The methods for estimating the size of slug populations may be divided into three types. Absolute estimates are expressed as numbers per unit area of habitat (i.e. density) and give the most reliable estimate of the size of the population. However, they tend to be time consuming and laborious. Relative estimates are usually related to some measure of slug activity, such as the catch per unit effort (e.g. time searching) or numbers obtained by some form of trapping. Since slug activity is largely governed by weather, relative estimates measure a variable and unknown fraction of the population. However, they still allow comparisons to be made of the spatial and temporal distributions of slug populations. Relative methods usually require only simple equipment and can be carried out more rapidly and enable more extensive investigations to be made. Finally, population indices can be calculated based on some artefact of the population. In the case of slugs, for example, the extent of damage to groups of wheat grains or potato tubers has been used. It is sometimes possible to relate relative estimates and population indices to absolute numbers, using regression analysis, provided the estimates are made simultaneously. Relative estimates or population indices may then be used for subsequent estimates, converting them to absolute numbers using correction factors.
KeywordsDigestive Gland Woodland Habitat Ancient Woodland Ground Flora Mixed Woodland
Unable to display preview. Download preview PDF.