John A. Litvaitis
Why study food habits? Probably one of the most fundamental questions that ecologists attempt to answer is, "What resources does a particular species require to exist?" Indeed, the first principle among wildlife ecologists is to have a thorough understanding of the food, cover, and water requirements of an animal before initiating any effort to alter the factors that may be limiting it. Information on food habits is therefore an important introduction to the natural history of any species. This has been a justification for many studies of food habits of vertebrates (Martin et al. 1961) and is still a valid reason to investigate the diet of any species when little information is available (Salas and Fuller 1996). Food habits have been investigated for a variety of other reasons. Such information is essential in understanding the potential competitive interactions among sympatric species (Jaksic et al. 1992; Wiens 1993) or in determining how the foraging patterns of individuals affect community composition. For example, how does grazing by wildebeests (Connochaetes taurinus) affect the diversity of grasses and forbs? Does predation by lions (Panthera leo) limit that same wildebeest population? A simple list of foods used by wildebeests or lions will not answer these questions. However, determining the biomass consumed and abundance of alternative forage or prey is an important first step in understanding how these two species influence community composition.
In human-dominated landscapes, information on the food habits of common terrestrial vertebrates has been useful in understanding the "economic food niche" of many species. Losses of livestock, agricultural crops, or game populations are serious economic concerns. Limiting these losses is a major charge of government wildlife management agencies. Historically, efforts to control depredating wildlife have included indiscriminate attempts to reduce populations of the offending species. Well-known examples of such an approach include the efforts to reduce large carnivores in North America. Detailed investigations have revealed that the actual nuisance individuals may be only a portion of the population and depredation may be restricted to a limited time period (Till and Knowlton 1983). As a result, control efforts can be more exact and cause less ecological damage.
A recent motivation to examine animal food is the emergence of environmental assessments during large-scale habitat alterations, such as road construction and commercial timber harvesting. Biologists must identify the important food and cover resources in an affected area if they are to mitigate the effects of these activities.
Regardless of the specific question being addressed, nearly all investigations of food habits can be distilled to two basic questions. What is the importance of a specific food to the fitness of an organism (especially survival and reproductive success)? How does the feeding niche of an organism affect community composition? In this chapter I compare the relative effectiveness of methods commonly used to investigate food habits and how appropriate each method is at answering these questions. I also consider several recent innovations and how potential improvements may improve our ability to understand the significance of food usage. Readers interested in learning more about the actual procedures should consult references by Korschegen (1980), Cooper-rider (1986), Reynolds and Aebischer (1991), and Litvaitis et al. (1994).
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