Biogeography

Announcements • Dr. Pepper’s farewell performance today.

Tue April 5

• I’ll remain available via email and office hours till summer.

Biogeography

• Midterm Exam 3 This Thursday April 6 ƒ Includes all lectures starting with March 2 (Neural systems). ƒ Also includes today’s lecture. ƒ Q&A review session with Drs. Masel & Pepper: tomorrow, Wed 4/5 4:00-5:00pm in Koffler 204.

Reading: Ch. 56 Quiz available online for extra credit through Wed. April 12

Outline

What is Biogeography? • The study of the distribution patterns of populations, species, and ecological communities

• Biogeographic Regions • History and Biogeography • Ecology and Biogeography • Terrestrial biomes (skim only) • Island Biogeography theory (whiteboard)

What are the basic patterns of biogeography?

ƒ Species distribution boundaries among different kind of organisms tend to overlap

Earth’s Biogeographic Regions

ƒ As a result, the major geographic areas of Earth have strikingly different biota (sets of species) ‰

Much of biogeography is concerned with explaining the origins, locations, and characteristics of these distinct biota.

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Figure 56.1 Major Biogeographic Regions

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 typically have distinctive endemic biotas because water barriers greatly restrict migration. (Madagascar is a good example.) • Most species are confined to a single biogeographic region, but Homo sapiens is the most widespread species on Earth today.

Mechanisms of Biogeography • Why does a species occur in a particular location? History and Biogeography

• There are only two possible answers: 1. It evolved there. 2. It moved there from somewhere else.

History and Biogeography • Past events influenced 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.

History and Biogeography • In 1912, Alfred Wegener proposed the idea of continental drift, based on several observations: ƒ The shapes of the continents (e.g., Africa and South America) seem to fit together like a puzzle. ƒ The alignment of mountain chains, rock strata, and glacial deposits suggest movement over time. ƒ The distribution of organisms on Earth is hard to explain if one assumes the continents never moved.

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Figure 22.15 Positions of the Continents during the Cretaceous Period

History and Biogeography Continental Drift: • 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 a northern land mass (Laurasia) and a southern land mass (Gondwana). • Throughout the history of life, continental drift has separated and combined biotas, greatly influencing the distribution of species.

Figure 56.3 Taxonomic Phylogeny to Area Phylogeny (Part 2)

History and Biogeography • 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. • Example: 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)

Figure 56.3 Taxonomic Phylogeny to Area Phylogeny (Part 3)

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Two causes of divided species ranges • A vicariant event is the appearance of a barrier that splits the range of a species. ƒ Vicariant events include sea level changes, mountain building, and continental movement. • A dispersal event is when members of a species cross an existing barrier and establish a new population on the other side.

History and Biogeography • By studying a single evolutionary lineage as well as distribution patterns among lineages, scientists can discover the roles of both vicariant events and dispersal events 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. ƒ 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.

Figure 56.4 A Vicariant Distribution Explained

History and Biogeography A Case Study of a single lineage: • Did two populations result from a vicariance or a dispersal event? • When several hypotheses can explain a pattern, the most parsimonious hypothesis (that which requires the least number of unobserved events to explain it) is generally preferred.

Figure 56.4 A Vicariant Distribution Explained

Vacariance or dispersal? • 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) separated the range of the weevil and the other animals than that individual crossings of Cook Strait did.

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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.

Geography Influences Ecology • 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 0.4°C for every degree of latitude. • Air temperature also decreases with elevation.

Figure 56.6 The Circulation of Earth’s Atmosphere

Ecology and Biogeography • 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

Ecology and Biogeography • Air masses descend at 30° north 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.

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Figure 56.7 A Rain Shadow

Ecology and Biogeography • 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 56.9 Biomes Have Distinct Geographic Distributions

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.

Terrestrial Biomes • Facts to know about key terrestrial biomes: • Major climatic features (temperature and rainfall). • Seasonal pattern of biological activity. • Dominant vegetation type. • Relative species richness.

Terrestrial Biomes • The key terrestrial biomes include: ƒ Tundra (arctic and alpine) ƒ Boreal forest ƒ Temperate deciduous forest ƒ Temperate grassland ƒ Cold desert ƒ Hot desert ƒ Chaparral

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Biomes: Tundra (Part 1)

Terrestrial Biomes: an example • 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 2)

Biomes: Tundra (Part 3)

Biomes: Tundra (Part 4)

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

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Biomes: Boreal Forest (Part 1)

Terrestrial Biomes: boreal forest • The boreal forest biome is found south of the tundra biome and at lower elevations on temperatezone 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 2)

Biomes: Boreal Forest (Part 3)

Biomes: Boreal Forest (Part 4)

Terrestrial Biomes: temperate deciduous • 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.

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Biomes: Temperate Deciduous Forest (Part 1)

Biomes: Temperate Deciduous Forest (Part 2)

Biomes: Temperate Deciduous Forest (Part 3)

Terrestrial Biomes: temperate grassland • 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)

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Biomes: Temperate Grasslands (Part 3)

Biomes: Temperate Grasslands (Part 4)

Biomes: Cold Desert (Part 1)

Terrestrial Biomes: cold desert • 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 seedproducing plants, birds, ants, and rodents.

Biomes: Cold Desert (Part 2)

Biomes: Cold Desert (Part 3)

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Biomes: Cold Desert (Part 4)

Terrestrial Biomes • The hot desert biome is found in two belts, centered around 30° north and 30° south 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)

Biomes: Hot Desert (Part 3)

Terrestrial Biomes • 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.

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Biomes: Chaparral (Part 1)

Biomes: Chaparral (Part 2)

Biomes: Chaparral (Part 3)

Biomes: Chaparral (Part 4)

Biomes: Thorn Forest and Tropical Savanna (Part 1)

Terrestrial Biomes • 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.

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Biomes: Thorn Forest and Tropical Savanna (Part 2)

Biomes: Thorn Forest and Tropical Savanna (Part 3)

Biomes: Thorn Forest and Tropical Savanna (Part 4)

Terrestrial Biomes • 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)

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Biomes: Tropical Deciduous Forest (Part 3)

Biomes: Tropical Deciduous Forest (Part 4)

Biomes: Tropical Evergreen Forest (Part 1)

Terrestrial Biomes • 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 2)

Biomes: Tropical Evergreen Forest (Part 3)

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Biomes: Tropical Evergreen Forest (Part 4)

Skipped: 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.

Skipped: 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.

Island Biogeography

• 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.

Island Biogeography

Data: Species richness on islands

• Observed patterns: Islands have fewer species per unit area than do large land masses (and smaller islands have fewer still). More remote islands also have fewer species. • A theory to explain these patterns was developed by R. MacArthur & E. O. Wilson. • A theory to explain these patterns will now be developed by you!

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Island Biogeography Theory • Assumptions: • Because speciation is relatively infrequent, it is of secondary importance in determining species richness. • The key factors are rates of colonization (e.g. from mainland) and of extinction.

(writing on board)

• Observed species richness results from an equilibrium between these two processes. • The rate at which colonists arrive is largely independent of island size. • Smaller populations are more likely to go extinct.

Island Biogeography (1)

Island Biogeography (2)

Island Biogeography: a “natural experiment”

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