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