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.



         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.