Chapter 3

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Biological Sciences
Marc Cadotte

Chapter 3: The Biosphere THE AMERICAN SERENGETI – TWELVE CENTURIES OF CHANGE IN THE GREAT PLAINS: A CASE STUDY  The Great Plains, bears little resemblance to the Serengeti  If not for a series of important environmental changes, however, the two ecosystems might superficially look very similar  Temperate and high latitude biological communities have been subjected to natural, long term climate change, leading to latitudinal or elevational shifts in their positions and species composition  The animal inhabitants of those earlier grasslands were strikingly different from today‘s  As the extensive grasslands of the Great Plains were developing, many of the large mammals of North America suddenly went extinct  Nearly all the animals that went extinct belonged to the same group: large mammals  Hypotheses have been proposed to account for the disappearance: - Changes in the climate during the extinction period were rapid and could have led to changes in habitat or food supply that would have negatively affected the animals - Arrival of humans in North America may have hastened the demise of the animals INTRODUCTION  Living things can be found in remarkable places  However, most living things occur in a range of habitats that cover a thin veneer of Earth‘s surface, from the tops of trees to the surface soil layers in terrestrial environments and within 200m of the surface of the oceans  Biosphere – the zone of life on Earth, is sandwiched between the lithosphere, Earth‘s surface crust and upper mantle, and the troposphere, the lowest layer of the atmosphere  Biomes – a large scale 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 3.1 Terrestrial Biomes are Characterized by the Growth Forms of the Dominant Vegetation TERRESTRIAL BIOMES  Biomes are the large scale biological communities shaped by the physical environment in which they are found - They reflect the climatic variation - Categorized by the most common forms of plants distributed across large geographic areas - Categorization of biomes relies on similarities in the morphological responses of organisms to the physical environment - Provides a useful introduction to the diversity of life on Earth, the biome concept provides a convenient biological unit for modelers simulating the effects of environmental change on biological communities as well as the effects of vegetation on the climate system  Tropical forests have multiple verdant layers, high growth rates, and tremendous species diversity  Polar deserts have a scattered cover of tiny plants clinging to the ground, reflecting a harsh climate of high winds, low temperatures, and dry soils  Terrestrial biomes are classified by the growth form (size and morphology) of the dominant plants - Characteristics of their leaves such as deciduousness (seasonal shedding of eaves), thickness and succulence (dev. Of fleshy water storage tissues) may also be used  Plant growth forms are good indicators of the physical environment, reflecting the climatic zones  Trees and shrubs invest E in woody tissues in order to increase their height and ability to capture sunlight and to protect their tissues from damage by wind or large amounts of snow  Perennial grasses can grow from the bases of their leaves and keep their vegetative and reproductive buds below the soil surface, which facilitates their tolerance of grazing, fire, sub- freezing temp. and dry soils  Convergence – the evolution of similar growth forms among distantly related species in response to similar selection pressures TERRESTRIAL BIOMES REFLECT GLOBAL PATTERNS OF PRECIPITATION AND TEMPERATURE  Climatic zones are major determinants of the distribution of terrestrial biomes  The tropics are characterized by high rainfall and warm, invariant temperatures - Subtropical regions – rainfall becomes more seasonal, with pronounced dry and wet seasons - Major deserts – zones of high pressure at about 30° N and S and with the rain shadow effects of large mountain ranges - Temperate and Polar zones – subfreezing temperatures during winter - The amount of precipitation N and S of 40° varies depending on proximity to the ocean and the influence of mountain ranges  The locations of terrestrial biomes are correlated with 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 temp. are important in determining the supply of nutrients in the soil, which is also an important control on plant growth form  While these 2 factors predict biome distributions reasonably well, this approach fails to incorporate seasonal variation in temperature and precipitation  Climatic extremes are sometimes more important in determining species distributions than mean annual conditions  Factors such as soil texture and chemistry as well as proximity to mountains and large bodies of water can influence biome distribution THE POTENTIAL DISTRIBUTIONS OF TERRESTRIAL BIOMES DIFFER FROM THEI ACTUAL DISTRIBUTIONS DUE TO HUMAN ACTIVITIES  Land Use Change – the alteration of terrestrial surface, including vegetation and land forms, by human activities such as agriculture, forestry and mining  As a result of human influences, the potential and the actual distributions of biomes are markedly different TERRESTRIAL CHARACTERISTICS BIOME Tropical - Found in low latitude regions Rainforests - Precipitation exceeds 2 000 mm - Seasonal climatic rhythms are generally absent - Plants grow continuously throughout the year - Include the most productive ecosystems on Earth - Characterized by broad-leaved evergreen and deciduous trees - Light is a key environmental factor determining the vegetation structure of this biome - Emergent trees rise above the majority of the other trees that make up the canopy of the forest - Below the canopy, plants that utilize trees for support and to elevate their leaves above the ground are found draped over or clinging to the canopy and emergent trees, including lianas (woody vines) and epiphytes (plants that grow on tree branches) - Understory plants grow in the shade of the canopy, further reducing the light that finally reaches the forest floor - Shrubs and forbs occupy the forest floor, where they must rely on light flecks that move across the forest floor during the day for photosynthesis Tropical Seasonal - Rainfall is seasonal, with pronounced wet and dry seasons associated with shifts Forests and in the ICTZ Savannas - This region is marked by a large gradient in climate primarily associated with the seasonality of rainfall - Vegetation: shorter stature, lower tree densities, and an increasing degree of drought deciduousness, with leaves dropping from the trees during the dry season, greater abundance of grasses and shrubs and fewer trees relative to rainforests - Vegetation types: tropical dry forests, thorn woodlands, and tropical savannas - The frequency of fires, which increases with the length of the dry season, influences the vegetation growth forms - Large herds of herbivores also influence the balance between trees and grasses and act as an important force promoting the establishment of savanna Hot Deserts - Deserts contain sparse populations of plants and animals, reflecting sustained period of high temperatures and low water availability - The subtropical position of hot deserts generally correspond to the descending air of Hadley cells - This descending air created zones of high pressure around 30° N and S, which inhibit the formation of storms and their associated precipitation - Low precipitation levels, coupled with high temperatures and high rates of evapotranspiration, result in a limited supply of water for desert organisms - Low water availability is an important constraint on the abundance of desert plants as well an important influence their form and function - Plants with succulent stems can store water in their tissues to help the plant continue to function during dry periods - Desertification – degradation of formerly productive land in arid regions resulting in loss of plant cover and acceleration of soil erosions Temperate - These vast, undulating expanses of grass-dominated landscape have often been Grasslands compared to a terrestrial ocean, with wind-driven ―waves‖ of plants bending to the gusts blowing through them - Temperate climates have greater seasonal temperature variation than tropical climates, with increasing periods of subfreezing temperatures toward the poles. - Precipitation in some grasslands is high enough to support forests - Frequent fires and grazing by large herbivores prevent the establishment of trees and thus maintain the dominance of grasses in these environments - In order to acquire enough water, grasses grow more roots than stems and leaves - The rich organic matter that accumulates in the soils as a result enhances their fertility, so grassland soils are particularly well suited for agricultural development - In more arid grasslands, rates of grazing by domesticated animals can exceed the capacity of the plants to produce new tissues. And grassland degradation, including desertification, may occur - As in deserts, irrigation of some grassland soils has resulted in salinization, decreasing their fertility over time Temperate - Woodlands (characterized by an open canopy of short trees) and shrublands Shrub lands and occur in regions with a winter rainy season Woodlands - Precipitation falls primarily in winter, and hot, dry weather occurs throughout the late spring, summer, and fall - The vegetation of Mediterranean-type climates is characterized by evergreen shrubs and trees. Evergreen leaves allow plants to be active during cooler, wetter periods and also lower their nutrient requirements, since they do not have to develop new leaves every year - Sclerophyllous leaves (tough, leathery, and stiff). These plants are well adapted to dry soils and may continue to photosynthesize and grow at reduced rates during the hot, dry summer - Their sclerophyllous leaves help to deter consumption by herbivores and prevent wilting as water is lost - Fire is a common feature: Some of the shrubs recover after fires by resprouting from woody storage organs protected from the heat below the ground surface. Other shrubs produce seeds that germinate and grow quickly after a fire - Regular disturbance by fire, combined with the unique climate of temperate shrublands, is thought to promote high species diversity - Their climates and nutrient-poor soils have limited the extent of agricultural and pastoral development Temperate - Deciduous leaves are a solution to the extended periods of freezing weather in Deciduous the temperate zone Forests - Temperate deciduous forests occur in areas with enough rainfall to support tree growth and where soils are fertile enough to supply the nutrients lost when leaves are shed in the fall - Temperate deciduous forests are primarily limited to the Northern Hemisphere - Similar species occur on each of these continents in this biome, reflecting a common biogeographic history - The vertical structure of the forest includes canopy trees as well as shorter trees, shrubs. And forbs below the canopy - Species diversity is lower than in tropical forests - Disturbances such as fire and outbreaks of herbivorous insects do not play a major role in determining the development and persistence of temperate deciduous forest vegetation, although they can be important in determining its boundaries, and els periodic outbreaks of herbivores do occur - The fertile soils and climate are conducive to the growth of crops - Forest clearing for crop and wood production has been widespread in this biome Temperate - Evergreen forests span a wide range of environmental conditions in the Evergreen temperate zone, from warm coastal zones to cool continental and maritime Forests climates - Precipitation also varies substantially - Temperate evergreen forests with high levels of precipitation = ―temperate rainforests‘‘ - Temperate evergreen forests are commonly found on nutrient-poor soils, whose condition is in part related to the acidic nature of the leaves of the evergreen trees - Temperate evergreen forests are found in both the Northern and Southern hemispheres between 30° and 50° latitude - Their diversity is generally lower than that of deciduous and tropical forests - The temperate evergreen forest biome has been subjected to extensive clearing, and little old-growth forest remains - In industrialized countries, the effects of air pollution have damaged some temperate evergreen forests Boreal Forests - Above 50° N, the severity of winters increases - Minimum temperatures of -50°C (-58°F) may last up to 6 months - Not only must the plants cope with low air temperatures, but soils may regularly freeze, leading to the formation of permafrost, defined as a subsurface soil layer that remains frozen year-round for at least 3 years - Although precipitation is low, the permafrost impedes water drainage, so soils are moist to saturated - The biome that occupies the zone between 50° and 65° N is the boreal (far northern) forest (taiga). It is composed primarily of coniferous species, including spruces, pines, and larches but also includes extensive deciduous birch forests in maritime locations - Boreal forest is found only in the Northern Hemisphere, it is the largest biome in area and contains one-third of Earth‘s forested land - The cold, wet condition of boreal forest soils limits the decomposition of plant material such as leaves, wood. And roots. Thus, the rate of plant growth exceeds the rate of decomposition, and the soils contain large amounts of organic matter - Soil fires may continue to burn slowly for several years, even through the cold winters. In the absence of fire, forest growth enhances permafrost formation by lowering the amount of sunlight absorbed by the soil surface - Boreal forests have been less affected by human activities than other forest biomes - Logging occurs in some regions, as does oil and gas development, including the mining of oil sands - the large store of organic matter in the soil makes boreal forests an important component of the global carbon cycle - climate warming may result in more rapid decomposition and thus higher rates of carbon release from boreal forest soils, enhancing atmospheric greenhouse gas concentrations and resulting in a positive feedback to global warming Tundra - Trees cease to be the dominant vegetation beyond approximately 65° latitude - The poleward decrease in temperature and precipitation across the tundra biome is associated with the zones of high pressure generated by the polar atmospheric circulation cells - Characterized by sedges, forbs, grasses, and low-growing shrubs such as heaths, willows and birches. Lichens and mosses are also important component - Although the summer growing season is short, the days are long, with continuous periods of light for 1 to 2 months of the summer - The plants and lichens survive the long winter by going dormant, maintaining living tissues under the snow or soil, insulated from the cold air temperatures - Despite the low precipitation, many tundra areas are wet, as the permafrost keeps the precipitation that does fall from percolating to deeper soil layers - Repeated freezing and thawing of surface soil layers over several decades results in sorting of soil materials according to their texture - This process forms polygons of soil at the surface with upraised rims and depressed centers - Where soils are coarser or permafrost does not develop, the soils may be dry, and plants must be able to cope with low wa
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