TEMPORAL PERSISTENCE AND TAXONOMIC UBIQUITY OF A CIRCUMEQUATORIAL BIODIVERSITY TROUGH

Although it is commonly believed that maximum biodiversity occurs at the equator, most marine taxa do not conform to this pattern either at the present day or through geologic time. Instead, the number of genera increases from the poles to midlatitudes—a typical latitudinal biodiversity gradient—but then declines from the midlatitude peak to the equator, creating a conspicuous circumequatorial trough. The width of this trough varies among taxa and over time (and may manifest as a unimodal midlatitude peak), as demonstrated here for ten well-preserved and abundant fossil taxa (Anthozoa, Bivalvia, Brachiopoda, Bryozoa, Cephalopoda, Crinozoa, Echinozoa, Gastropoda, Ostracoda, and Porifera including Archaeocyatha) using data derived from the Census of Marine Life and the Paleobiology Database. This same pattern is present in many independent studies that examined specific time intervals, and it is present even after sample-standardization, making it very unlikely that the pattern results from uneven sampling. Terrestrial fossil taxa may also have exhibited a circumequatorial biodiversity trough, although the pattern is less pronounced or absent in terrestrial taxa today. According to the metabolic theory of ecology, species richness is predicted to continuously increase with temperature; however, these data indicate that the relationship of richness and temperature is humped. Maximum richness occurs at relatively cold temperatures (2-8°C for these ten taxa) and declines toward higher and lower temperatures, regardless of latitude. For example, tropical corals (defined here as those occurring within 10° latitude of the equator), considered a paragon of warm-water marine life, are actually most diverse at 6°C. High ocean temperatures and low dissolved oxygen in the tropics may limit taxonomic richness by increasing metabolic cost; however, several contraindicative patterns complicate this explanation. Further study of the dynamics of the circumequatorial biodiversity trough among taxa and through time may help explain how biodiversity is generated and maintained.