Читать книгу Practical Field Ecology - C. Philip Wheater - Страница 20

In population and density studies, it is the number of plants or animals of particular species that is of importance. Such studies may look at the number per unit area (i.e. the density) or calculate estimates of population sizes. Densities are taken from the estimated population size divided by the size of the area sampled. However, for mobile organisms it may be difficult to identify the spatial limits of the population (e.g. in studies of butterflies, some species may be highly mobile with individuals not being restricted to defined small sites). Under such circumstances, densities may be less useful than estimations of population sizes of the animals using particular sites. If populations of several species are being studied, then it is important to ensure that the sampling methods used are appropriate to all the species being monitored. For example, in rainforests, some species of butterflies are found mainly within the canopy and are only occasionally caught at ground level and, conversely, some are predominant at ground level. Clearly, any survey comparing such study sites should incorporate sampling at both levels.

The spatial distribution of organisms can also be of interest, whether that is of static organisms (see Chapter 3) or more mobile individuals, populations, or communities. In the latter case, movement obviously complicates the monitoring techniques. It is often the case that individuals (and indeed populations) are restricted to defined areas either because of limits to their ability to disperse and colonise, or because of resource limitations (including competition between members of their own or other species). Biogeographical studies can provide information about distributions in geographic space and over time, whilst a landscape ecology study would look at spatial patterns over a range of scales in relation to landscape function and processes. At the smallest scale, individual home ranges and territories can be studied. Plotting the movements of animals diurnally and seasonally can indicate how individuals and groups of individuals make use of the environment in which they live.