Looking backwards: Assessing the projections of a transition matrix model

Analyses of population projection models are increasingly being used by con servation biologists and land managers to assess the health of sensitive sp ecies and to evaluate the likely effects of management strategies, harvesti ng, grazing, or other manipulations. Here I describe some of the limitation s of this approach and illustrate how these limitations may affect its usef ulness. I do this by comparing the results of such an analysis, performed i n 1979 on two populations of a perennial plant, Arisaema triphyllum, with n ew information about the size and structure of these same populations gathe red in 1994, 15 years later. While one population changed as the model proj ected it would, the other behaved quite differently from the projection. In stead of increasing in size, this population decreased between 1979 and 199 4. Possible shortcomings in the data and in the model include: too few plants to provide accurate transition probabilities; too few years to capture accu rately the complete range of year-to-year environmental variability; and th e failure of the most commonly used form of the model to account for densit y-dependent vital rates. In addition, the asymptotic growth rates (lambda) these models yield may sometimes be irrelevant and even misleading if one's primary interest is in a population's short-term prospects for survival, a s is often the case in studies of sensitive species. These shortcomings may apply to many studies involving the use of projection models, and they hav e important implications for the value of this approach in conservation bio logy and species management decisions.