PLANT-CHEMICAL DEFENSE AND TWIG SELECTION BY SNOWSHOE HARE - AN OPTIMAL FORAGING PERSPECTIVE
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| Authors: | SCHMITZ, OJ; HIK, DS; SINCLAIR, ARE |
| Year: | 1992 |
| Journal: | Oikos 65: 295-300 Article Link (DOI) |
| Title: | PLANT-CHEMICAL DEFENSE AND TWIG SELECTION BY SNOWSHOE HARE - AN OPTIMAL FORAGING PERSPECTIVE |
| Abstract: | We examined preferences of snowshoe hares for twigs of 2 winter forage plants: chemically defended balsam poplar Populus balsamifera and comparatively less defended gray willow Salix glauca. We used an optimal foraging model that explicitly incorporates the effects of secondary plant chemicals to predict foraging preferences of hares. The model predicted that the foraging preferences should be conditional: preference should depend on the ratio of digestible nutrient content in the defended and comparatively less defended plants relative to the ratio of the wet mass/dry mass of defended and comparatively less defended plants. We conducted a feeding experiment in which we presented hares fresh twigs of P. balsamifera and S. glauca (controls). We also manipulated the wet mass/dry mass ratio and nutrient contents of P. balsamifera and S. glauca twigs to alter the above condition. (1) Drying the P. balsamifera twigs, to reduce their wet mass/dry mass ratio, resulted in a two fold increase in their consumption while the consumption of the undried, S. glauca remained similar to control levels. The efficacy of the plant defense was predictably reduced. (2) Consistent with model predictions, we reversed this behavior by presenting hares with dried twigs of both species. In this case, consumption did not differ from control levels indicating the defense was again effective. (3) The defense was also rendered less effective against hare browsing by increasing the nitrogen content of P. balsamifera twigs. It appears that hares do not exclusively select twigs to maximize the intake of a limiting nutrient or avoid plants containing secondary compounds. Preference is conditional upon the relative nutritional, physical and chemical attributes of highly defended and less defended browse species available in a habitat. |
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