ISLANDS - STABILITY, DIVERSITY, CONSERVATION

Islands present both a diversity and a stability 'paradox'. They are o ften highly species-poor but have considerable biological interest in terms of extraordinary endemic genera and taxonomically isolated group s. They appear to be stable, as in some cases these organisms have per sisted for many millions of years, and having an oceanic climate, extr eme climatic events may be comparatively rare. However, when subject t o extrinsic (anthropogenic) disturbance they do not appear to be stabl e, but often suffer catastrophic ecological change. These apparent par adoxes are resolved when it is realized that all these features are co nsequences of the same island characteristics: biotic isolation and oc eanicity. As a result of these two characteristics, far oceanic island s are quantitatively different from continental systems in the nature of their ecological processes, which appear to give rise to an extreme punctuated equilibrium model of evolutionary change. Endemics may be ancient relict endemics displaying prolonged stasis and persistence, o r products of adaptive radiation representing rapid punctuational even ts. A process-based definition of a relict endemic (palaeoendemic) is one whose founding lineage (i.e. the original continental source taxon ) has not left any descendents. A corollary of this definition is that the time of divergence between an endemic and its continental sister- group should predate the colonization of the island by the now endemic lineage. An example is Dicksonia arborescens which has been on St Hel ena for at least 9 Myrs and no longer occurs in the likely source area of Africa. These relict endemics, frequent on islands, are important as the last remnants of tranches of biodiversity that have vanished el sewhere. Island conservation strategies require an integrated understa nding of both sides of the diversity and stability paradox so that bot h island processes and island organisms can be conserved.