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Species-area relationship: new challenges for an old pattern

Final Activity Report Summary - SPAR (Species-area relationship: New challenges for an old pattern.)

The species-area relationship pattern (SPAR), the increase in species number with increasing area, is considered one of ecology's few laws and recently the validity of the relationship has been proved even for bacteria. Our goal is to successfully partition all the issues related to SPAR, in an effort towards a deeper understanding of this ubiquitous, but still evading, ecological regularity. Whilst syntheses have been attempted previously, the present study is more detailed in exploring the properties of the data sets, based on more data, and by adopting a more rigorous and powerful analytical approach than previous studies.

The main questions addressed in the present project were, and still are addressed using the largest database used so far for documentation of species diversity patterns on islands, and especially the species-area relationship, 600 data sets in total. This database consists of a wide range of animal, plant and other taxa, from oceanic, continental and land-bridge islands. In addition to numbers of species and areas, in most of the data sets assembled, we have located several other relevant factors, such as elevation, isolation, environmental heterogeneity, climate, productivity, altitude, climate, human disturbance, etc.

Our results are offering a number of significant insights regarding the factors shaping biodiversity: Following the usage that a biological province is a region whose species have for the most part evolved within it, rather than immigrating from somewhere else, we propose that islands can be considered equivalent to biological provinces for single island endemic species. We show that single island endemic species-area slopes are quite similar to those previously calculated for inter-provincial species-area relationship, indicating that, when speciation becomes the dominant process adding to the species richness of assemblages, high z-values should be anticipated, regardless of the biogeographical scale of the study system.

Additionally, we showed that despite that increasing area means higher population sizes and thus more different species that can be accommodated; it is clear that quantification of other factors (e.g. climate, habitat diversity, evolutionary history) that partially co-vary with area, is necessary if we are to develop a more predictive model of species number variation across insular systems. This is evident because even within single archipelagos, increasing the accuracy of the estimation of physical space does not significantly improve model fits, in contrast to models incorporating crude measures of habitat diversity.

Furthermore, in the context of the current fellowship we presented a general dynamic theory of oceanic island biogeography, extending the most influential work in island biogeography (MacArthur and Wilson, 1967); that aims to provide a general explanation of biodiversity patterns on oceanic islands through describing the relationships between fundamental biogeographical processes - speciation, immigration, extinction - through time and in relation to island ontogeny. The general dynamic theory is a deliberately simplified representation of diversity dynamics on oceanic islands, which offers the promise of a more integrative and dynamic island biogeographical theory, and a framework for a renewed effort to integrate ecological and evolutionary approaches to the study of island biotas.