The impact of nutria (Myocastor coypus) on marsh vegetation in the Willamette Valley, Oregon
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
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.
Advisor:Kuhn, Lee W.
School:Oregon State University
School Location:USA - Oregon
Source Type:Master's Thesis
Keywords:coypu marsh ecology freshwater plants oregon willamette river valley
Date of Publication:05/06/1971