Background/Question/Methods
Perhaps because of their apparent generality and superficial simplicity, gradients in biological diversity are among the most studied patterns in ecology. And while there are a variety of scales and grain sizes at which such questions can be studied, it is the large-grain latitudinal gradient in biological diversity of diverse taxa that is most conspicuous. As has been made clear by every review on the causes of latitudinal diversity gradients, general answers have been few while potential explanations have continued to diversify. In turn, none but the most outrageous mechanisms for diversity gradients have been rejected. One or several theories may yet find consensus support as explanations for diversity gradients. However, alternative scenarios for different taxa are also feasible.  It is possible that the data necessary to derive a consensus explanation about the general drivers of diversity gradients are unobtainable and therefore that all existing methods (e.g., simulation models, correlational approaches, and phylogenetic/fossil tests of hypotheses) will fail to allow rejection of existing hypotheses.  In this scenario, the answer to “what causes gradients in biological diversity” will continue to be relativistic and depend on the region and  taxon studied, the approach used and more generally the perspective of the investigator.
Results/Conclusions
I will present case examples from recent studies of insect diversity gradients and the results of interviews with biogeographers to explore the question of whether the causes of diversity gradients are knowable (or relativistic).  My results suggest that while biogeographers may have come closer to understanding which drivers of diversity can realistically operate, that there is little evidence of consensus as to which of these drivers are most important.  I conclude by considering how, despite the inherent problems in testing diversity theories, we might still make progress in understanding aspects of diversity gradients.