Academic journal article The American Midland Naturalist

Seasonal Variation in Assimilated Diets of American Beavers

Academic journal article The American Midland Naturalist

Seasonal Variation in Assimilated Diets of American Beavers

Article excerpt

ABSTRACT.-American beavers (Castor canadensis) forage on various aquatic and terrestrial plant species. We used stable isotope analysis of carbon (C) and nitrogen (N) to estimate source contributions of seasonal assimilated beaver diets in Voyageurs National Park, Minnesota, from Apr. 2007 to Nov. 2008. Mean (±95% confidence interval) annual beaver diets were estimated as 45.5 ± 11.4% terrestrial and 55.5% aquatic vegetation (22.0 ± 14.5 emergent and 33.5 ± 7.9 floating leaf). Percentages of floating leaf and terrestrial vegetation were similar between winter and summer assimilated diets, but emergent vegetation increased 45% in summer. Although δ^sup 15^N was 7% greater in summer, δ^sup 15^N and δ^sup 13^C were similar by age class and sex, as were assimilated percentages of emergent, floating leaf or terrestrial vegetation. Variation in total assimilated aquatic vegetation did not affect subadult and adult seasonal changes in body mass, tail thickness or tail area, but kit body condition was negatively related to total assimilated aquatic vegetation. Aquatic vegetation accounted for more assimilated diet during winter than previously reported.

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INTRODUCTION

American beavers (Castor canadensis) are classified as choosy generalists (i.e., they select for certain species from among many species consumed; Baker and Hill, 2003). For example, beavers in Massachusetts selected deciduous species over coniferous species (Busher, 1995). Beaver diets vary seasonally, with terrestrial and aquatic herbaceous vegetation consumed when available and woody vegetation typically consumed during winter (Belovsky, 1984; Roberts and Arner, 1984; Baker and Hill, 2003). For example, winter diets, mostly derived from food stored in a cache (Baker and Hill, 2003), were 70 to 90% woody vegetation (Ohio: Svendsen, 1980; Mississippi: Roberts and Arner, 1984), and summer diets were 30 to 50% aquatic vegetation (Svendsen, 1980).

Terrestrial and aquatic vegetation contain similar calories (Gorham and Sanger, 1967; Belovsky and Jordan, 1978), but aquatic leaves, stems, and tubers generally have higher digestibility (Belovsky, 1984; Doucet and Fryxell, 1993); higher mineral and protein content (Fraser et al., 1984); and lower amounts of cellulose, lignin, and secondary metabolites (Doucet and Fryxell, 1993). Thus, aquatic vegetation offers higher rates of nutrient assimilation than terrestrial forages, and may therefore be a crucial part of beaver diets. Beaver total body mass and tail size fluctuates seasonally as fat stores are depleted through winter, and beavers must gain mass over summer to offset winter losses (Aleksiuk, 1970; Smith and Jenkins, 1997). Higher relative assimilation of aquatic vegetation could facilitate Ulis necessary increase in body mass and fat.

Beavers require less search time and incur lower risk of prédation when foraging on aquatic vegetation than when foraging on terrestrial vegetation (Doucet and Fryxell, 1993). For example, beavers obtained 2.5 times more food per unit effort when foraging on aquatic vegetation (Belovsky, 1984), ostensibly safe from terrestrial prédation. Predators of beavers include the gray wolf (Cants lupus), coyote (C. latrans), and bears (Ursus spp.; Voigt et al, 1976; Smith and Peterson, 1988; Smith et al., 1994; Baker and Hill, 2003). Beavers may perceive direct or indirect cues from large terrestrial predators (Severud et ai, 2011), and in response increase foraging on aquatic vegetation or terrestrial vegetation closer to shore (Smith et al, 1994). Time spent foraging on aquatic vegetation during summer was higher for kits and subadults than for adults (Svendsen, 1980), and may in part reflect differential vulnerability to prédation. Males and females of all age classes consume similar diets year round (Roberts and Arner, 1984).

Stable isotope analysis of carbon (C) and nitrogen (N) can be used to reconstruct diets (Kelly, 2000), with isotopie signatures of herbivores reflecting the stable isotope ratios of plants assimilated (Stewart et al, 2003). …

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