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 variation in species composition and diversity among geographic locations.
- Patterns of species composition and diversity at different spatial scales are connected to one another in a hierarchical way.
- The global spatial scale includes the entire world, a huge geographic area over which there are major differences in climate and in species diversity and composition.
- 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 limitation to that region.
- The local spatial scale encompasses the smallest geographic area and is essentially equivalent to a community.
- Beta diversity is the change in species number and composition, or turnover of species, across the landscape from one local community to another.
- Studies show that regional species pools largely determine the numbers of species present in local communities, but that local conditions are also important and cannot be discounted.
Concept 17.2 Global patterns of species diversity and composition are influenced 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 biotas of the six biogeographic regions reflect a history of isolation due to continental drift caused by the movements of Earth’s tectonic plates.
- 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 tropics and declines at higher latitudes.
- A number of hypotheses, involving species diversification rate, species diversification time, and productivity, have been proposed to explain the latitudinal gradient in species diversity.
Concept 17.3 Regional differences in species diversity are influenced by area and distance, which determine the 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 in island-like areas.
- According to the theory, larger islands closer to a source of species have more species than smaller islands that are more distant from a source of species because they have 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 and in island-like areas.