Latitudinal Gradients in Species Diversity
Robert KinlockeDepartment of Geography and GeologyUWI, Mona
GEOG2202 – The Biosphere
IntroductionTropical regions have the highest levels of
diversity Polar regions have comparatively low levels
of diversity Gradual changes reflected in a polar to
tropical transition
Latitudinal gradients in diversity among three groups
Taxon Latitude 8o N
66o N
Land mammals
160 20
Reptiles 600 50
Bats 80 1
Latitudinal Gradients ofSelected Vertebrates
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:Explain why some of the explanations given
for latitudinal gradient in species diversity are considered circular
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 gradientCompetition (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
• 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
Predation (Paine, 1966; Pianka, 1966; Harper, 1969; Janzen, 1970; Lubchenco and Menge, 1978)
Based on predator-prey interactionsThere are more predators and microorganisms in the
tropicsThese 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)Twenty-eight
thousand species of tropical plants grow not on the ground but on tree trunks and branches.
High Epiphyte Load cont’dDisturbance 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.
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
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
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?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 problemsNo explanation of gradient as it relates to the
gradual change from higher to lower latitudesThey also do not offer any explanation fro
gradients in the marine environment
Boredom ends here!