ABSTRACT
In biogeography, environmental tolerances and limits are often deduced from an examination of a species distribution map in conjunction with maps of environmental variables such as isotherms and isohyets. A number of butterflies and moths have distributions associated with downland pastures, developed particularly on the chalk outcrops of southern England. For organisms in tidal habitats there is the added complication of deciding whether or not their distributions are controlled by aerial or marine environmental factors. Nobody would deny that environmental factors such as temperature and salinity have a profound affect on species distributions. Simple causal relationships imply the independence of environmental factors, which is rarely the case. Sea temperature is clearly the most important environmental factor influencing marine distribution patterns. The high concentration of dissolved salts in the marine environment means that both plants and animals must have osmo-regulatory devices to limit the loss of water from their tissues into the surrounding media.
