Through much of the Neogene at least, speciation has evidently been easier within low-latitude, high-diversity ecosystems than within high-latitude, low-diversity ecosystems, creating an abundance of young lineages in low latitudes. This dynamic implies that resources are more easily obtained by new species in high-diversity than in low-diversity ecosystems, a counterintuitive observation, particularly in light of apparent resistance of tropical habitats to invasion from high latitudes over both the short and the long term. Such a latitudinal trend in origination could be promoted by ecological feedbacks, such as proposed by advocates of niche construction (e.g., Odling-Smee et al. 2003; Erwin 2007), but the question of why there should be a gradient in feedbacks remains. The rapid diversity rebound in the New World Tropics following a late Neogene extinction pulse (see Todd et al. 2002; O'Dea et al. 2007) might have been partially fueled from higher latitudes, but the huge pool of tropical lineages available as diversification sources would have swamped the much smaller number of potential extratropical invaders. Combined molecular and paleontological analyses are needed to assess biogeographic sources of the marine Neotropical evolutionary rebound. Adaptations to the high-latitude environment, whether biotic, physical, or both combined, seem to result in the suppression of per-taxon origination relative to lower latitudes. This would explain the progressive failure of generic lineages to penetrate into increasingly higher latitudes where ecological generalists are most successful and are incumbent. Indeed, such preemptive interactions may be a significant factor in molding the LDG to the gradient of those diversity-dependent environmental conditions that correlate with latitude. The role played by extinction in the dynamics of the LDG would thus be to reduce the incumbent fauna sufficiently below its carrying capacity to provide "openings" for the invasion of new species (Walker and Valentine 1984), some of which would add genera to the region. If a polar extinction spike could be found, our hypothesis might be tested by whether there was a compensatory replacement of lost genera from the temperate biota. At any rate, the lack of any clear difference between temperate and polar extinction rates accords well with the dynamic outlined above. There is no evidence in the data reported here that the extinction rate directly affects carrying capacity or is the primary control on the LDG, although it may push diversity below regional carrying capacity and thus drive temporal or spatial divergence from an idealized trend. The hypothesis we develop here thus goes counter to classic suggestions that equatorward range limits are set by biotic interactions and poleward limits are set by physical factors (e.g., Dobzhansky 1950; MacArthur 1972). Instead, we suggest that physical environmental factors determine regional differences in the niche breadth of taxa throughout the LDG, with resulting variations in biotic resistance to invasion and local origination. The LDG, and by implication other large-scale spatial variations in diversity, is thus primarily an evolutionary rather than an ecological phenomenon, albeit, as in most evolutionary cases, driven by adaptations to ecological conditions. We suggest that marine latitudinal gradients in diversity-dependent ecological factors produce an OTT dynamic, with a low-latitude source and higher-latitude filters, where the filtering processes are mediated by the incumbency of local populations that belong on average to progressively older and ecologically more generalized lineages at progressively higher latitudes. Moreover, the role of biotic interactions as proximate factors in structuring spatial and temporal diversity patterns will be severely underestimated if, as proposed here, the nature of those interactions varies with physical environmental parameters. Diversity-dependent factors do not only come into play at the high end of the diversity scale. Depending on regional history and environmental context, we suspect that biotas at any diversity level can be at, or distant from, a state in which diversity dependence is a significant force.
|Original language||English (US)|
|Number of pages||10|
|State||Published - Mar 1 2008|
All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics
- Agricultural and Biological Sciences(all)