SUMMARY

CONCEPT 18.1 Patterns of species diversity and distribution vary at global, regional, and local spatial scales.

18.1.1 Define biogeography and explain how patterns of species diversity and composition are connected across different spatial scales.

Biogeography is the study of variation in species diversity and composition among geographic locations.

Patterns of species diversity and composition at the global, regional, and local spatial scales are connected to one another in a hierarchical way.

18.1.2 Outline the processes important to global-, regional-, and local-scale biogeography.

Global-scale biogeography is the result of variations in speciation, extinction, and dispersal at latitudinal and continental spatial scales and evolutionary time scales.

Regional-scale biogeography (gamma diversity) 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.

Local-scale biogeography (alpha diversity) is equivalent to a community and is determined by dispersal, physical conditions,

and species interactions.

Beta diversity is the change in species number and composition, or turnover of species, across the landscape from one local community to another.

18.1.3 Analyze the relative importance of species pools versus local-scale processes in determining local community species diversity.

Studies show that regional species pools largely determine the numbers of species present in local communities but that local physical conditions and species interactions are also important.

CONCEPT 18.2 Global patterns of species diversity and composition are influenced by geographic area and isolation, evolutionary history, and global climate.

18.2.1 Describe the two major biogeographic patterns— biogeographic regions and latitudinal gradients in species diversity—at the global scale.

Earth's land mass can be divided into biogeographic regions that vary markedly in species diversity and composition.

For most taxonomic groups, species diversity is greatest in the tropics and declines at higher latitudes.

18.2.2 Explain the underlying forces thought to be important in creating biogeographic regions.

The biotas of the biogeographic regions reflect an evolutionary 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.

18.2.3 Outline the hypotheses proposed to explain the latitudinal gradient in species diversity pattern.

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 18.3 Regional differences in species diversity are influenced by area and distance, which determine the balance between immigration and extinction rates.

18.3.1 Graph and explain the species-area relationship and know why it differs between islands and mainland areas.

Species-area relationships show that species richness increases with the area sampled and decreases with distance from a source of species.

Most species-area relationships have been documented for “islands,” which include isolated areas surrounded by dissimilar habitat but also mainland areas.

Species-area curves are steeper for islands compared to mainland areas because of the greater difference in species richness among isolated islands.

18.3.2 Explain regional species diversity for islands and island­like areas using the equilibrium theory of island biogeography. 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.

REVIEW QUESTIONS

1.

2. Describe the factors that Alfred Russel Wallace believed created biogeographic regions on land and in the oceans.

3. Latitudinal gradients in species diversity and composition are strong global features of biogeography. Describe three hypotheses proposed to explain why species diversity is higher in the tropics and decreases toward the poles for the majority of taxonomic groups.

HONE YOUR PROBLEM-SOLVING SKILLS

One study from the Biological Dynamics of Forest Fragments Project (BDFFP) considered the number of understory bird species living in different-sized forest fragments surrounded by deforested land (see the Ferraz et al. [2003] study in the Case Study Revisited). This study involved counting the number of bird species in the fragments at the start of the experiment and then over a handful of years afterward. A scaling factor was estimated to determine the time it takes to lose half of the bird species (t₅₀) in the different fragment sizes. Below is a table with the results of the study, organized by the fragment area:

Source: G. Ferraz et al. 2003. Proc NatlAcad Sci USA 100: 14069-14073.

1. Graph the initial numbers of bird species by fragment area. Do the fragments follow the species-area relationship?

2. Assuming that the species loss is linear over time, use the table to calculate the percentage loss of species per year in the 1-, 10-, and 100-ha fragments. Which fragment has the

greatest species loss per year, and which has the least?

3. Now use the percentage loss per year to calculate the number of species in each of the fragment sizes 9 years after the start of the experiment. Graph the number of species by fragment area on the graph you developed for Question 1.

4. If you were to draw linear regressions for the species-area data points at the start of the experiment and 9 years after fragmentation, which species-area relationship would have the steepest slope (z)? Explain why.

LIST OF KEY TERMS

alpha diversity beta diversity biogeographic regions biogeography continental drift endemic equilibrium theory of island biogeography gamma diversity local scale regional scale regional species pool species-area relationship turnover vicariance