BIO 360: Ecology

Lecture 31: Habitat destruction and loss of biodiversity

I.  The biodiversity of islands

    A.  A consistent pattern for many island archipelegos is the positive relationship between island area and diversity

        1.  The relationship can be represented mathematically as S = cA^z, where S is the number of species, A is the area of the island, and c and z are constants describing the slope and intercept of a line on a log-log plot


       2.  Bird extinctions in the eastern US

                Assumed z = 0.25, 50% forest loss
                Predicted 26 extinctions, an overestimate (4 actual)
                Not all species required deciduous or coastal pine forest
                Predicts local extinctions, not global extinctions
                Considering only species restricted to eastern US (28 species), 4/28 = 14% of species went extinct
                16% of species went extinct

        3.  Another consistent pattern on islands is that more remote islands (distant from other islands) have fewer species


    C.  Island Biogeography

        1.  More species on large islands and on less isolated islands

        2.  Less species on small islands and on more isolated islands

        3. MacArthur and Wilson (1967) theorized that these differences could be explained by differences in colonization of and extinction on islands of different sizes

        4.  The number of species on an island is at the point where loss due to extinction balances the gain due to immigration

            -- This can be represented graphically by plotting the number of species on the x axis, and the rate of immigration and extinction on the y axis.
            -- The rate of immigration decreases with number of species already on the island.
            -- The rate of extinction increases with the number of species on the island

        5.  Wilson and Simberloff tested this hypothesis by first eliminating all insects on small mangrove islands, and then observing recolonization of the islands.

           - Measured insect diversity
           - Eliminated all insects
           - Measured insect diversity over time
           - Observed “equilibrium level of diversity”

II.  Applications to conservation biology

    A.  Keep in mind that there are two aspects to habitat loss
        1.  Loss of habitat area

        2.  Habitat fragmentation, or the division of habitat area into smaller, more isolated fragments

    B.  These observations, and the theory of island biogeography, have led to many new insights for conservation of biodiversity

    C.  Does the theory of island biogeography apply to "islands" of habitat within continents?

                Reserve area does affect populations in reserves

                Degrading effect of fragmentation accumulate over decades

                Extinction Debt   Extinctions may occur long after habitat loss