56 Biogeography. 56 Introduction Earth’s Biogeographic Regions History and Biogeography Ecology...

56Biogeography

56 Biogeography

• Introduction

• Earth’s Biogeographic Regions

• History and Biogeography

• Ecology and Biogeography

• Terrestrial Biomes

• Aquatic Biogeography

• Regional Patterns of Species Richness

• Biogeography and Human History

56 Introduction

• Darwin predicted that species that are widespread would be more abundant and variable than species with narrow distributions.

• Widespread species are often more abundant locally, but no species is found everywhere.

• Biogeography is the study of the patterns of distribution of populations, species, and communities across Earth.

56 Earth’s Biogeographic Regions

• The question of why a species occurs in a particular location has two possible answers:

It evolved there or it moved there from elsewhere.

If a species is absent, either it was never there or it was once present but no longer lives there.

• Biogeographers interpret a wide array of information to explain the distribution of the organisms.

• This includes information about evolutionary history, continental drift, glacial advances/retreats, sea level changes, and mountain building.

56 Earth’s Biogeographic Regions

• Earth can be divided into several major biogeographic regions.

• A species found only in a certain region is endemic to that region.

• Remote islands such as Madagascar typically have distinctive endemic biotas because water barriers greatly restrict migration.

• Most species are confined to a single biogeographic region, but Homo sapiens is the most widespread species on Earth today.

Figure 56.1 Major Biogeographic Regions

56 History and Biogeography

• Past events influence the distribution of species on Earth.

• Early biogeographers, such as Linnaeus, believed that the continents were fixed in their positions, and that all organisms were created in one place from which they later dispersed.


• In 1912, Alfred Wegener proposed the idea of continental drift, based on several observations:

The shapes of the continents (e.g., Africa and South America) seem to fit together like a puzzle.

The alignment of mountain chains, rock strata, and glacial deposits suggest movement over time.

The distribution of organisms on Earth is hard to explain if one assumes the continents never moved.


• About 280 million years ago in the Permian period, the continents were united in a land mass called Pangaea.

• By 100 million years ago during the Cretaceous period, Pangaea had separated into northern (Laurasia) and southern (Gondwana) land masses.

• Throughout the history of life, continental drift has separated and combined biotas, greatly influencing the distribution of species.

Figure 22.15 Positions of the Continents during the Cretaceous Period


• Area phylogenies are used to describe when and where evolutionary lineages originated.

• To generate an area phylogeny, the names in a taxonomic phylogeny are replaced with the names of the places where those taxa live or lived.

• An area phylogeny suggests that horses speciated as they moved from Africa to Asia.

• To infer the approximate times of separation of lineages, biogeographers use molecular difference between species, fossils to determine how long a taxon has been in an area, and the distribution of living species.

Figure 56.3 Taxonomic Phylogeny to Area Phylogeny (Part 1)


• A vicariant event is the appearance of a barrier that splits the range of a species.

• Vicariant events include sea level changes, mountain building, and continental movement.

• If members of a species cross an existing barrier and establish a new population, the species’ disjunct range is the result of dispersal.


• By studying a single evolutionary lineage as well as distribution patterns among lineages, scientists can discover the relative roles of vicariant events and dispersal in determining today’s distribution patterns.

• The longer an area has been isolated from other areas, the more endemic taxa it is likely to have.

• Australia has been separated from other continents for 65 million years and has the most distinct biota on Earth.

• North America and Eurasia were joined together for much of Earth’s history and have very similar biotas.


• When several hypotheses can explain a pattern, the parsimonious hypothesis (that which requires the least number of unobserved events to explain it) is generally preferred.


• An example is found in the distribution of the New Zealand flightless weevil.

• The weevil and other flightless insects are found on the north and south islands of New Zealand.

• Geological evidence suggests that the tip of the north island was once connected to the south island.

• Therefore, it is more likely that a vicariant event (separation of the land) allowed the dispersal of the weevil and the other animals than that individual crossings of Cook Strait did.

Figure 56.4 A Vicariant Distribution Explained

56 Ecology and Biogeography

• The climate of a region is the average of the atmospheric conditions found there over time.

• Climates vary greatly on Earth and influence the geographic distribution of species.


• Solar energy inputs drive global climates.

• Every place on Earth receives the same total number of hours of sunlight each year, but not the same amount of energy.

• The rate at which solar energy arrives at the Earth’s surface depends primarily on the angle of the sunlight. At high latitudes, solar energy inputs vary greatly throughout the year.

• Mean air temperature decreases about 0.4C for every degree of latitude.

• Air temperature also decreases with elevation.


• Earth’s climates are strongly influenced by global air circulation patterns which result from global variation in solar input.

• Air rises when heated and releases moisture. Warm air rises in the Tropics and is replaced by air flowing towards the equator from north and south. The intertropical convergence zone is where these air masses come together.

• Heavy rains usually fall in a region when it is close to the intertropical convergence zone.

• This zone shifts latitudinally with the seasons, resulting in patterns of rainy and dry seasons.

Figure 56.5 Rainy and Dry Seasons Change with Latitude


• Air masses descend at 30north and south. This air is cool and has lost its moisture. Many deserts are located at these latitudes.

• The movements of air masses are responsible for global wind patterns.

• The spinning of Earth on its axis also influences surface winds. Air masses are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Figure 56.6 The Circulation of Earth’s Atmosphere


• When air encounters mountain ranges, it rises, cools, and drops moisture on the windward slopes resulting in a precipitation distribution called a rain shadow where the leeward slopes are dry.

Figure 56.7 A Rain Shadow


• Global wind circulation patterns drives the circulation of ocean water.

• Ocean water generally moves in the direction of the prevailing winds.

• Winds blowing toward the equator cause warm water to converge at the equator and move west until it encounters a landmass.

• When warm equatorial water encounters a landmass, it splits and moves north or south; this is a major mechanism of heat transfer to high latitudes.

Figure 56.8 Global Oceanic Circulation

56 Terrestrial Biomes

• Ecologists classify communities of organisms into biomes.

• Biomes are major ecosystem types based on the structure of the dominant vegetation.

• The vegetation of a biome has a similar appearance wherever that biome is found on Earth.

• The distribution of biomes on Earth is influenced by annual patterns of temperature and rainfall.

• Each biome has a characteristic climate, seasonality, and vegetation, and typical patterns of species richness.

Figure 56.9 Biomes Have Distinct Geographic Distributions


• The tundra biome is found in the Arctic and high on mountains.

• In the Arctic, permanently frozen soil (permafrost) underlies tundra vegetation.

• Plants grow only during the short summers when the first few centimeters of permafrost melt.

• Lowland Arctic tundra is very wet because water cannot drain through the permafrost.

• Arctic tundra animals either migrate into the area for the summer only or are dormant for most of the year.

Biomes: Tundra (Part 1)

Biomes: Tundra (Part 2)


• Tropical alpine tundra is not underlain by permafrost, so photosynthesis and other biological activities continue throughout the year and more plant forms are present.


• The boreal forest biome is found south of the tundra biome and at lower elevations on temperate-zone mountains.

• Winters are long and very cold, while summers are short and warm.

• The short summer favors trees with evergreen leaves.

• Boreal forests have only a few tree species.

• Northern Hemisphere forests are dominated by coniferous evergreen gymnosperms.

• Southern Hemisphere forests are dominated by beech trees.

Biomes: Boreal Forest (Part 1)

Biomes: Boreal Forest (Part 2)


• The temperate deciduous forest biome is found in eastern North America, eastern Asia, and western Europe.

• Temperatures fluctuate dramatically from season to season.

• Precipitation is evenly distributed throughout the year.

• Deciduous trees lose their leaves during the winter.

• Many more tree species are present relative to boreal forests.

Biomes: Temperate Deciduous Forest (Part 1)

Biomes: Temperate Deciduous Forest (Part 2)


• The temperate grassland biome is found in many parts of the world, all of which are relatively dry much of the year.

• Most grasslands have hot summers and cold winters.

• Grasslands are structurally simple, but they are rich in species of perennial grasses, sedges, and forbs. Grassland plants are adapted to grazing and fire.

• Most of the grassland biome has been converted to agriculture.

Biomes: Temperate Grasslands (Part 1)

Biomes: Temperate Grasslands (Part 2)


• The cold desert biome is found in dry regions at middle to high latitudes.

• Cold deserts are also found at high altitudes in the rain shadows of mountain ranges.

• Seasonal temperatures vary greatly.

• Cold deserts are dominated by a few species of low-growing shrubs.

• The most common taxa in the biome are seed-producing plants, birds, ants, and rodents.

Biomes: Cold Desert (Part 1)

Biomes: Cold Desert (Part 2)


• The hot desert biome is found in two belts, centered around 30north and 30south latitudes.

• Central Australia and the middle of the Sahara Desert are the driest regions within the biome.

• Except in the driest regions, hot deserts have richer and more diverse vegetation than cold deserts do.

• Succulent plants that store large quantities of water in their stems are common. Annual plants germinate and grow when rain falls.

Biomes: Hot Desert (Part 1)

Biomes: Hot Desert (Part 2)


• The chaparral biome is found on the west sides of continents at moderate latitudes, where cool ocean waters flow offshore.

• The Mediterranean region of Europe, coastal California, and central Chile are examples of chaparral.

• Low-growing shrubs and trees with evergreen leaves are the most common plants in chaparral. The vegetation is adapted to periodic fires.

• Large populations of small seed-eating rodents are present in the biome.

Biomes: Chaparral (Part 1)

Biomes: Chaparral (Part 2)


• Thorn forests are found on the equatorial sides of hot deserts. The climate is semi-arid with little or no rain in winter, but sometimes heavy rain in summer.

• The dominant plants are spiny shrubs and small trees. Acacia is common.

• Savannas are found in dry tropical and subtropical regions of Africa, South America, and Australia.

• The savanna biome is characterized by its vast expanses of grassland and scattered trees, and by huge numbers of grazing and browsing mammals.

Biomes: Thorn Forest and Tropical Savanna (Part 1)

Biomes: Thorn Forest and Tropical Savanna (Part 2)


• The tropical deciduous forest biome is found closer to the equator relative to thorn forests and has a long summer rainy season.

• Species richness is moderate for plants and high across all other categories, including mammals, birds, reptiles, and amphibians.

• The tropical deciduous forest biome has some of the best soils in the tropics for agriculture. Most of it has been cleared.

Biomes: Tropical Deciduous Forest (Part 1)

Biomes: Tropical Deciduous Forest (Part 2)


• Tropical evergreen forests are found in equatorial regions where total rainfall exceeds 250 cm annually.

• The biome is the richest on Earth in both plant and animal species.

• Overall productivity of tropical evergreen forests is the highest among terrestrial ecological communities.

• There are many epiphytes, plants that grow on other plants and derive nutrients and moisture from air and water.

Biomes: Tropical Evergreen Forest (Part 1)

Biomes: Tropical Evergreen Forest (Part 2)

56 Aquatic Biogeography

• Three-fourths of Earth’s surface is covered by water.

• The oceans represent one large interconnected water mass with no obvious barriers for dispersal.

• Fresh water is divided into river basins and thousands of relatively isolated lakes.

• Terrestrial habitats are a barrier to dispersal of freshwater aquatic organisms.


• About 2.5 percent of Earth’s water is found in ponds, lakes, and streams.

• Freshwater ecosystems contain about 10 percent of all aquatic species.

• More than 25,000 insect species such as dragonflies have at least one aquatic stage in their lives (usually the larva).

• Most families of freshwater fishes are restricted to a single continent due to the saltwater barrier presented by Earth’s oceans.


• Ocean water moves in great circular patterns which determine biogeographic patterns.

• Most marine organisms have restricted ranges.

• Water temperature and salinity can be barriers to dispersal of marine organisms.

• Deep ocean waters prevent the dispersal of marine organisms that live only in shallow water.

• Richness of shallow-water species near isolated islands of the Pacific decreases with distance from the larger islands of Indonesia.

Figure 56.10 Oceanic Biogeographic Regions are Determined by Ocean Currents

Figure 56.11 Generic Richness of Reef-Building Corals Declines with Distance from Indonesia

56 Regional Patterns of Species Richness

• Species richness increases with area sampled.

• If the sampling area crosses a biogeographic boundary, the rate at which new species are counted increases.

Figure 56.12 Species Richness Increases with Area Sampled


• One of the first geographic patterns of species richness observed was that more species are present in low latitudes than high latitudes.

• More species are found in mountainous regions than in relatively flat areas because more vegetation types and climates exist in the mountains.

Figure 56.13 The Latitudinal Gradient of Species Richness of North American Mammals


• Species richness on islands is always less than an equivalent area of the mainland.

• Species richness on islands is positively correlated with island size and inversely correlated with distance from the mainland.

Figure 56.14 Small, Distant Islands Have Fewer Bird Species


• Over periods of a few hundred years, species richness is influenced by immigration of new species and the extinction of existing species.

• The MacArthur-Wilson model relates species richness to immigration and extinction on an island.

• The rate of arrival of new species and the rate of extinctions of species already present determine the equilibrium number of species on an island.

• The rate of immigration and extinction on an island is affected by the size of the island and distance from the mainland.

Figure 56.15 MacArthur and Wilson’s Model of Species Richness on Islands (Part 1)


• MacArthur and Wilson’s model can be used to predict how species richness will vary among islands of different sizes and distance from the mainland.

• The number of species should be highest for islands that are relatively large and closest to the mainland.

Figure 56.15 MacArthur and Wilson’s Model of Species Richness on Islands (Part 2)


• Major disturbances can sometimes serve as “natural experiments.”

• The eruption of Krakatau in 1883 destroyed all life on the island’s surface, but it provided a test of the MacArthur and Wilson model.

• By 1933 the island was again covered by a tropical evergreen forest.

• While forest canopy was recovering, there were high rates of colonization. Today, rates of colonization are not as fast, but colonization and extinctions are still occurring.

Table 56.1 Number of Species of Resident Land Birds of Krakatau

56 Biogeography and Human History

• The distributions of land masses and species on Earth have had a strong influence on human history.

• In recent times, human populations from Eurasia have come to dominate other cultures. Biogeography contributed to this.

• Eurasia happened to have a large number of plants (large-seeded grasses) and animals suitable for domestication.

• Thirteen large mammal species, including pigs, horses, cattle, sheep, goats, and camels, were domesticated in Eurasia.


• No animals were domesticated in Africa, and only the llama was domesticated in the Americas.

• For domestication, large mammals needed three characteristics: herding lifestyles, male-dominated hierarchies, and a lack of territoriality. The large mammals of Africa all lacked at least one of these traits.

• Domesticated animals provided food and labor for farming.


• Domestication of large mammals also introduced diseases such as smallpox and measles to the human population.

• Eurasian people acquired immunity to these diseases.

• When Europeans colonized the New World, they exposed the indigenous people to smallpox and measles, and without immunity, many indigenous people died.


• In Eurasia, most mountain ranges are oriented east–west; therefore, dispersal of people was relatively easy.

• Humans dispersed only recently into North America across the Bering Land Bridge. The only domesticated animal they brought was the dog.

• There were few species of grasses with large seeds in North America. Maize came to dominate, but it was difficult to domesticate.

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