Ecological biogeography of Southern Ocean islands: The importance of considering spatial issues

Understanding patterns of among-island variation in species richness has lo ng been an important question in ecology and biogeography. However, despite the clear spatial nature of the data used for such investigations, the spa tial distribution of the different sampled locations is rarely explicitly c onsidered, which may be critical for statistical and biological reasons. In a recent study, Chown et al. (1998) investigated the relationships between species richness of different indigenous and introduced taxonomic groups a nd a variety of variables characterizing Southern Ocean islands, and here, we use these data to address spatial issues. As predicted, we found spatial autocorrelation in species richness for terrestrial taxa with high dispers al ability or for terrestrial taxa that had time to disperse locally (intro duced land birds and indigenous taxa) but not for taxa that had low opportu nity to disperse to nearby islands (introduced plants, insects, and mammals ), which suggests that colonization from nearby islands has played an impor tant role in shaping present-day patterns of among-island variation in spec ies richness. Interestingly, in several cases, the estimated effect of vari ables changed when spatial covariance was incorporated. Moreover, the absen ce of autocorrelation of some variables allowed us to confirm some importan t results of Chown et al. (1998), notably those involving the potential imp act of human presence on the biodiversity of these islands. Overall, our re sults illustrate the importance of considering spatial structures in ecolog ical studies. This is notably the case when dispersal processes can be expe cted to explain some of the observed patterns.