Latitudinal distribution of surface area
- Surface area declines from equator to poles.
- Distribution of land v. oceans has nearly reversed over geologic time.
- Influences climatic gradients and area of different environments.
Diversity Gradients Have Three Major Influences
- Ecological interactions and tolerances to environment: species inhabit areas with appropriate resources and climate.
- Evolutionary processes: speciation and extinction rates vary over time and space.
- Earth history, including plate motions, climate change, and landscape change, determines the geography of environmental gradients and barriers to geographic-range shifts.
Earth History and Diversity Gradients
- We observe high diversity in tropical latitudes today, and the climatic tropics are restricted to low latitudes.
- 50 million years ago, climatically tropical environments extended over much of the earth, encompassing a much larger area of land and sea than today.
- Global cooling caused tropical ecosystems to shrink to low latitudes, temperate ecosystems to expand, and boreal ecosystems to appear.
- Modern tropical diversity reflects the legacy of a long existence and large area of this ecosystem.
- High-latitude boreal forest and tundra ecosystems are geologically young.
Summary
- Species diversity displays strong geographic gradients, especially the increase in species diversity at low latitudes and in regions of topographic complexity.
- These gradients in species diversity follow latitudinal temperature gradients today.
- Over geologic time, these gradients were much shallower, corresponding to shallow temperature gradients.
- Both the age and the historically large area of tropical ecosystems contribute to high tropical diversity today.