Chapter 17 Summary

CONCEPT 17.1 Patterns of species diversity and distribution vary at global, regional, and local spatial scales.

• Biogeography is the study of species distribution and diversity at different geographic locations and spatial scales (global, regional, and local).
• The global spatial scale includes the entire world, a huge geographic area over which there are major differences in climate and species distributions.
• The regional spatial scale encompasses a smaller geographic area in which the climate is roughly uniform and the species contained therein are bound by dispersal to that region.
• The local spatial scale encompasses the smallest geographic area and is essentially equivalent to a community.
• Different spatial scales of species diversity are connected to one another in a hierarchical way, with one dependent on another.
• Studies show that the regional species pools can largely determine the number of species present in communities but local conditions are also important and cannot be discounted.

CONCEPT 17.2 Global patterns of species diversity and composition are controlled by geographic area and isolation, evolutionary history, and global climate.

• Earth’s land mass can be divided into six biogeographic regions that vary markedly in species diversity and composition.
• The biota of the six biogeographic regions reflect a history of isolation due to continental drift.
• Tracing the threads of vicariance over large geographic areas and long time periods provided important evidence for early theories of evolution.
• Species diversity is greatest in the lower-latitude tropical zones and declines at higher latitudes.
• A number of hypotheses, involving geographic area and climatic stability, evolutionary history, and productivity, have been proposed to explain the latitudinal gradient in species diversity.

CONCEPT 17.3 Regional differences of species diversity are controlled by area and distance due to a balance between immigration and extinction rates.

• Species richness tends to increase with the area sampled and tends to decrease with distance from a source of species.
• Most species–area relationships have been documented for islands, which include all kinds of isolated areas surrounded by dissimilar habitat.
• The equilibrium theory of island biogeography predicts that a balance between immigration and extinction rates controls species diversity on islands or island-like areas.
• Larger islands closer to a source of species have more species than smaller islands that are more distant from a source of species, possibly due to higher immigration rates and lower extinction rates.
• The same species–area relationship observed on islands also holds for mainland areas, but the rate of increase in species richness with increasing area is lower than on islands.