 Models like Lande’s, and population viability analysis (PVA) more generally, often are susceptible to several criticisms (see Beissinger &amp; McCullough (2002) for a discussion of PVA, and Wimsatt (1981) for a discussion of the robustness and fragility of models). They exhibit parameter and structural sensitivity; if we change the parameter values or change the assumptions of the models, we often find very different results. For example, Patrick Foley examined a model of grizzly bears Ursus arctic in Yellowstone National Park that incorporated environmental stochasticity along with intrinsic growth rates and carrying capacity (Foley 1994). The model for estimated values of the above predicted the expected time to extinction was 12, 000 years. However, he later examined a model which included demographic and environmental stochasticity but set demographic stochasticity to zero (Foley 1997). The expected time to extinction was approximately 50 years. Thus, PVA models can be very fragile with respect to their parameter values and functional forms.