Relationship Between Area and Species
The type of distribution of plants living together in a space is called sociability. Developing patterns depend on the dispersal mechanisms of
Characterisation of areas
The study of areas considers the results of plant dispersal, the plant species distribution. Distribution maps (mainly individual point distributions or grid maps) show the areas occupied by the species; these areas are characterised according to size. Plants distributed across climatic zones, over very large areas, are called cosmopolitans. They are often phy-logenetically old species with particularly effective dispersal mechanisms having no specific environmental requirements. Species occurring in microspaces, e.g. only on an island or in an isolated mountain area, are called endemics. Areas are also characterised by their form with particular distinction drawn between closed and disjunct distribution areas. In the latter, the area of distribution consists of several partial areas. Disjunct areas may be explained by climate changes and tectonic events. Disjunct areas also show that they are subject to changes over time (dynamic); they can grow if the local conditions change in their favour, or shrink if the opposite occurs.
Species occupying similar areas are characterised as area types (floristic elements or geo-elements; e.g. Atlantic or boreal types). Area types are the basis for the organisation of large floristic provinces and regions, and finally of the six plant kingdoms of the earth, the northern hemisphere Holarctic, the Palaeotropics and Neotropics, as well as the southern hemisphere Australis, Capensis and Antarctic (see Fig. 4.2.11).
participating species, rates of reproduction, competitiveness, etc. In nature, clumped distribution is particularly frequent, i.e. distribution is not uniform, rather an "island-like" distribution exists, where larger or smaller spaces are covered more densely, so that there are differences in abundance. Ecologists seeking the origin of such developing patterns need to know how many species (and individuals) are able to live in a certain space, and whether - and if yes, which - interactions exist between number of species and size of area, the so-called species area interaction.
Interactions of the process of biological distribution, establishment of organisms and the size of areas have been particularly analysed for islands. Islands have fewer species than mainland areas of the same size, as in the latter plants are able to quickly establish from the surrounding area. Due to their spatial isolation, islands are rich in endemic species and are therefore special cases. They are clearly delimited, easy to comprehend, with a limited number of different habitats under relatively uniform climatic conditions. They occur in various sizes and are situated at different distances from the mainland and therefore serve as examples to clarify the following basic relations between number of species and size of area:
• How do plants establish on islands?
• What limits the number of species on islands?
The first question is relatively easily answered. Vectors for dispersal on islands situated far from the mainland, and thus from the closest source of propagules, can only be by long-distance dispersal; various types of autochory are not applicable. Transport by sea birds on various established routes may be a method of dispersal (particularly epichoric dispersal), but also abiotic vectors are possible, such as air and sea currents, or exceptional events, e.g. tropical cyclones. For the Pacific islands and groups of islands the proportion of participating vectors was estimated with morphological characteristics of propagules. On islands near the mainland birds are very important, for larger distances movement by wind and seas are dominating. Propagules are carried to islands from all directions. Almost half of all species on Hawaii originate from the Indo-Pacific region, more than a quarter from the Holarctic and 17% originate from cold temperate southern hemisphere areas (Fenner 1985).
It can also be shown how plants establish on islands; Arrhenius (1921) expressed the relation between the size of an island and number of species by the following equation:
where S is the number of species of a taxon on the island, A the area, z a parameter without dimension (constant), which changes little worldwide (slope of the linear regression, when log S is plotted against log A, values between 0.17 and 0.4). C is the gradient dependent on the dimensions in which A was measured, the biogeo-graphical area and the taxonomic group.
The relationship between species and area shown in equation 4.2 results in the fact that the number of species of the group considered is halved if the area is reduced by a factor of 10. This attempt to provide a mathematically comprehensible theoretical expression relating the number of species to the size of island is one of the important bases of the island biogeography steady-state theory. Darlington (1957) proved this empirically for reptiles on West Indian islands, and Johnson and Simberloff (1974) confirmed this for plant species of the Scottish Islands (Fig. 4.2.10). These relationships are explained by two hypotheses. The habitat diversity hypothesis (Gorman 1979), suggesting that on larger islands there is a greater number of diverse habitats, and the area alone hypothesis (Kohn and Walsh 1994) assuming a direct relationship between island size and number of species.
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