CHAPTER 3 - Textbook Notes.docx

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University of Toronto Scarborough
Biological Sciences
Marc Cadotte

CHAPTER 3 THE BIOSPHEREThe American SerengetiTwelve Centuries of Change in the Great Plains A Case Studyy Temperate and highlatitude biological communities have been subjected to natural longterm climate change leading to latitudinal or elevational shifts in their positions and species compositiony A diverse collection of megafauna animals larger than 45 kg or 100 pounds existed in North America rivalling the diversity found today in the Serengeti y About 1000013000 years ago as the extensive grasslands of the Great Plains were developing many of the large mammals of North America suddenly went extincty An unusual aspect of this extinction was that nearly all of the animals that went extinct belonged to the same group large mammalsy The causes of this extinction are a mystery to palaeontologists Several hypotheses have been proposed changes in climate and the arrival of humans in North America controversial and evidence is weak for support of humans driving so many species of large mammals to extinctionIntroductiony The biosphere the zone of life on Earth is sandwiched between the lithosphere Earths surface crust and upper mantle and the troposphere the lowest layer of the atmosphere y Biomea largescale terrestrial biological community shaped by the regional climate soil and disturbance patterns where it is found usually classified by the growth form of the dominant plants Concept 31 Terrestrial biomes are characterized by the growth forms of the dominant vegetationTerrestrial Biomesy Biomes are largescale biological communities shaped by the physical environment in which they are found y Biomes are categorized by the most common forms of plants distributed across large geographic areas The categorization of biomes does not take taxonomic relationships among organisms into account instead it relies on similarities in the morphological responses of organisms to the physical environmenty Terrestrial communities vary considerably from the warm wet tropics to the cold dry polar regionsy Tropical forests have multiple verdant layers high growth rates and tremendous species diversity y Lowland tropical forests in Borneo have an estimated 10000 species of vascular plants and most other tropical forest communities have 5000 speciesy Polar deserts have a scattered cover of tiny plants clinging to the ground reflecting a harsh climate of high winds low temperatures and dry soilsy Terrestrial biomes are classified by the growth form size and morphology of the Figure 33Pg 51 dominant plants for example trees shrubs or grasses Characteristics of their leaves such as deciduousness seasonal shedding of leaves thickness and succulence development of fleshy water storage tissues may also be usedy Why use plants rather than animals to categorize terrestrial biomes Plants are immobile so in order to occupy a site successfully for a long time they must be able to cope with its environmental extremes as well as its biological pressures such as competition for water nutrients and light In addition animals are a less visible component of most large landscapes and their mobility allows them to avoid exposure to adverse environmental conditionsy Since their emergence from the oceans about 500 million years ago plants have taken on a multitude of different forms in response to the selection pressures of the terrestrial environment These selection pressures include aridity high and subfreezing temperatures intense solar radiation nutrientpoor soils grazing by animals and crowding by neighboursy Similar plant growth forms appear in similar climatic zones on different continents even though the plants may not be genetically relatedy The evolution of similar growth forms among distantly related species in response to similar selection pressures is called convergence Terrestrial biomes reflect global patterns of precipitation and temperaturey The tropics between 235 N and S are characterized by high rainfall and warm invariant temperaturesy In the subtropical regions that border the tropics rainfall becomes more seasonal with pronounced dry and wet seasonsy The major deserts of the world are associated with the zones of high pressure at about 30 N and S with the rainshadow effects of large mountain rangesy Subfreezing temperatures during winter are an important climatic feature of the temperate and polar zones The amount of precipitation north and south of 40 varies depending on proximity to the ocean and the influence of mountain rangesy The locations of terrestrial biomes are correlated with these variations in temperature and precipitation Temperature influences the distribution of plant growth forms directly through its effect on the physiological functioning of plants Precipitation and temperature act in concert to influence the availability of water and its rate of loss by plants Water availability and soil temperature are important in determining the supply of nutrients in the soil which is also an important control on plant growth formThe potential distributions of terrestrial biomes differ from their actual distributions due to human activitiesy The effects of land conversion and resource extraction by humans are increasingly apparent on the land surface These human effects are collectively described as land use change
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