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Article Excerpt
American beaver (Castor canadensis Kuhl) occurs naturally in nearly every available aquatic habitat in North America, with the exceptions of the arctic tundra, high montane environs, southwestern deserts, and peninsular Florida (Hall 1981; Jenkins & Busher 1979). Beavers have been categorized by diet as vegetarians, choosy generalists, opportunistic herbivores, or obligate vegetarians (Johnson 1927; Harper 1969; Jenkins 1978, 1981; Busher 1996; Baker and Hill 2003; Schmidly 2004). They consume a variety of plant parts (bark, shoots, foliage, nuts, and roots) but preferentially select foods based on species, size class, distance to source, and nutritional quality (Jenkins 1975, 1980, 1981; Pinkowski 1983; Belovsky 1984; McGinley & Whitham 1985; Fryxell & Doucet 1991; Fryxell 1992; Doucet & Fryxell 1993). Beavers prefer a greater variety of herbaceous vegetation in their diet than woody plants during all seasons (Henderson 1960; Jenkins 1979, 1981; Svendsen 1980; Roberts & Arner 1984), but woody plants are the limiting dietary factor for their presence on first through third order streams with moderate to low flow rates (Slough & Sadlier 1977).
Woody plants in beaver diets vary geographically relative to plant availability. In northeastern North America, beavers preferred aspen (Populus tremuloides), and Johnson (1927) suggested an evolutionary relationship existed between the geographic origin of C. canadensis and Populus sp. Beavers frequently fed on yellow poplar (Liriodendron tulipifera), American hornbeam (Carpinus carolinia), red maple (Acer rubrum), and sugar maple (A. saccharinum) in the Midwest (Svendsen 1980). Willow (Salix sp), poplar (Populus balsamifera), and alder (Alnus crispa) were common foods in the diet of beavers in the Canadian Northwest Territories (Aleksiuk 1970).
Beavers rely on woody vegetation for the majority of their diet from October through April (Svendsen 1980; Hill 1982; Roberts & Arner 1984). Denney (1952) ranked preference of woody plants by beavers in North America as aspen, willow, cottonwood (Populus deltoides), and alder. However, caution is warranted when making conclusions relating to preference; extensive use does not imply preference when a greater diversity of trees produces complex patterns of food selection (Jenkins 1981). Little is known about beaver food preference for most regions of its range, especially southern populations, with only relative food preferences documented for a few areas of northeastern and western North America (Jenkins 1981). Beavers select forage resources at different levels of resolution, but little information is available concerning species preference, selective foraging effects on structure and composition of riparian vegetation, and the mechanisms involved (Jenkins 1981).
As a species portraying aspects of central place foraging theory, beavers exert choices upon a variety of prey sizes. Studies on prey size selection by foraging beavers in various habitats suggested selection for trees of smaller diameters with increasing distance from water (Jenkins 1980; Pinkowski 1983; Belovsky 1984; McGinley & Whitham 1985; Donkor & Fryxell 1999, 2000; Gallant et al. 2004). Donkor and Fryxell (1999, 2000) found fewer stems selected by beavers as a function of distance from water, and Fryxell and Doucet (1993) demonstrated size selection for aspen and alder varied as availability changed. Basey et al. (1988) documented that the greater selectivity for prey sizes displayed by beavers was consistent with central place foraging theory (Fryxell & Doucet 1991).
There is a paucity of information on foraging and food habits for beavers in Texas. Only general information on the variety of foods consumed by beavers is available (Schmidly 2004), and this information is restricted to winter foods of beavers in central Texas. The objectives of this study were to assess forage preference of woody vegetation by beavers, determine size class preference, and investigate niche breadth with respect to forage resources on the San Marcos, Blanco, and San Gabriel rivers in central Texas.
METHODS
Study area. -- The study was conducted in the riparian zone along 8 km stretches used as foraging sites by beavers on the San Marcos (Hays County), Blanco (Hays County), and San Gabriel (Williamson County) rivers in central Texas (Fig. 1). These rivers originate near the southeastern perimeter of the Edwards Plateau Ecological Region and flow southeasterly across the Balcones Escarpment into the Blackland Prairie Ecological Region (Gould et al. 1960). Riparian zones of these rivers vary in width based on the extent of adjacent land-use practices.
The San Marcos River is a spring-fed, perennial stream with a relatively constant flow and temperature (Tupa & Davis 1976, Groeger et al. 1997). The study area extended from Spring Lake in San Marcos (29[degrees] 52' 23" N, 97[degrees] 56' 06" W) to the confluence of the river with the Blanco River. Along this stretch, the floodplain was slightly rolling with relatively steep river banks. The river was moderately used for recreation throughout the year, but the stretch within the city was heavily used, especially in spring and summer. Outside the city, land-use adjacent to the river was agricultural. Woody species of the riparian zone included bald cypress (Taxodium distichum), black willow (Salix nigra), American and cedar elm (Ulmus americana, U. crassifolia), pecan (Carya illinoensis), chinaberry (Melia azedarach), hackberry (Celtis sp.), eastern cottonwood (Populus deltoides), boxelder (Acer negundo), sycamore (Platanus occidentalis), privet (Ligustrum...
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