Testing the time-for-speciation effect in the assembly of regional biotas


Correspondence author. E-mail: drabosky@berkeley.edu


1. Rates of evolutionary diversification play a fundamental role in the assembly of regional communities, but the relative balance of diversity-dependent and diversity-independent rate control remains controversial. Recent studies have reported a significant relationship between the amount of time a geographic region has been occupied and species richness, implying that feedbacks between species interactions and diversification rates may be less important than diversity-independent mechanisms in generating regional species pools.

2. Previous analyses of the regional age-diversity relationship have used a range of metrics to quantify the amount of ‘evolutionary time’ that a region has been occupied, but the relative performance of these metrics has not been quantified.

3. Here, I evaluate the performance of the most commonly used methods and data transformations for assessing the regional age-diversity relationship.

4. I find that process-based models of diversification are more appropriate than process-independent models for evaluating the influence of time on species richness. I also demonstrate that time should not be log-transformed when testing the regional time-for-speciation hypothesis, as in some recent studies.

5. Application of this framework to patterns of elevational richness in several recent studies provides support for a logistic model of diversity accumulation within elevational bands and implies that evolutionary age alone cannot fully account for current species richness.

6. These results indicate that process-based models, in concert with appropriate data transformation, provide a robust foundation for inference on the causes of regional diversity gradients.