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Lecture 3

Lecture 3 - The Biosphere

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Western University
Biology 2483A
Hugh Henry

LECTURE 3: THE BIOSPHERE  An overview of what the world is made of biologically  The biosphere is the zone of life on Earth  Biomes are large-scale biological communities shaped by the physical environment, particularly climate.  Biomes are categorized by dominant plant forms, not taxonomic relationships  Plants occupy sites for a long time and are good indicators of the physical environment, reflecting climatic conditions and disturbances. o From an ecological standpoint, this means that plants need to be able to tolerate their surroundings no matter what as they cannot move  Terrestrial biomes are characterized by growth forms of the dominant plants, such as leaf deciduousness or succulence Plant Growth Forms  Plants have taken many forms in response to selection pressures such as aridity, extreme temperatures, intense solar radiation, grazing, and crowding  Similar growth forms can be found on different continents, even though the plants are not genetically related  Convergence: evolution of similar growth forms among distantly related species in response to similar selection pressures  Temperature has direct physiological effects on plant growth form  Precipitation and temperature act together to influence water availability and water loss by plants  Water availability and soil temperature determine the supply of nutrients in the soil Global Biome Distribution  Human activities influence the distribution of biomes  Land use change: conversion of land to agriculture, logging, resource extraction, urban development  The potential and actual distributions of biomes are markedly different  There are nine major terrestrial biomes  Climate diagrams show the characteristic seasonal patterns of temperature and precipitation at a representative location Tropical Rainforests  High biomass, high diversity – about 50% of Earth’s species  Light is a key factor – plants must grow very tall above their neighbors or adjust to low light levels  Emergents rise above the canopy  Lianas (woody vines) and epiphytes use the trees for support  Understory trees grow in the shade of the canopy, and shrubs and forbs occupy the forest floor  Tropical rainforests are disappearing due to logging and conversion to pasture and croplands  About half of the tropical rainforest biome has been altered  Recovery of rainforests is uncertain: soils are nutrient-poor, and recovery of nutrient supplies may take a very long time Tropical Seasonal Forests and Savannas  Wet and dry seasons associated with movement of the ITCZ o Period of drought occurs  Shorter trees, deciduous in dry seasons, more grasses and shrubs  Fires promote establishment of savannas; some are set by humans  In Africa, large herbivores – wildebeests, zebras, elephants, and antelopes – also influence the balance of grass and trees o Can convert areas of forest to areas of savannas  On the Orinoco River floodplain, seasonal flooding promotes savannas  Less than half of seasonal tropical forests and savannas remain  Human population growth in this biome has had a major influence  Large tracts have been converted to cropland and pasture Hot Deserts  High temperatures, low moisture  Often in areas of subsidence, high pressure systems  Sparse vegetation and animal populations  Low water availability constrains plant abundance and influences form o Plants are always in a state of moisture stress  Many plants have succulent stems that store water  Convergence of this form is shown by cacti (Western Hemisphere) and euphorbs (Eastern Hemisphere)  Humans use deserts for agriculture and livestock grazing  Agriculture depends on irrigation, and results in soil salinization  Long-term droughts and unsustainable grazing can result in desertification – loss of plant cover and soil erosion Temperate Grasslands  Warm, moist summers and cold, dry winters  Grasses dominate; maintained by frequent fires and large herbivores such as bison  Grasses grow more roots than stems and leaves, to cope with dry conditions o Have deep root systems o This results in accumulation of organic matter and high soil fertility  Most fertile grasslands of central North America and Eurasia have been converted to agriculture because of the rich organic component to the soil  In arid grasslands, grazing by domesticated animals can exceed capacity for regrowth, leading to grassland degradation and desertification  Irrigation in some areas cause salinization Temperate Shrublands and Woodlands  Reversal of growing season – summers are very dry, plant growing season starts in the fall, despite the cool weather, when precipitation increases  Evergreen leaves allow plants to be active during cooler, wetter periods  They also lower nutrient requ
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