Rapoport's rule: an explanation or a byproduct of the latitudinal gradientin species richness?

A recent explanation of the declining species richness gradient with increa sing latitude away from the tropics implicated broad scale habitat variabil ity, an associated range expansion, and a resulting increase in niche bread th. The niche breadth in turn was thought to affect richness by competition and rescue effect. While all three factors appear to be correlated, neithe r the postulated nor alternative causal mechanisms have been tested. We con duct such a test using a system which has all the attributes of the large s cale pattern but which, in contrast to continental scale observations, allo ws for estimation or control of crucial variables such as taxonomic composi tion, habitat heterogeneity, habitat variability, exact species distributio n, and local richness. Specifically, we test the alternative that the corre lation between the geographical range of species and local diversity is a f unction of differential species survival and link this phenomenon to habita t variability. We use 40 species of aquatic invertebrates inhabiting a land scape of 49 miniature rock pools on the coast of Jamaica. The system we exa mined exhibits a gradient of increasing richness with decreasing habitat va riability, analogous to the broad scale latitudinal pattern. Furthermore, s pecies with broader ecological ranges are also broadly distributed. Superfi cially, this appears to be in agreement with the older explanations but two facts suggest different causes. First, there is no evidence of a 'rescue e ffect' maintaining high richness in many habitats despite their proximity t o species sources. Second, ecologically broad species coexist with habitat specialists without reducing richness in jointly occupied habitats.