Biodiversity & Conservation

, Volume 6, Issue 3, pp 477–493 | Cite as

Islands: stability, diversity, conservation

  • Q. C. B. Cronk


Islands present both a diversity and a stability ‘paradox’. They are often highly species-poor but have considerable biological interest in terms of extraordinary endemic genera and taxonomically isolated groups. They appear to be stable, as in some cases these organisms have persisted for many millions of years, and having an oceanic climate, extreme climatic events may be comparatively rare. However, when subject to extrinsic (anthropogenic) disturbance they do not appear to be stable, but often suffer catastrophic ecological change. These apparent paradoxes are resolved when it is realized that all these features are consequences of the same island characteristics: biotic isolation and oceanicity. As a result of these two characteristics, far oceanic islands 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, or products of adaptive radiation representing rapid punctuational events. 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 Helena 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 elsewhere. Island conservation strategies require an integrated understanding of both sides of the diversity and stability paradox so that both island processes and island organisms can be conserved.

palaeoendemism endemic plants climate change invasive organisms adaptive radiation. 


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  1. Bazara'a, M., Guarino, L., Miller, A. and Obadi, N. (1991) Dirachma socotrana — back from the brink? Oryx 25, 229–32.Google Scholar
  2. Bennett, K.D. (1990) Milankovitch cycles and their effects on species in ecological and evolutionary time. Paleobiology 16, 11–21.Google Scholar
  3. Beydoun, Z.R. (1982) The Gulf of Aden and N.W. Arabian Sea. In The Ocean Basins and Margins, vol. 6 The Indian Ocean (A.E.M. Nairn and F.G. Stehli, eds) pp. pp. 284–8. New York: Plenum Press.Google Scholar
  4. Beydoun, Z.R. and Bichan, H.R. (1970) The geology of Socotra Island, Gulf of Aden. Quart. J. Geol. Soc. Lond. 125, 413–46.Google Scholar
  5. Carlquist, S. (1974) Island Biology. Columbia: Columbia University Press.Google Scholar
  6. Carlquist, S. (1995) Introduction. In Hawaiian Biogeography (W.L. Wagner and V.A. Funk, eds) pp. 1–13. Washington: Smithsonian Institution Press.Google Scholar
  7. Cree, A., Daugherty, C.H. and Hay, J.M. (1995) Reproduction of a rare New Zealand reptile, the tuatara, Sphenodon punctatus, on rat-free and rat-inhabited islands. Conserv. Biol. 9, 373–83.Google Scholar
  8. Cronk, Q.C.B. (1980) Extinction and survival in the endemic vascular flora of Ascension Island. Biol. Conserv. 17, 207–19.Google Scholar
  9. Cronk, Q.C.B. (1986a) The decline of the St Helena ebony Trochetiopsis melanoxylon. Biol. Conserv. 35, 159–72.Google Scholar
  10. Cronk, Q.C.B. (1986b) Botanical Survey of Socotra for Conservation and Plant Genetic Resources. In WWF Conservation Yearbook 1985/1986, pp. 516–7. Gland, Switzerland: World Wide Fund for Nature.Google Scholar
  11. Cronk, Q.C.B. (1989) The past and present vegetation of St Helena. J. Biogeogr. 16, 47–64.Google Scholar
  12. Cronk, Q.C.B. (1990) The history of the endemic flora of st Helena: late Miocene Trochetiopsis-like pollen from St Helena and the origin of Trochetiopsis. New Phytol. 114, 159–65.Google Scholar
  13. Cronk, Q.C.B. (1992) Relict floras of Atlantic islands: patterns assessed. Biol. J. Linnean Soc. 46, 91–103.Google Scholar
  14. Cronk, Q.C.B. (1994) Extinction and conservation in the St Helena flora: the palaeobiological and ecological background. Bol. Mus. Mun. Funchal (suppl. 2), 69–76.Google Scholar
  15. Cronk Q.C.B. (1995a) A new species and hybrid in the St Helena endemic genus Trochetiopsis. Edin. J. Bot. 52, 205–13.Google Scholar
  16. Cronk, Q.C.B. (1995b) Images of nature. Nature 376, 652–3.Google Scholar
  17. Cronk, Q.C.B. and Fuller, J.L. (1995) Plant Invaders. London: Chapman and Hall.Google Scholar
  18. Dalgaard, C. (1994) Checklist of chromosome numbers counted in Madeiran flowering plants, with notes on polyploidy, life form, endemism and evolution. Nordic J. Bot. 14, 241–55.Google Scholar
  19. Darwin, C. (1859) On the Origin of Species by Means of Natural Selection. London: Murray.Google Scholar
  20. Faith, D. (1995) Phylogenetic pattern and the quantification of organismal biodiversity. In Biodiversity. Measurement and Estimation (D.L. Hawksworth, ed.) pp. 45–58. London: Chapman and Hall.Google Scholar
  21. Favarger, C. and Contandriopoulos, J. (1961) Essai sur l'endémisme. Berichte der Schweiz Bot. Gesellsch. 71, 384–408.Google Scholar
  22. Foote, D. (1995) Patterns of diversity in island soil fauna: detecting functional redundancy. In Islands (P.M. Vitousek, L.L. Loope and H. Adsersen, eds). Ecological Studies 115. Berlin: Springer.Google Scholar
  23. Francisco-Ortega, J., Jansen, R.K., Crawford, D.J. and Santos-Guerra, A. (1995) Chloroplast DNA evidence for intergeneric relationships of the Macaronesian endemic genus Argyranthemum (Asteraceae). Syst. Bot. 20, 413–22.Google Scholar
  24. Goodfriend, G.A., Cameron, R.A.D. and Cook, L.M. (1994) Fossil evidence of recent human impact on the land snail fauna of Madeira. J. Biogeogr. 21, 309–20Google Scholar
  25. Gould, S.J. (1985a) Taxonomy of death. Nature 313, 505–6.Google Scholar
  26. Gould, S.J. (1985b) The paradox of the first tier: an agenda for paleobiology. Paleobiology 11, 2–12Google Scholar
  27. Grubb, P.J. (1971) Interpretation of the ‘Massenerhebung’ effect on tropical mountains. Nature 229, 44–5.Google Scholar
  28. Gwynne, M.D. (1968) Socotra. Acta Phytogeogr. Suec. 54, 179–85.Google Scholar
  29. Harrison, S. (1991) Local extinction in a metapopulation context: an empirical evaluation. Biol. J. Linn. Soc. 42, 73–88.Google Scholar
  30. Knox, E.B. and Palmer, J.D. (1995) The origin of Dendrosenecio within the Senecioneae (Asteraceae) based on chloroplast DNA evidence Am. J. Bot. 82, 1567–73.Google Scholar
  31. McLain, D.K., Moulton, M.P. and Redfearn, T.P. (1995) Sexual selection and the risk of extinction of introduced birds on oceanic islands. Oikos 74, 27–34.Google Scholar
  32. Martin, P.S. (1984) Catastrophic extinctions and late Pleistocene blitzkrieg: two radiocarbon tests. In Extinctions (M.H. Nitecki, ed.) pp. 153–89. Chicago: University of Chicago Press.Google Scholar
  33. Milberg, P. and Tyrberg, T. (1993) Naive birds and noble savages — a review of man-caused prehistoric extinctions of island birds. Ecography 16, 229–50.Google Scholar
  34. Moser, C.K. (1918) Isle of Frankincense. Nat. Geog. Mag. 33, 266–78.Google Scholar
  35. Mosiman, J.E. and Martin, P.S. (1975) Simulating overkill by palaeoindians. Am. Sci. 63, 304–13.Google Scholar
  36. Paulay, G. (1994) Biodiversity on oceanic islands — its origin and extinction. Am. Zool. 34, 134–44.Google Scholar
  37. Percy, D. and Cronk, Q.C.B. (1996) Conservation in relation to the mating system in Nesohedyotis arborea (Rubiaceae), a rare endemic tree from St Helena. Biol. Conserv. (in press).Google Scholar
  38. Polans, N.O. (1983) Enzyme polymorphisms in Galapagos finches. In Patterns of Evolution of Galapagos Organisms (R.I. Bowman, M., Berson and A.E. Leviton, eds) pp. 219–36. San Francisco: AAAS.Google Scholar
  39. Popov, G.B. (1957) The Vegetation of Socotra. J. Linnean Soc. London 55, 706–20.Google Scholar
  40. Sandys Winsch, C. and Harris, P.J.C. (1994) Green development on the Cape Verde Islands. Environ. Conserv. 21, 225–30.Google Scholar
  41. Schilling, E.E., Pamero, J.L. and Eliasson, U.H. (1994) Evidence from chloroplast DNA restriction site analysis on the relationships of Scalesia (Asteraceae: Heliantheae). Am. J. Bot. 81, 248–54.Google Scholar
  42. Simberloff, D (1986) The proximate causes of extinction. In Patterns and Processes in the History of Life (D.M. Raup and D. Jablonski, eds) pp. 259–76. Berlin: Springer.Google Scholar
  43. Steadman, D.W. (1995) Prehistoric extinctions of Pacific island birds — biodiversity meets zooarchaeology. Science 267, 1123–31.Google Scholar
  44. Sunding, P. (1979) Origins of the Macaronesian flora. In Plants and Islands (D. Bramwell, ed.) pp. 13–40 London: Academic Press.Google Scholar
  45. Tkatsch, J. (1934) Socotra. In Encyclopaedia of Islam, vol. 4 (M.T. Houtsma, ed.), pp. 476–81.Google Scholar
  46. Towns, D.R. and Ballantine, W.J. (1993) Conservation and restoration of New Zealand island ecosystems. Trends Ecol. Evol. 8, 452–7.Google Scholar
  47. Vane-Wright, R.I., Humphries, C.J. and Williams, P.H. (1991) What to protect? Systematics and the agony of choice. Biol. Conserv. 55, 235–54.Google Scholar
  48. Vollesen, K. (1994) Delimitation of Angkalanthus (Acanthaceae: Justiceae) and the new genus Chorisochora. Kew Bull. 49, 469–79.Google Scholar
  49. Whittaker, R.J. (1995) Disturbed island ecology. Trends Ecol. Evol. 10, 421–5.Google Scholar
  50. Williams, P.H., Humphries, C.J. and Vane-Wright, R.I. (1991) Measuring biodiversity: taxonomic relatedness for conservation priorities. Aust. Syst. Bot. 4, 665–9.Google Scholar
  51. Williams, P.H. and Gaston, K.J. (1994) Measuring more of biodiversity: can higher taxon richness predict wholesale species richness? Biol. Conserv. 67, 211–7.Google Scholar
  52. Wulff, E.V (1943) An Introduction to Historical Plant Geography. Waltham, MA: Chronica Botanica.Google Scholar
  53. Wyles, J.S. And Savich, V.M. (1983) Are the Galapagos iguanas older than the Galapagos? In Patterns of Evolution in Galapagos Organisms (R.I. Bowman, M. Berson and A.E. Leriton, eds) pp. 177–99. San Francisco: AAAS.Google Scholar
  54. Zavada, M.S. and Benson, J.M. (1987) First fossil evidence for the primitive angiosperm family Lactoridaceae. Am. J. Bot. 74, 1590–4.Google Scholar

Copyright information

© Chapman and Hall 1997

Authors and Affiliations

  • Q. C. B. Cronk
    • 1
  1. 1.Royal Botanic GardenEdinburghU.K

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