Latitudinal Gradients in Species Diversity 2

1. GEOG2202 – The Biosphere Latitudinal Gradients in Species Diversity 2 Robert Kinlocke Department of Geography and Geology UWI, Mona

2. Environmental Stability and Predictability • The tropics are characterized by stable and predictable environments (...well sort of) • The stability of the tropics the allows more species to specialize as the evolve and exploit restricted areas. • While the number of individuals in the tropics and temperate regions may be the same, more species can inhabit a unit of habitat in in the tropics

3. In high latitudes the environment is more harsh and variable. • Organisms specialize less in these environments and must be able to live through seasonal changes (at higher latitudes seasonal differences become more extreme, for example: longer periods of dark and sub- zero temperatures) • Implications for migration and species range?

4. No consistent correlation has been found between environmental stability and species richness • Some areas in tropical environments are highly unpredictable but nevertheless show extremely high diversity • e.g. Coral reef ecosystems are subject to changes in temperature and salinity but show high diversity

5. Productivity • Diversity increases with productivity • Productivity increases towards the tropics and with it environmental complexity • With increased complexity there is a trade off between abundance, that is a large number of individuals which make demands on the same resources and therefore inefficient use of total resources, and specialists who efficiently make use of the complex environment

6. No consistent relationship has been found between high productivity and species richness • e.g. Coastal salt marshes

7. Spatial Heterogeneity • Increase in complexity from polar to tropical regions and so tropical environments will support more diverse communities • Greater topographical diversity in the tropics (low latitude habitats can range from lowland tropical to high altitude boreal habitats and some of these habitats are entirely absent from high latitude regions) • These conditions facilitate coexistence as species are able to exploit various ecological niches

8. Figure 1: The relationship between bird species diversity and the structural complexity of the habitat

9. Diverse communities coexist in the same communities in the tropics • There is no evidence that diversity at the micro-level changes with latitude • “paradox of the plankton” (Hutchinson, 1961) - coexistence of many species in water masses with seemingly little structural heterogeneity.

10. The angle of incidence of light • The crowns of trees in high latitudes tend to be conical because of the light regime • Light admitted at sharply inclined angles hinders the growth of a second layer of vegetation • Tropical trees tend to have shallow dome shaped crowns which allow light to enter at shallower angles

11. Area Effects • Island Biogeography • Larger areas support more diverse habitats • The tropics cover a wider area than temperate regions (common map projections don’t necessarily depict this) • But land area in Southern Asia is smaller than that of Northern Asia which has a much lower species diversity

12. Latitudinal Range • Observed correlation between the latitudinal range and the latitude that species occupy • “Rapoport’s rule” (Rapoport, 1982) • Species ranges which centred on high latitudes tended to have wider distributions

13. High latitude species had to withstand larger seasonal variations in environmental conditions • They have a wider range of tolerance and are less restricted than tropical organisms in habitat use

14. The rule does not hold for all species • A consequence rather than a cause of the observed gradient? • Fewer species in higher latitudes = less competition and restriction to narrow ranges

15. That’s it!