Biodiversity patterns on islands: a general dynamic model and the island species–relationship

In a recent paper in Journal of Biogeography (June 2008), we presented a theoretical model of rates of immigration, extinction, and speciation on oceanic islands as a function of island ontogeny (life cycle). The central premise of the model is that the ecological capacity of remote volcanic islands typically describes a hump shaped trend as an island grows to maturity and then declines through erosion and subsidence, eventually disappearing again.
The model generates several predictions for emergent biogeographical patterns within archipelagos comprised of such islands, focusing on numbers and proportions of endemics, relative importance of adaptive and non-adaptive processes, etc. Here we (i) outline the key properties of this general dynamic model, and (ii) present some additional tests of the model using snapshot data based on numbers of endemic species.

The increase of species richness as sampling area increases, i.e. the species–area relationship, is one of ecology’s few laws. Thousands of studies have reported the pattern, for more than 91 years but still necessity for synthesizing the available knowledge exists. We conducted the most extensive quantitative analysis of the form taken by the island species–area relationship (i.e. each data point is tallied independently of every other and so the form of the relationship can vary; ISAR). We amassed 601 data sets from strictly geographical islands and employed an information-theoretic framework to compare the 20 available species–area functions.
Overall, we conclude that over most scales of space, ISARs are best represented by simple models, with the power model, being the overall best but not the single-best one. Biological significance can be assigned to the parameters of the logarithmic form of the power model. The general form of the ISAR is convex upwards without an asymptote; consideration may be given to fitting sigmoid models when the spatial range is around, or exceeds, three orders of magnitude. While the form of the ISAR varies considerably between study systems, part of this variation can reasonably be related to the array of different mechanisms and processes that constrain the ecological space available within an island system and the geographical context within which the archipelago is located.