PARENT SESSION
Oral Session #38: Herbivory: Herbivore Response to Plants. Presiding: P. Kleintjes.
Tuesday, August 7, 2001. 1:00 PM to 4:30 PM. Hall of Ideas J.
Gain functions for blue duikers in small patches: Does plant geometry matter?
Vandervelde, Thea1, Shipley, Lisa1, Searle, Kate2, Hobbs, N.2, Wunder, Bruce3, 1 2 3
ABSTRACT- Foraging by herbivores is often characterized by periods of movement interrupted by pauses for feeding. We define small patches as the area where animals feed during these pauses. Marginal value theory has been widely applied to predict residence time of herbivores in small patches. However, this application depends on the function that relates cumulative energy gains to patch residence time. In particular, this function must be smoothly decelerating. We conducted experiments to examine the strength of evidence for linear, stepped, and decelerating non-linear models representing the cumulative gain of dry matter by blue duikers as a function of their residence time in small patches. In particular, we asked the question: 'How does the geometric arrangement of plant biomass in small patches affect the support for alternative gain functions?' We offered a constant biomass of fresh alfalfa to experimental animals in small patches while varying the number of plants composing that biomass. The strength of evidence for non-linear models was greatest for small patches composed of a few, large plants, while support for stepped and linear models was greatest for patches composed of many small plants. The mechanism causing deceleration in non-linear and stepped models appeared to result from decreases in bite mass cropped as patch biomass was depleted. We conclude that plant geometry will affect the ability of marginal value models to portray herbivore decisions on patch residence time.
KEY WORDS: foraging, model selection, herbivore, gain function