Gaps in maps: disjunctions in European plant distributions

Abstract

Abstract Maps of species distribution are normally used for information regarding occurrence. Valuable as such information is, the converse of knowing where species do not occur can also be meaningful. This is particularly the case where there are no obvious reasons for absence. There is a long history of hypotheses in botanical writing as to the causes of gaps in distribution, which vary in their geographical dimensions from limited local absences to more extensive non-occurrences. Examples of this latter situation are usually described as disjunct distributions. Some of the commonest disjunctions are found in the distribution of the species commonly referred to as Arctic-Alpines. The evolutionary relationship between the occurrences of these species in the Arctic as compared with more southern montane habitats has long been a source of speculation. The main questions have been whether or not these disjunct distributions are merely accidents of dispersal or the remnants of former widespread and contiguous distributions that have suffered either from physiological failure or other aspects of evolutionary maladaptation to a changing environment. The advent of molecular methods and their role in the discovery of previously unsuspected migration routes now makes it possible to reconsider the causes of some of these gaps.