BIOL 1911 Chapter Notes - Chapter 54.4: Evapotranspiration
54.4 – Biogeographic factors affect community diversity
• Large-scale biogeographic factors also contribute to the tremendous range of diversity observed in biological
communities
• Two main biogeographic factors
o Latitude of a community
o Area it occupies
Latitudinal Gradients
• Plant and animal life is generally more abundant and diverse in the tropics than in other parts of the globe
o Observation noted since the 1850s
o Confirmed by many studies
• Two key factors contributing to latitudinal gradients species richness: evolutionary history and climate
• Evolutionary history
o Over the course of evolution, a series of speciation events may lead to increased species richness in a
community
o Tropical communities are generally older than temperate or polar communities, “
”
o Therefore, species diversity may be highest in the tropics simply because there has been more time for
speciation to occur in tropical communities than in temperate or polar communities
• Community
o Two main climatic factors correlated with diversity = sunlight and precipitation
o At high levels in the tropics
o C ’ ,
and plants
▪ A function of solar radiation, temperature and water availability
▪ Much higher in hot areas with abundant rainfall than in areas with low temperatures or low
precipitation
o Potential evapotranspiration, a measure of potential water loss that assumes water is readily available, is
determined by the amount of solar radiation and temperature and is highest in regions where both are plentiful
o Species richness of plants and animals correlates with both measures (evapotranspiration and potential
evapotranspiration)
Area Effects
• Species-area curve: all other factors being equal, the larger the geographic area of a community, the more species it has
o Described as one of the first patterns of species richness in 1807 by Alexander von Humboldt
o Explanation: large areas offer a greater diversity of habitats or microhabitats
o Developing species-area curves for key taxa in a community helps ecologists predict how the loss of a given
area of ’
• Robert MacArthur and E.O. Wilson (1960s) tested the predictions of the species-area relationship
o Examined the number of animals and plants on different island chains
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