997, we constructed fences around one randomly selected plot from each pair to exclude geese; the other plot acted as a control and was open to grazing by geese. We clipped plant samples and estimated plant and flower densities in August of each year, at the time of peak biomass.

In 1997, we measured rates of net nitrogen mineralization in open and exclosed plots in soil cores incubated in situ in buried bags. In addition, we measured nitrogen fixation rates of cyanobacteria growing on the surface of plots using the acetylene-reduction technique. Cores from plots were frozen and brought back to the lab for measurement of gross nitrogen mineralization (pool-dilution technique), microbial carbon and nitrogen, and organic nitrogen content.

Use by geese changed plant community composition and species abundance with no overall reduction in net above-ground primary productivity. We found that the abundance of preferred forage plants was increased, decreased, or not affected by herbivory, depending on the species and on whether above-ground or below-ground parts were consumed. There is a gradual loss of P. maritima (not detected until the third year of the study) on plots open to grazing, probably because below-ground parts are consumed. However, there is a substantial increase in C. ramenskii in open plots. Although geese may have to shift to feeding on alternative forage plants, our data do not indicate that the marshes are in danger of being overgrazed at present goose population levels.

While no differences in rates of nitrogen fixation were detected between open and exclosed plots, the rate of net nitrogen mineralization was greater in plots open to graz- ing. This indicates that use by geese increases nitrogen- cycling rates and nitrogen availability, probably through fecal nitrogen input. Increased nitrogen availability may be important to plant growth and to offset the effects of herbivory. Other soil analyses are in progress at this time.

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