Area and Endemism

There are three major difficulties encountered by those dealing with the phenomenon of endemism: a semantic problem, the absence of a clear conceptual framework, and an analytical problem. First, the terms endemic and endemism are used in the literature in unclear or contradictory ways. Often neither the title nor the abstract of an article makes the meaning clear. Following the usage that tends to prevail among Anglo-American zoogeographers, a species or other taxon is regarded here as endemic to an area if it occurs only in that area. To speak of a taxon as endemic in this context without specifying an area is meaningless. Since geographic ranges of taxa change with time, time must also be specified, or at least understood. Second, a conceptual model is provided in which only changes in ranges (occuring frequently) and speciation (occuring relatively rarely) are seen to change the percentage of endemism in any given area. At a subsidiary level, many complex factors influence areographic changes and speciation. Among the more important of these are: distance from source to target area, size of area, geological age of area, time since isolation, environemntal variety and stability, and vagility and ecological tolerance of organisms being considered. These are not all independent factors. This complexity leads to the third and still largely unresolved problem namely how to analyse a global biological system involving processes on both a shorter-term ecological time scale and a longer-term evolutionary time scale.