Marginal areas can be defined superficially in terms of geographical isolation. However, the range of peripheral situations increases when biological boundaries are substituted for the frontiers of physical geography. The sedentary nature of vegetation and the facility with which distribution can be mapped makes the study of limits to plant survival an unending subject of enquiry and speculation. Habitats become marginal for plant populations whenever survival is threatened. Consequently, peripheral areas for plant survival can be defined biologically in terms of demography, physiological requirements, and genetic variation. Each of these perspectives makes its own assumptions as to why plant populations are limited in their distribution and what constitutes a marginal area. In seeking explanations in terms of cause and effect it is therefore possible that more than one explanation may be valid, as in different situations the same species may be limited by contrary causes.
Physiological limitations are a fundamental aspect of survival but are not always readily observed in natural situations due to interference from other species competing for space or resources and oscillations in environmental conditions. If long-term observations are possible, demographic data may discern the causes for failure of populations to maintain their numbers.
Genetic studies, especially when combined with historical information concerning sites and past distributions, can provide equally cogent explanations for distribution limits. Any discussion of marginal areas has therefore to accept that in any particular case study it is probable that there will be more than one kind of limitation. In short, the recognition of boundaries presents many possibilities and depends in large measure on what the observer is capable of seeing.
Fig. 2.1 Biodiversity in a marginal area as seen in a Norwegian mountain pasture.
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