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GEOG2202 – The Biosphere. Latitudinal Gradients in Species Diversity. Robert Kinlocke Department of Geography and Geology UWI, Mona. Introduction. Tropical regions have the highest levels of diversity Polar regions have comparatively low levels of diversity

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latitudinal gradients in species diversity

GEOG2202 – The Biosphere

Latitudinal Gradients in Species Diversity

Robert Kinlocke

Department of Geography and Geology

UWI, Mona

introduction
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
slide4

Latitudinal

Gradients of

Selected

Vertebrates

pitfalls to be avoided when looking for common causes
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
at the end of this session you should be able to
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
a circular argument
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
explanations for gradient
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
slide9

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
slide10

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
high epiphyte load strong 1977
High Epiphyte Load (Strong, 1977)
  • Twenty-eight thousand species of tropical plants grow not on the ground but on tree trunks and branches.
slide12

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.
slide13

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
slide14

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
slide15

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
circularity
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?
other problems
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