Conservation of Biological Diversity in Botanical Gardens
Conservation is already, and very appropriately, recognized as being a major activity for botanical gardens in both their research and educational programs. In this field, arboreta and botanical gardens have a particular and important potential, which I discuss in this chapter.
In nature, plants frequently exist in small populations. Examples include many rare endemics, such as those of mountain peaks and many in the Mediterranean dry sclerophyll scrublands, especially in the Cape Province of South Africa and in Southwest Australia, and those of certain rain forests. Over the relatively short time we realistically have had to work as conservation managers, extremely small stands have been found to persist in nature. Higher plants, being sedentary, are often highly site-specific. This facilitates the development of logical plans for demarcating minimal areas for in situ conservation based on ecological knowledge and principles of island biogeography. On the whole, the most favorable sites are a few environmentally heterogeneous reserves of sufficient size to minimize edge effects (e.g., changes in species composition at the periphery caused by in- and out-migrations from adjacent unprotected lands). Ideally, these would be loosely connected by small stepping stones or corridors to allow for the exchange of genes (Diamond, 1975). Identification and immediate protection of sites of high conservation value must be our highest priority in the absence of even the grossest information upon which to base plans, including basic inventory as well as distributional and ecological data on many of the richest biota. This underlines the vital necessity of increasing inventory and ecological information as a prerequisite to developing any logical plan for conservation. In practice, of course, the luxury of regional planning often does not exist. The conservationist only succeeds in raising awareness when the plant is reduced to endangerment in one or a few isolated localities or, at best, is offered a patchwork of lands for which the farmer and the planner have failed to find other uses. As development proceeds and natural habitats become increasingly fragmented, extinction accelerates (Wilcox and Murphy, 1985). The most endangered floras are those of the arable lands; the current distribution of preserves takes little account of this.
Even when the luxury of time for planning does exist and centers of species richness and endemism can be identified and conserved, many locally endemic plant species refuse to follow the rules and occur in isolated areas where, overall, conservation priorities are low. Even under ideal circumstances, though, decreases in and fragmentation of natural areas is certain to lead, as predicted by the theory of island biogeography, to substantial increases in extinction rates, though it is uncertain whether these rates would be on the scale calculated for large animals (e.g., Schonewald-Cox, 1983; Soulé et al., 1979; Simberloff and Abele, 1984). This is therefore a case for some form of selective program of ex situ conservation, that is, conservation through cultivation.