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!