Latitudinal Gradients in Species Diversity

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

2. Introduction • Tropical regions have the highest levels of diversity • Polar regions have comparatively low levels of diversity • Gradual changes reflected in a polar to tropical transition

3. Latitudinal gradients in diversity among three groups

4. Latitudinal Gradients of Selected Vertebrates

5. Pitfalls to be avoided when looking for common causes: • Do not confuse causation and correlation • The explanation must hold true for all types of organisms

6. At the end of this session you should be able to: • Explain why some of the explanations given for latitudinal gradient in species diversity are considered circular

7. A Circular Argument? • One that seeks to support the premise with the premise. It leads nowhere! • It is always true not because it is rational but rather because it is structured as if it were true

8. Explanations for gradient • Competition (Dobzhansky, 1950; Pianka, 1966; Huston, 1979) • Natural selection in temperate regions is directed by environmental factors such as cold and drought which may cause widespread mortality. • These factors are less important in tropical regions • Biological stresses are more important in natural selection and evolution

9. Competition cont’d... • Intense competition among species in the tropics leads to greater habitat and food restriction • Competition, especially diffuse competition (that from many species of competitors) tends to hold the population of each species in check • More species coexist in the habitat • Competition leads to finer adaptations and specialization

10. Predation (Paine, 1966; Pianka, 1966; Harper, 1969; Janzen, 1970; Lubchenco and Menge, 1978) • Based on predator-prey interactions • There are more predators and microorganisms in the tropics • These reduce prey populations (e.g.: epidemics) • Reduction in prey population reduces competition among prey and allows new prey species to enter • Encourages new predators to enter (both as a result of dispersal and evolutionary processes) • An upper limit to this process is set by the productivity of the tropics

11. High Epiphyte Load (Strong, 1977) • Twenty-eight thousand species of tropical plants grow not on the ground but on tree trunks and branches.

12. High Epiphyte Load cont’d • Disturbance is a major feature of tropical forests • Disturbance may be due to high epiphyte load which may cause tree fall • Cleared patches created in forest • Succession reverts to an earlier stage which is characterized by diversity rather than the dominance associated with climax stage.

13. Mutualism (Dobzhansky, 1950; Piane, 1966; Janzen, 1970; Menge and Sutherland 1976) • Mutualists promote the coexistence of their symbiots, thus, the empirically observed high frequency of mutualism in the tropics promotes a higher diversity of symbiots

14. Host Diversity (Rhode, 1989) • Greater diversity among hosts results in greater diversity among parasites • The tropics are inhabited by more host species and this explains the greater diversity amongst parasites

15. Niche width (Ben- Eliahu and Safriel 1982; Brown and Gibson 1983) • Tropical species tend to be more specialized (i.e. Narrower niches) • More species can be packed into tropical habitats

16. Circularity? • Diversity is being used as an explanation of diversity • The arguments do not address the main issue which is reasons for the gradient • Essence of argument: Biological competition greater in the tropics because there are more species in the tropics • Why are there more predators, more micro organisms and heavier vine loads?

17. Other problems • No explanation of gradient as it relates to the gradual change from higher to lower latitudes • They also do not offer any explanation fro gradients in the marine environment

18. Boredom ends here!