Much the same factors which appear to determine the distribution of plant species are relevant to the distribution of animals. The proximity of the Continent of Europe to the eastern chalk in Kent and Sussex is undoubtedly related to the particular richness of the fauna in these counties. Relatively few species seem to be more numerous or only present in western Britain, although the insects of the Cornish peninsula, particularly associated with vegetation of the calcareous maritime serpentine rocks at the Lizard, probably include many species which only occur in this region in Britain. Species such as the Small Blue butterfly Cupido minimus are undoubtedly commoner in Western Britain than in the East.
Many animal species have distributions in Britain which are markedly southern. Some occur only along the south coast of England, others are not found north of the Thames while others again occur only up to the Midlands. It is often difficult, especially in the case of predacious species, to determine whether these are restricted to limestone formations or not. Limestone in southern England warms up quickly and shares with arenaceous and cindery substrates an abundance of thermophilous animal species which is not found so frequently on the colder clay soils.
The richness of phytophagous animals on limestone in southern Britain is frequently easily related to the occurrence of particular foodplants and the abundance of these. It seems probable that the physical characteristics of limestone rocks and soils, the occurrence of these rocks and soil abundantly in southern England, and the growth and diverse vegetation on limestones are the most important factors in determining the distribution and abundance of the animals. Chemical factors are of over-riding importance in some cases, for example the occurrence of many terrestrial Mollusca. The proximity of the sea seems to be a powerful modifying influence, with many maritime species being particularly associated with limestone cliff tops and cliff faces.
The history of grasslands in Britain whether in the long term (i.e. throughout the Pleistocene), medium term (Prehistoric) or recent (Historic) has been important in determining the composition of the fauna. Palaeoecological studies have shown how different the fauna of Britain has been in recent geological time. The creation of grasslands from the primaeval forest is reflected in the fossil faunas collected from different sites. In historic times the introduction of the Rabbit (Oryctolagus cuniculus), and the extinction of the Great Bustard (Otis tarda) and near extinction of the Large Blue_ butterfly (Maculinea anion) are examples of the greatly modifying influence directed by human activity on the fauna of calcareous grasslands.
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