Before one starts

Objectives

The definition of objectives is particularly important in studies of the abundance of animals or plants because it will determine whether one needs to make a full count of the individuals present in an area (or, at least, an estimate of that number) or whether an index of numbers is satisfactory. By an index, we mean a measurement that is related to the actual total number of animals or plants – such as the number of eggs of an insect species found on a sample of leaves of its host plant (as an index of numbers of adult insects) or the number of rabbit droppings in a sample area (as an index of the number of rabbits). Because accurate counts are extremely difficult to make, one may have to make do with an index even when a count would be preferable. Indeed, so long as an index is sufficient for one's purposes, a reliable index is preferable to an unreliable count.

The objectives will also determine the extent to which a population should be divided by sex, age or size in one's study: some work on the structure of communities may require only that total numbers of each species are known, whereas population dynamicists will often need to know the composition of the population by sex and age.