Biogeographic patterns are interconnected across a hierarchy of scales ranging from global to local. Three strong biogeographic patterns can be seen: (1) biomes or groupings of ecologically similar organisms are shaped by the environment, (2) diversity varies from continent to continent, forming biogeographic regions, and (3) diversity varies with latitude.
learning outcomes
You should be able to:
Describe the principle characteristics of each of the seven major biomes featured in this section, including climate, latitude and/or continent(s), and representative plants and animals.
Use the observations and insights of Alfred Russel Wallace to explain how biogeography might influence the distribution of plants and animals.
Compare and contrast the hypotheses posed for latitudinal patterns of species diversity, including species diversification rate, species diversification time, and productivity, giving examples of situations that could be used to explain each one.
Describe the information used to characterize biomes. What are the differences and similarities between the rainforest biome and the temperate evergreeen forest biome?
Biomes are characterized using information about the growth forms of their dominant plants, which reflect the evolution of those plants under annual patterns of temperature and precipitation. Tropical rainforests are found in equatorial regions where conditions are consistently wet and warm. The dominant vegetation is tropical trees, with up to 500 species per square kilometer. Temperate evergreen forests, by contrast, grow at middle to high latitudes, where winters are mild and wet and summers are cool and dry. The dominant trees are a handful of conifer species. The two forest types are similar in that they thrive under wet conditions and do not lose their leaves or needles on a seasonal basis.
Refer to Figure 53.14. Why did Wallace’s line help explain what controls the formation of biogeographic regions?
Wallace saw dramatically different terrestrial species inhabiting the adjacent islands of Bali and Lombok, even though they were separated by 24 kilometers and had similar physical environments. This observation led him to hypothesize that different biota had been separated by a barrier (in this case, a deep channel), which kept them apart over evolutionary time. Wallace surmised that biogeographic regions are the result of the isolation that species experienced by residing on different continents or islands.
Over Earth’s history, ice sheets extended into temperate latitudes. Does this fact better support the species diversification rate or the species diversification time hypothesis proposed to explain latitudinal variation in diversity? Explain.
The fact that ice sheets extended into temperate regions, where species diversity is currently lower than in the tropics, better supports the species diversification time hypothesis. This hypothesis proposes that the amount of time in which speciation has taken place is greater in the tropics, where severe climate conditions did not slow down speciation or increase extinction. Thus the tropics have accumulated more species over time simply because of the lack of dramatic changes in climate.
As you have seen, an important aspect of biogeography is the relationship between diversity and geographic area. We have considered this relationship at global scales. We turn now to smaller regional scales, to understand in more detail the importance of geographic area to patterns of biogeography.