Читать книгу Ecology - Michael Begon - Страница 113

As we have noted, the overall value of R, calculated from a population projection matrix (or integral projection model), reflects the values of the various elements in that matrix, but their contribution to R is not equal. We are often interested in these relative contributions, because, for example, we may wish to increase the abundance of a threatened species (ensure R is as high as possible) or decrease the abundance of a pest (ensure R is as low as possible) and wish to know, therefore, which phases in the life cycle are the most important, since it is there that should be the focus of our efforts. In fact, there have been two distinct, though related approaches to this decomposition of R.

The term life table response experiment (LTRE) was initially used to describe studies in which the varied effects of a factor, for example a pollutant, on growth, survival and reproduction were combined to generate a meaningful overall response – the effect on R (Caswell, 1989). The key here is that the pollutant exerts its directly measurable effects on the growth, survival and reproduction of individuals, but we may be most interested in the overall, combined effect at the population level, that is, R. Subsequently, and now much more commonly, the term LTRE analysis has been used to describe retrospective analysis of populations subjected to different levels of a factor, with a view to determining the respective contributions of growth, survival and reproduction to overall differences in R. The contrast is between combination in the first case and decomposition in the second.