For short-term ecological studies, an annual fitness measure based on number of gene copies that persist through time is used for estimating the effects of experimental treatments or relatively rapid environmental changes. Such fitness estimates do not necessarily reflect the success of individuals over their lifetimes, and will not reveal carry-over or residual effects of treatments later in life. Lifetime estimates of fitness are often used for estimation of longer-term influences of environmental variables. Two commonly used measures for such studies are lifetime reproductive success (LRS) and individual fitness (estimated via matrix methods). The latter estimates have the advantage of taking population demography into account.
Results/Conclusions
We give three examples of using fitness estimates to inform ecological studies, from our long-term field research on Columbian ground squirrels. The first example used a gene copy estimate to evaluate the influence of experimental ectoparasite removal on expected fitness differences of parasitized and unparasitized reproductive female ground squirrels. This example used both annual and individual fitness approaches. The second example used annual fitness to evaluate population success during 20 years of changes in climatic variables. The third example quantified the influence of kin cooperation on fitness, using an individual fitness approach to estimate direct and indirect fitness (about 60% and 40% of total fitness, respectively). We compare two methods of lifetime fitness estimation, LRS and individual fitness, the latter calculated from matrix methods. LRS does not directly account for age structure of populations and is not easily adapted to adjustment for changes in population size. Individual fitness accounts for population age structure and can be adjusted for changes in population size in a couple of different ways. Unfortunately, LRS and individual fitness show only moderate correspondence. Choice of an appropriate fitness estimate in ecological research, therefore, requires some careful thought.