One of the longest-standing and principal debates in ecology is the relationship between diversity and latitude and its underlying causal mechanisms.  Most commonly, studies document a strong negative linear relationship between diversity and latitude, with diversity peaking in the tropics.  Recent studies indicate that these pervasive linear relationships are scale-dependent, with stronger negative patterns dominating at regional spatial scales and weaker negative patterns dominating at local scales.  Interestingly, positive and modal relationships have also been found, but are usually dismissed as exceptions.  Identifying the fundamental nature of the latitudinal diversity gradient and how the relationship changes with spatial scale is critical to understanding how both ecological and evolutionary dynamics interact to shape global patterns of biodiversity.  However, few other studies to date have tested the scale-dependence of the latitudinal diversity gradient within a single study system across both a broad geographic gradient and range of spatial scales.  To explore the scale-dependence of the latitudinal diversity gradient, I surveyed taxonomic diversity of sessile marine invertebrate communities on spatially nested arrays of settlement panels in four regions along a latitudinal gradient.  Focal regions were Connecticut (41°N), Virginia (37°N), Florida (27°N), and Belize (16°N), spanning approximately 25 degrees of latitude.  Diversity was estimated at four spatial scales, ranging from 4cm² quadrats to 35km-long stretches of coastline. 

Results/Conclusions

Results indicated a clear shift in the diversity-latitude relationship.  As expected, a negative linear relationship occurred at the largest spatial scale, with the highest diversity occurring in Belize.  However, contrary to expectations, a strong unimodal relationship was present at the smallest spatial scale, with the highest diversity occurring in Florida.  These results indicate not only that the strength of the diversity-latitude relationship changes with scale, as has been shown in previous studies, but also that the fundamental nature of the relationship may shift with scale from modal to linear.  Such scale-dependence may offer a unifying framework for studies that document disparate diversity-latitude patterns.  These results further suggest that there is not a single causal mechanism underlying the latitudinal diversity gradient, but rather a complex combination of different ecological mechanisms that operate at different spatial scales.