Number of Species
Figure 8.7 The influence of distance of an island from the source on the equilibrium number of species. EXT curve, Extinction curve. IMMIG curve, Immigration curve.
Example 3: Bird community in an oak wood in Surrey, England Long-term studies of a bird community in an oak wood in Surrey, England, also support the view that isolation can influence the avifauna of habitat islands. A rough equilibrium number of 32 breeding species were found in that community, with a turnover of three additions and three extinctions annually. It was projected that if the woods were as thoroughly isolated as an oceanic island, it would maintain only five species over an extended period: two species of tits (same genus as titmice), a wren, and two thrushes (the English Robin and Blackbird).
Island biogeography theory can be a great help in understanding the effects of habitat fragmentation. It does not, however, address other factors that can greatly influence which birds reside in a fragment. Some of these include whether nest-robbing species are present in such abundance that they could prevent certain invaders from establishing themselves, whether the fragment is large enough to contain a territory of the size required by some members of the pool of potential residents, or whether other habitat requirements of species in that pool can be satisfied. To take an extreme example of the latter, Acorn, Nuttall's, Downy, or Hairy Woodpeckers would not colonize a grass-covered, treeless habitat in California, even if they were large, and all four woodpeckers are found in adjacent woodlands.
Island biogeography theory at the light of ecosystem principles
In general terms, the Island Biogeography Theory explains therefore why, if everything else is similar, distant islands will have lower immigration rates than those close to a mainland, and ecosystems will contain fewer species on distant islands, while close islands will have high immigration rates and support more species. It also explains why large islands, presenting lower extinction rates, will have more species than small ones. This theory forecasts effect of fragmenting previously continuous habitat, considering that fragmentation leads to both lower immigration rates (gaps between fragments are not crossed easily) and higher extinction rates (less area supports fewer species).
The Ecological Law of Thermodynamics equally provides a sound explanation for the same observations. Let us look in first place to the problem of the immigration curves. In all the three examples, the decline in immigration rates as a function of increasing isolation (distance) is fully covered the concept of openness introduced by Jorgensen (2000a). Once accepted the initial premise that an ecosystem must be open or at least non-isolated to be able to import the energy needed for its maintenance, islands' openness will be inversely proportional to its distance to mainland. As a consequence, more distant islands have lower possibility to exchange energy or matter and decreased chance for information inputs, expressed in this case as immigration of organisms. The same applies to fragmented habitats, the smaller the plots of the original ecosystem the bigger the difficulty in recovering (or maintaining) the original characteristics. After a disturbance, the higher the openness the faster information and network (which may express as biodiversity) recovery will be.
The fact that large islands present lower extinction rates and more species than small ones, as well as less fragmented habitats in comparison with more fragmented ones, also complies with the Ecological Law of Thermodynamics. All three examples can be interpreted in this light. Actually, provided that all the other environmental are similar, larger islands offer more available resources. Under the prevailing circumstances, solutions able to give the highest exergy will be selected, increasing the distance to thermodynamic equilibrium not only in terms of biomass but also in terms of information (i.e. network and biodiversity). Moreover, after a disturbance, like in the case of the Krakatau Island, the rate of re-colonization and ecosystem recovery will be a function of system's openness.
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