This chapter initiates the third section of this book, illustrating applications of the GMM model to particular issues in herbivore ecology and management. We address first the assessment of habitat suitability, as manifested by the abundance or performance of a herbivore population in a particular region. Wildlife managers usually think of the habitat ‘carrying capacity’, or maximum population that can be sustained. Livestock ranchers seek the optimal stocking density that would yield the highest production of meat, wool or other products. Caughley (1976b) termed the former ‘ecological carrying capacity’, the latter ‘economic carrying capacity’. Theoretical ecologists symbolize the zero growth density by the constant K in the logistic equation (May 1981), while acknowledging its vague reality. Ecological analysis has focussed largely on the feedbacks regulating populations around some density (Sinclair 1989). Less attention has been paid to the environmental determinants of the density attained.

The basic utility of the ‘carrying capacity’ concept was questioned by McLeod (1997) for real-world environments where population abundance fluctuates widely over time. Is it represented by the mean density? The peak density attained between disrupting events? Or by some remote upper level, rarely reached (cf. Ellis and Swift 1988)? Densities also differ widely regionally, and change numerically with enlargements in the scale of the area encompassed (Pastor et al. 1997).

Many factors contribute to habitat suitability, including not only the availability of suitable food and other resources, but also shelter from extreme conditions, and security against predators and other hazards.