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Lecture 17

Lecture 17: "Biogeography"

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Western University
Biology 2483A
Hugh Henry

Ecology Lecture No. 17: Biogeography th Tuesday November 6 , 2012 Introduction: -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 communities. -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 Biogeography: -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 future. -The productivity or carrying capacity states that produ
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