y] types and areas of natural distributions 205 



Endemic Areas 



In contrast with the plants exhibiting various types of discon- 

 tinuous range, and which may be widely scattered or at all events 

 polyendemic (polytopic — see next section), are those whose range in 

 each case is confined to a single restricted area, not extending beyond 

 some one region, island, or other circumscribed tract. Such plants 

 are called endemics, although this term again is largely relative. An 

 endemic area is the area of a species or other taxon that, in its dis- 

 tribution, is limited to some single natural region or habitat, the 

 history or conditions of which mark it off from others. Islands and 

 mountain massifs are particularly pertinent in this connection. 



Of endemics there are two main types. There are the old ones 

 whose range was once far more extensive than it is today, and which, 

 being remnants or survivors of former floras, may be called relic 

 endemics or epibiotics. These may make up a large proportion of 

 the species of ancient islands or mountain massifs, being said to 

 involve 72 per cent, of the thousand or so native vascular species 

 of New Zealand and 85 per cent, of those of St. Helena. A good 

 example of this type of endemism is furnished by the giant Redwoods 

 of the western United States, which used to be extremely wide- 

 spread in the northern hemisphere {cf. Fig. 43). Their drastic 

 contraction in range seems the more remarkable when we recall 

 that they include what are probably the oldest individual living 

 organisms today, some being reported to exceed 3,000 years in age. 

 The other main type of endemic is made up of the relatively young 

 taxa, usually below the rank of species, and then usefully termed 

 micro-endemics, which are characteristic of newer portions of the 

 earth's surface. Thus, when ecological conditions change within 

 the limits of some natural region, there is a tendency for new forms 

 to evolve, and these may be closely bound to the region owing to 

 its special habitat conditions or because they are physically unable to 

 spread beyond its confines. Such plants may be called neo-endemics. 



The determination of the proportion of these main types of 

 endemism in a particular flora is an important factor in its analysis, 

 capable of telling us much about its age and history. Relic 

 endemics are particularly useful in indicating antiquity, isolation, 

 and diversification of habitats, for these factors all tend to produce 

 additional endemics and help in their survival, as probably do also 

 suitable conditions for the development of vegetation. Such 

 endemics tend to be deficient in biotypes and are usually recognized 



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