Ecology Lecture No. 17: Biogeography
Tuesday November 6 , 2012
-Biogeography is the study of patterns of species composition and diversity across geographic locations.
In general, the lower latitudes have many more, and different, species than higher latitudes. For
example, The Amazon rainforest is the most species-rich forest in the world, with approximately 1,300
tree species. In contrast, the boreal forests of Canada have only two tree species that cover vast areas.
-Species richness and composition also vary from continent to continent. The same community type or
biome can vary in species richness and composition depending on its location on Earth. Ecologists have
worked to understand the processes that control these broad patterns. A number of hypotheses have
been proposed, which are highly dependent on spatial scale.
Spatial Scale & Diversity:
-The global scale constitutes the entire world. Here species have been isolated from one another, on
different continents or in different oceans, by long distances and over long periods Rates of speciation,
extinction, and dispersal help determine differences in species diversity and composition. Regional
scales encompass areas with uniform climate where the species are bound by dispersal to that region. A
regional species pool contains all the species contained within a region (gamma diversity) and provides
the raw material for local assemblages, while setting up the theoretical limit for species diversity in
-Landscape is the topographic and environmental features of a region. Species composition and diversity
vary within a region depending on how the landscape shapes rates of migration and extinction. Local
scale is equivalent to a community. Species physiology and interactions with other species are important
factors in the resulting species diversity (alpha diversity). Beta diversity describes the change in species
number and composition, or turnover of species, from one community type to another and connects
local and regional scales.
-The actual area of different spatial scales depends on the species and communities of interest.
-Global patterns of species diversity and composition are controlled by geographic area and isolation,
evolutionary history, and global climate.
Global Patterns Of Species Distribution:
-Alfred Russel Wallace overlaid species distributions and geographic regions and revealed two global
patterns: There is a gradient of species diversity with latitude and Earth’s land mass can be divided into
six biogeographic regions (each corresponding roughly to one of Earth’s six major tectonic plates). Earth’s Land Mass & Continental Drift:
-The legacy of continental drift can be found in the fossil record and in existing taxonomic groups.
Vicariance is the evolutionary separation of species by barriers such as those formed by continental
drift. For example, after isolation on different continents, the world’s flightless birds evolved unique
characteristics, but retained their large size and inability to fly.
The Latitudinal Gradient In Species Diversity:
-In compiling numerous studies regarding the latitudinal gradient in species diversity, negative
relationships between latitude and diversity (lower diversity at higher latitude) were by far the most
common. By measuring the number of families along multiple north–south transects, Gaston et al.
(1995) found that the number of families increased at low latitudes, but also depended on longitude.
These so-called hot spots, or areas of high species richness, occur at particular longitudes.
Global Patterns Of Species Richness:
-Global patterns of species richness should be controlled by three processes: Speciation, extinction, and
dispersal. If we assume dispersal rates are similar everywhere, then species richness should reflect a
balance between extinction and speciation. Subtracting extinction rate from speciation rate gives the
rate of species diversification (the net increase or decrease of species over time). Mittelbach et al. (2007)
summarized the hypotheses explaining latitudinal diversity patterns in three categories: Species
diversification rate, species diversification time and productivity or carrying capacity.
Latitudinal Diversity Patterns Of Species Richness:
-The species diversification rate states that the tropics have the most land area on Earth and their
temperatures are very stable. Large, thermally stable areas should decrease extinction rates and make
speciation by geographic isolation more likely.
-The species diversification time states that the tropics are thought to have been more climatically
stable over time, giving species more time to evolve. Temperate and polar regions are thought to have
undergone severe climatic changes such as glaciation, thereby disrupting species diversification.
Another idea is that most species originate in the tropics and then move to other regions during warm
climatic periods (as discovered by Jablonski et al. (2006) who found evidence of this in marine bivalves).
Loss of biodiversity in the tropics could cut off the supply of new species to higher latitudes in the
-The productivity or carrying capacity states that produ