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Lecture 17

Lecture 17 - Biogeography

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Biology 2483A
Hugh Henry

LECTURE 17: BIOGEOGRAPHY Introduction  Physical factors and species interactions are important regulators of species distributions at local scales  But global and regional scale processes are also important in determining the distributions and diversity of species on Earth  Biogeography – the study of patterns of species composition and diversity across geographic locations o For example: The Amazon rainforest is the most species-rich forest in the world, with approximately 1,300 tree species o In contrast, the boreal forests of Canada have only two tree species, that cover vast areas  In general, the lower latitudes have many more, and different, species than higher latitudes  Species richness and composition also vary from continent to continent  The same community type of 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 scales are interconnected in a hierarchical way, with the patterns of species diversity and composition at one spatial scale setting the conditions for patterns at smaller spatial scales  Global scale – the entire world o Species have been isolated from one another, on different continents or in different oceans, by long distances and over long periods o Rates of speciation, extinction and dispersal help determine differences in species diversity and composition  Regional scale – areas with uniform climate; the species are bound by dispersal to that region o Regional species pool – all the species contained within a region (gamma diversity) o The regional species pool provides the raw material for local assemblages and sets the theoretical upper limit on species diversity for communities  Landscape scale – topographic and environmental features of a region o Species composition and diversity vary within a region depending on how the landscape shapes rates of migration and extinction  Local scale – equivalent to a community o Species physiology and interactions with other species are important factors in the resulting species diversity (alpha diversity)  Beta diversity – change in species number and composition, or turnover of species, from one community type to another o Beta diversity connects local and regional scales  Actual area of different spatial scales depends on the species 2nd 4om2unities of interest o Example – terrestrial plants might have a local scale of 10 –10 m , but for bacteria, the local scale might be more like 10 cm 2 Global Biogeography  Global patterns of species diversity and composition are controlled by geographic area and isolation, evolutionary history and global climate  Alfred Russel Wallace (1823–1913) is the father of biogeography. His main contribution was the study of species distributions across large spatial scales  He overlaid species distributions and geographic regions and revealed two global patterns: o There is a gradient of species diversity with latitude o Earth’s land mass can be divided into six biogeographic regions  The six biogeographic regions correspond roughly to Earth’s six major tectonic plates  The legacy of continental drift can be found in the fossil record and in existing taxonomic groups  Vicariance—evolutionary separation of species by barriers such as those formed by continental drift o Example – the large flightless birds (ratites) had a common ancestor from Gondwana. After isolation on different continents, they evolved unique characteristics, but retained their large size and inability to fly  The latitudinal gradient in species diversity observed by Wallace has been documented repeatedly over the last 200 years o Willig et al. (2003) compiled results of 162 studies on many taxonomic groups o Negative relationships between latitude and diversity were by far the most common  Gaston et al. (1995) measured number of families along multiple north–south transects. o Number of families increased at low latitudes, but also depended on longitude o So-called hot spots, or areas of high species richness, occur at particular longitudes  Many hypotheses have been proposed to explain patterns of species richness, but there is little agreement  There are multiple and confounding latitudinal gradients in area, evolutionary age, and climate  Manipulative experiments are impossible because of the global spatial scale and evolutionary time scale  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: 1. Species diversification rate  The tropics have the most land area on Earth and temperatures are very stable  Large, thermally stable areas should decrease extinction rates; speciation by geographic isolation would be more likely 2. Species diversification time  The tropics are thought to have been more climatically stable over time, and species have had more time to evolve  Temperature and polar regions have undergone severe climatic changes such as glaciation, disrupting species diversification  Another idea is that most species originate in the tropics and then move to other regions during warm climatic periods  Jablonski et al. (2006) found evidence of this in marine bivalves. Most extant taxa originated in the tropics and spread towards the poles  Thus the tropics could be seen as a “cradle” of diversity  But they can also be a “museum”—species that diversify there tend to stay there  Loss of biodiversity in the tropics could cut off the supply of new species to higher latitudes in the future 3. Productivity or carrying capacity  Productivity is highest in the tropics (t
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