Only three processes can directly change the number of species in a given place: speciation, extinction, and dispersal. This gives us a set number of hypotheses that can be tested for any system, including biogeographic regions or habitats as different as coral reefs and the deep sea.
The latitudinal diversity gradient is the tendency for species richness to increase as we move closer to the equator. Like any species richness pattern, this gradient must be explained at the most fundamental level by the timing or frequency of speciation, extinction and/or dispersal. If the timing matters, perhaps the tropics are more diverse because clades started diversifying there and didn't disperse to the temperate and polar zones until much later (less time for speciation). If the frequency matters, then perhaps speciation happens more often in the tropics (higher speciation rate).
In a study published in Global Ecology and Biogeography, we showed that the timing of colonization explained species richness of freshwater fishes better than speciation rates. Tropical lineages have been diversifying since the Mesozoic. In contrast, lineages that live in North America and Eurasia today did not begin diversifying until the Cenozoic (after the extinction of the dinosaurs). Even at a similar or faster rate of speciation, temperate lineages have not had enough time to "catch up" to the tropics. Slow and steady may be the way to build extreme diversity.
The latitudinal diversity gradient is the tendency for species richness to increase as we move closer to the equator. Like any species richness pattern, this gradient must be explained at the most fundamental level by the timing or frequency of speciation, extinction and/or dispersal. If the timing matters, perhaps the tropics are more diverse because clades started diversifying there and didn't disperse to the temperate and polar zones until much later (less time for speciation). If the frequency matters, then perhaps speciation happens more often in the tropics (higher speciation rate).
In a study published in Global Ecology and Biogeography, we showed that the timing of colonization explained species richness of freshwater fishes better than speciation rates. Tropical lineages have been diversifying since the Mesozoic. In contrast, lineages that live in North America and Eurasia today did not begin diversifying until the Cenozoic (after the extinction of the dinosaurs). Even at a similar or faster rate of speciation, temperate lineages have not had enough time to "catch up" to the tropics. Slow and steady may be the way to build extreme diversity.
Key publications:
Miller, E. C., C. M. Martinez, S. T. Friedman, P. C. Wainwright, S. A. Price, and L. Tornabene. 2022. Alternating regimes of shallow and deep-sea diversification explain a species richness paradox in marine fishes. Proceedings of the National Academy of Sciences, 199, e2123544119.
Miller, E. C. 2021. Comparing diversification rates in lakes, rivers, and the sea. Evolution, 75: 2055–2073.
Miller, E. C. and C. Román-Palacios. 2021. Evolutionary time best explains the latitudinal diversity gradient of living freshwater fish diversity. Global Ecology and Biogeography 30: 749–763.
Miller, E. C., K. T. Hayashi, D. Song, and J. J. Wiens. 2018. Explaining the ocean’s richest biodiversity hotspot and global patterns of fish diversity. Proceedings of the Royal Society B 285: 20181314.
Miller, E. C. and J. J. Wiens. 2017. Extinction and time help drive the marine-terrestrial biodiversity gradient: is the ocean a deathtrap? Ecology Letters 20: 911–921.
New York Times article about species richness patterns in the world's oceans
Miller, E. C., C. M. Martinez, S. T. Friedman, P. C. Wainwright, S. A. Price, and L. Tornabene. 2022. Alternating regimes of shallow and deep-sea diversification explain a species richness paradox in marine fishes. Proceedings of the National Academy of Sciences, 199, e2123544119.
Miller, E. C. 2021. Comparing diversification rates in lakes, rivers, and the sea. Evolution, 75: 2055–2073.
Miller, E. C. and C. Román-Palacios. 2021. Evolutionary time best explains the latitudinal diversity gradient of living freshwater fish diversity. Global Ecology and Biogeography 30: 749–763.
Miller, E. C., K. T. Hayashi, D. Song, and J. J. Wiens. 2018. Explaining the ocean’s richest biodiversity hotspot and global patterns of fish diversity. Proceedings of the Royal Society B 285: 20181314.
Miller, E. C. and J. J. Wiens. 2017. Extinction and time help drive the marine-terrestrial biodiversity gradient: is the ocean a deathtrap? Ecology Letters 20: 911–921.
New York Times article about species richness patterns in the world's oceans