Diversity Over Space and Time

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.