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

by Wentz, William Alan

Abstract (Summary)
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.

Bibliographical Information:

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

ISBN:

Date of Publication:05/06/1971

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