Graduate Thesis Or Dissertation
 

The impact of nutria (Myocastor coypus) on marsh vegetation in the Willamette Valley, Oregon

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  • The relation of nutria (Myocastor coypus) feeding to total abundance, species composition, and seasonal use of the marsh flora on the William L. Finley National Wildlife Refuge was studied during 1969 and 1970. Nutria numbers were estimated by livetrapping. Food habits data were compiled from observations of feeding nutria. Phenology, distribution, and abundance of the marsh vegetation were systematically studied to estimate the availability of plant species. Ten one-milacre exclosure plots were used to evaluate the relation of nutria feeding to total abundance of vegetation. Nutria densities on the refuge varied with water levels. During the winter high water periods densities were as low as 0.26 nutria per acre. During the summer nutria concentrated along permanent water areas when most ponds and streams went dry. Summer densities as high as 56.0 nutria per acre were found. Of the 40 species of plants eaten by nutria the 15 most heavily used species accounted for 81.2 percent of the 438 observations. Salix spp. accounted for 12.3 percent of the observations and was the most heavily used species. Other important food plants were Ludwigia palustris (9.3%), Sparganium simplex (8.9%), and Bidens cernua (7.5%). Forty-seven other plant species that occurred on the study area were not eaten. Forage ratios were used to express the relation of a food item in the nutria's diet to its relative abundance in the environment. Sagittaria latifolia, Polygonum hydropiperoides, and Polygonum hydropiper had the highest forage ratios and were among the least available plants. Nutria feeding significantly reduced the total abundance of vegetation and the effects of feeding were greatest under the highest populations. Nutria feeding is responsible for the disappearance of Sagittaria latifolia from the refuge. Other species are being affected to lesser degrees. The elimination of excess plant biomass, the rapid recycling of nutrients, and the creation of openings in dense vegetation are beneficial results of nutria feeding because they slow natural plant succession and the filling of the marsh.
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