Population biology and life-history variation of gamebirds.
This chapter considers further aspects of the population biology of gamebirds and discusses how some of the patterns observed might have evolved through the process of natural selection. Ecologists usually consider such studies under the general heading of 'life-history strategies'. Essentially, the studies aim to ascertain how different species maximise their lifetime reproductive success, defined as the total number of offspring produced within its lifetime, that survive to the age of first reproduction. As this is determined by a variety of different facets of the species' ecology, many life-history studies are concerned with whether there are consistent trends in the way that survival and fecundity rates vary between and within species.
The study of how natural selection has tuned the survival and fecundity of different species is particularly important in the case of managed populations. Many aspects of management aim to increase or decrease certain population parameters in relation to those that have evolved for the species in the unmanaged state. For example, hen pheasants are caught and placed in laying pens where they produce large numbers of eggs which are then incubated artificially (35 eggs per female is not uncommon). Such a level of egg production is much greater than the average clutch size recorded for 'wild' pheasants. Management of wild populations also tends to increase population densities to levels far greater than in the unmanaged habitat. On some estates red grouse have been recorded at densities of 115 pairs km-2, while in the natural state density is probably closer to 3 pairs km-2 (Hudson and Watson, 1985). In biological terms management may be thought of as a subtle form of experimental manipulation that reveals more about the population biology of the species. For example, populations which are harvested at different intensities may be used to determine whether increased mortality leads to increases in fecundity or changes in the rates of dispersal.