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Biology 2483A
Mark Moscicki

Biogeography Nov 12 2013 Introduction  Physical factors and species interactions are important regulators of species distributions at local scales. However, we must also look at the processes operating on larger scales. Global and regional scale processes are also important in determining the distributions and diversity of species on Earth.  Biogeography is the study of patterns of species composition and diversity across geographic locations.  If you took a trip across the world to visit every forest biome on earth, you would see that species composition and diversity differs from biome to biome. 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. Biogeographic Patterns on Earth 1. Species richness and composition vary with latitude. In general, the lower warmer latitudes have many more, and different, species than higher polar latitudes (closer you are to the equator, the higher the species richness and diversity is) 2. Species richness and composition also vary from continent to continent (even where latitude or longitude is similar)  In the northern hemisphere, we have temperate evergreen forests in the Pacific Northwest and in Southern California. In the southern hemisphere, we have temperate evergreen forests on the Northern and Southern islands of New Zealand. Although temperate evergreen forests in the Northern and Southern hemispheres are similar in some ways (low species richness compared to tropics), they are made up of completely different species assemblages (New Zealand is endemic meaning majority of its species are only found there!). 3. The same community type or biome can vary in species richness and composition depending on its location on Earth. What Now?  Ecologists have worked to understand the processes that control these broad patterns.  A number of hypotheses have been proposed to explain biogeographic variation in species composition and diversity. These hypotheses are highly dependent on the spatial scale at which they are applied. Different Scales  Patterns of species diversity and composition vary at global, regional, and local spatial scales.  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.  It is difficult to place area values on regional and local scales. The answer is highly dependent on the species/community of interest.  Terrestrial plants might have a 2 4 2 local scale of 10 –10 m , but for bacteria, the local scale might be 2 2 more like 10 cm . Global Scale  The global scale includes the entire world, in which there are major variations in latitude and longitude.  Important with regards to conservation  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 (also over evolutionary time scales. Regional Scale  Regional scale includes smaller geographic areas with uniform climate. The species are restricted by dispersal limitation (Polar bears originated in the Arctic and refuse to cross the tropical regions into Antarctica even though they could survive there)  All of the species contained in a region is known as the Regional species pool. This is sometimes referred to as the gamma diversity of the region. Gamma is broadest scale of species.  The regional species pool provides the raw material for local assemblages and sets the theoretical upper limit on species diversity for communities.  Earth's regions differ in species diversity and composition due to differences in the rates of speciation, extinction and dispersal at the global scale (Driving force that explains why the tropics have more species than Northern Canada) Landscape Scale  The physical geography of a region, such as the number, area and distance from one another of mountains, valleys , deserts, islands, lakes, etc is referred to as the Landscape (topographic and environmental features of a region) is critical to the biogeography within a region.  Species composition and diversity vary within a region depending on how the landscape shapes rates of extinction and migration to and from local habitats.  The connection between regional and local scales across a landscape is described as the Beta diversity. Beta diversity is a measurement that describes the change in species number and composition, or turnover of species, from one community type to another. (see diff species as you move across a landscape from community to community) Local Scale  The local scale is equivalent to a community.  It reflects how suitable the biotic and abiotic conditions of that area are for the species of the "regional pool" that disperse to that specific habitat  Species physiology and interactions with other species are important factors in the resulting species diversity (alpha diversity-relates to diversity in a community). Physical forces are another main driving force. Global Biogeography  Global patterns of species diversity and composition are controlled by geographic area and isolation, evolutionary history, and global climate. We want to know why we get these patterns.  Alfred Russel Wallace (1823–1913) is the father of biogeography. Although he is best known as the co-discoverer of natural selection, his main contribution was the study of species distributions across large spatial scales.  He travelled to Malay Archipelago (modern day Philippines) in 1852 where he noticed that the mammals of the Philippines were more similar to those in Africa (5,500 km away) than they were to those in New Guinea (750km away)  He overlaid species distributions and geographic regions and revealed two global patterns: 1) There is a gradient of species diversity with latitude. Species diversity is greatest in the tropics and decreases toward the poles (further N & S=less diversity) 2) Earth’s land mass can be divided into six biogeographic regions containing distinct biotas that differ in species composition and diversity.  The six biogeographic regions correspond roughly to Earth’s six major tectonic plates (sections of earths crust that move through action of currents generated deep within earths mantle).  The 6 biogeographic ranges are the Nearctic (North America), Neotropical (Central and South America), Palearctic (Europe and parts of Asia & Africa), Ethiopian (most of Africa), Oriental (India, China, Southeast Asia), and Australasian (Australia, Indo-Pacific, New Zealand)  There are 3 types of boundaries between tectonic plates. Mid ocean ridges are when molten rock flows from earths mantle and cools to form new crust, thus pushing the plates apart (seafloor spreading). Subduction zones occur where 2 plates meet and one is forced under the other (associated with volcanoes, earthquakes and mountain formation). When 2 plates meet, and they slide horizontally past one another, a fault has been created.  Consider the movement of plates since the Permian period (251 million years ago when all of the earths land masses made up one continent called Pangea) About 144 million years ago during the Cretaceous period, Pangea split into 2 continents (Laurasia to the north and Gondwana to the south). During the early Tertiary period (60 million years ago), Laurasia and Gondwana broke up to form today's continents.  NOTE: Biogeographical regionalism is also seen in oceans. Despite the appearance of connectivity, oceans have significant impediments to the exchange of biotas in the form of continents, currents, salinity, oxygen gradients, water depth  The legacy of continental drift can be found in the fossil record, genetic analyses, and existing taxonomic groups.  The evolutionary separation of species by barriers such as those formed by continental drift (oceans) is called vicariance. Tracing vicariance over large geographic areas and long periods of time provided important evidence for the theories of evolution  The large flightless birds (ratites) had a common ancestor from Gondwana. After isolation on different continents (Gondwana broke up), they evolved unique characteristics in isolation, but retained their large size and inability to fly.  New Zealand has 2 species. You expect the Kiwi and Moa to be most closely related but they are not! (Kiwis likely split off in Australia and somehow dispersed to New Zealand at a later date) Species Diversity Varies With Latitude  The latitudinal gradient in species diversity observed by Wallace has been documented repeatedly over the last 200 years. Plant species diversity and community composition changed dramatically with latitude (highest in tropics/decreasing toward poles).  Willig et al. (2003) compiled results of 162 studies on many taxonomic groups extending over broad spatial scales.  Negative relationships between latitude and diversity were by far the most common (meaning diversity decreased towards the poles).  Unimodal relationships were also evident (increasing toward mid latitudes and then decreasing toward the poles)  NOTE: The previous figure shows that not all groups of organisms show decreases in species richness at higher latitudes. Some groups display the opposite pattern.  Seabirds have their highest diversity at temperate and polar latitudes. Richness declines in the tropics and sub tropics. This pattern may be correlated with marine productivity which is higher in temperate and polar oceans.  Same pattern observed in marine benthic communities  In addition to strong latitudinal gradient, an important pattern of longitudinal variation has been observed. Gaston et al. (1995) measured the number of families along multiple north–south transects.  Number of families increased at low latitudes (tropics), but also depended on longitude. This pointed to the observation of hot spots. These so-called hot spots, or areas of high species richness, occur at particular longitudes. This is where we should focus ecological efforts. Latitudinal Gradients have Multiple, Interrelated Causes  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. Zero is no diversification (set # species). Positive means species gain and negative means species loss.  We want to know what controls this rate? Do diversification rates vary with geographic location? Many hypotheses have been proposed to explain patterns of species richness, but there is little agreement. Part of the reason for this is the fact that there are multiple and confounding latitudinal gradients in area, evolutionary age, and climate that are correlated with species diversity gradients (all changing at same time). In addition, manipulative experiments are impossible because speciation and extinction occur at a global spatial scale and over an evolutionary time scale.  Three hypotheses have been composed to explain latitudinal gradients in species richness Species Diversification Rate  Based on assumption that the rate of species diversification in the tropics is greater than that of temperate regions.  The tropics have the most land area on Earth and temperatures are very stable (land temps are remarkably uniform over a wide area between 25°N and S. but then drop off rapidly at higher latitudes).  It was suggested that the large area and uniform temperatures combine to decrease extinction rates and increase speciation rates.  By increasing population size, we decrease the risk of extinction due to chance events. A larger thermally stable area also allows for an expanded geographic range. Thus, risk of extinction is further decreased by spreading the risk over a larger geographic area  Speciation should increase as a result of having larger geographic ranges. We have a greater chance of reproductive isolation of populations leading to speciation.  Note: Graph below shows southern hemisphere with smallest land amount because it is mostly water. Species Diversification Time  Suggests that rates of diversification in the tropics and at higher latitudes are similar, but that the evolutionary time available for diversification has been much greater
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