THE EFFECT OF TEMPERATURE AND HABITABLE AREA ON THE LATITUDINAL DIVERSITY GRADIENT

A remarkable feature of the marine latitudinal diversity gradient is that the position of peak biodiversity has moved over time, and has only ever occupied its current position in the tropics for a small portion of the Phanerozoic Eon. Two hypotheses that have been advanced to explain this shift are, first, that peak biodiversity occurs where shallow shelf area is greatest (larger areas house more taxa), and, second, that peak biodiversity occurs where temperature conditions are most favorable (just outside the “pejus zone”, a physiologically stressful area of warm temperatures and low dissolved oxygen centered on the equator). Both of these mechanisms predict variation in the position of peak biodiversity over time. For most of the Phanerozoic, the predictions of the two hypotheses coincide and as such are indistinguishable. However, the predictions diverge during four time intervals for which we have sufficient data, allowing us to tease apart the relative contribution of each of these factors, using sample-standardized latitudinal diversity gradients of brachiopods through the Phanerozoic. During the cool climates of the middle Cambrian, late Ordovician, and the Recent, the temperature hypothesis predicts that peak biodiversity will occur at low latitudes despite shallow shelf area being greatest at mid- or high latitudes. During the warm climate of the late Permian, the temperature hypothesis predicts that peak biodiversity will occur at mid-latitudes despite shallow shelf area being greatest at low latitudes. In each case, the results favor temperature as the dominant control over the position of peak biodiversity at these times, with habitable area exerting a secondary effect.