Print PageLatitudinal gradients of species diversity are pervasive. Numerous hypotheses have been proposed to explain the increase in number of species toward the equator. One prominent and well-tested hypotheses is the mid-domain effect, which proposes that random placement of species’ geographic ranges within bounded continents creates peaks in diversity at the equator and latitudinal decreases toward the poles. Typically, this hypothesis is tested by estimating some measure of goodness of fit between species richness patterns based on empirical data and those generated from a null model. Nonetheless, it has recently been pointed out that such curve fitting represents only weak tests of macroecological hypotheses. One means of strengthening macroecological inference is to examine interchangeability. Two characteristics are interchangeable if they are identical from the point of view of the hypothesis. Biodiversity is multifaceted thereby providing numerous characteristics from which to assess interchangeability of mid-domain effects. We examined the degree to which a mid-domain model recapitulates latitudinal gradients in other forms of biodiversity: phylogenetic, phenetic and functional characteristics. Added support for the mid-domain hypothesis would come from demonstration of good fit of empirical gradients of other characteristics of biodiversity with those produced by a null model generating a mid-domain effect.
Results/Conclusions
Mid-domain models exhibited good fit to richness gradients but poorer fit to all other biodiversity characteristics and this suggests that geometric constraints may affect richness gradients more that other characteristics of biodiversity. Moreover, mid-domain effects likely are not the only determinants of gradients in biodiversity. Fuller understanding will come from simultaneous examination of random, contemporary and historical mechanisms to better discern relative effects on distribution and abundance of the current biota.
