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Aquatic Patch Creation in Relation to Beaver Population Trends

Johnston, Carol A., Naiman, Robert J.
Ecology 1990 v.71 no.4 pp. 1617-1621
Castor canadensis, aerial photography, animals, boreal forests, geomorphology, ponds, population growth, Minnesota
The creation of aquatic patches by beaver (Castor canadensis) in the boreal forest of northern Minnesota, USA, was studied to determine how the population dynamics of a disturbance—causing animal are linked to rates of patch formation and growth over a period of population expansion and stabilization. Using six series of aerial photographs taken between 1940 and 1986, we determined the size and growth rates of individual patches, and the numbers, area, density, and establishment rate of the patch population. The rate of patch formation was much higher during the first two decades of colonization than during the subsequent two decades. The average area of all ponds sites, which included both filled and drained ponds, remained at °ha throughout the time period, but the average area of new ponds decreased significantly over time. Ponds established by 1961 constituted 75% of the total number and 90% of the total pond—site area as of 1986. When pond sites of similar age but different pond cohort (i.e, decade of establishment) were compared, the average area per pond site was always significantly larger for the earlier cohort. Although the rate of pond creation paralleled the increase in number of beaver colonies between 1961 and 1986, the rate of new pond creation prior to 1961 greatly exceeded the increase in number of beaver colonies. We conclude that the rate of patch formation after the first two decades of beaver colonization was constrained by geomorphology, which limited the availability of sites at which a beaver dam could impound a large area of water.