Chapter 56
Biogeography
Biogeography
Introduction
Earths Biogeographic Regions
History and Biogeography
Ecology and Biogeography
Terrestrial Biomes
Aquatic Biogeography
Regional Patterns of Species Richness
Biogeography and Human History
Introduction
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.
Earths Biogeographic Regions
The question of __________ a species occurs in a particular location has two possible answers:
It __________ there or it __________ 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.
Earths 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
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 __________ proposed the idea of continental drift, based on several observations:
The shapes of the continents (e.g., Africa and
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.
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
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 todays distribution patterns.
The longer an area has been __________ from other areas, the more endemic taxa it is likely to have.
North
America and
History and Biogeography
When several hypotheses can explain a pattern, the __________ 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
The weevil
and other flightless insects are found on the north and south islands of
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
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 Earths 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.
Ecology and Biogeography
Earths 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 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.
[What is this called? And, what does it have to do with hurricanes?]
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 __________ 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
In the
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
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 __________.
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 __________ desert biome is found in two belts, centered around 30north and 30south latitudes.
Central
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.
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
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.
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
The savanna biome is characterized by its vast expanses of grassland and scattered trees, and by huge numbers of grazing and browsing mammals.
Terrestrial Biomes
The tropical __________ 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.
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.
Aquatic Biogeography
Three-fourths of Earths 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.
Aquatic Biogeography
About 2.5 percent of Earths 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 Earths oceans.
Aquatic Biogeography
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
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.
Regional Patterns of Species Richness
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 __________ regions than in relatively flat areas because more vegetation types and climates exist in the mountains.
Regional Patterns of Species Richness
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.
Regional Patterns of Species Richness
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.
Regional Patterns of Species Richness
MacArthur and
The number of species should be highest for islands that are relatively large and closest to the mainland.
Regional Patterns of Species Richness
Major __________ can sometimes serve as natural experiments.
The
eruption of
By 1933 the island was again covered by a tropical evergreen forest.
While forest canopy was recovering, there were high rates of __________. Today, rates of colonization are not as fast, but __________ and __________ are still occurring.
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
Thirteen
large mammal species, including pigs, horses, cattle, sheep, goats, and camels,
were domesticated in
Biogeography and Human History
No animals
were domesticated in Africa, and only the llama was domesticated in the
For
domestication, large mammals needed three characteristics: herding lifestyles,
male-dominated hierarchies, and a lack of territoriality. The large mammals of
Domesticated animals provided food and labor for farming.
Biogeography and Human History
Domestication of large mammals also introduced diseases such as __________ 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 __________.
Biogeography and Human History
In
Humans
dispersed only recently into
There were
few species of grasses with large seeds in
Animation 56.1 Rain Shadow
Animation 56.2 Biomes
Video 56.1 Global oceanic circulation and temperature
Video 56.2 A redwood forest and succession in a forest gap
Video 56.3
Portrait of a grassland ecosystem: The Serengeti Plains of
Video 56.4 Portrait of a rainforest ecosystem: The Amazon of South America and other locations
Video 56.5
Portrait of a marine ecosystem: A coral reef in the