23 - Ecosystems Public

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OCR A Level Biology Module 6 Chapter 23

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Defining ecosystems Ecosystems are made of all living organisms that interact with each other in a defined area plus the physical factors present in the region The boundaries of an ecosystem are defined by the person or team carrying out the study Factors affecting ecosystems Biotic factors = living factors competition for food, space and breeding partners Abiotic factors = non-living factors Light --> plants develop strategies to deal withe different light intensities such as large leaves in areas with low intensity or reproductive systems that only operate if there is optimum light intensity Temperature --> greatest effect is on the enzymes of the organism --> changes to the temperature can cause migration or hibernation, leaf fall, dormancy or flowering Water --> a lack of water usually leads to water stress and/or death --> xerophytes and camels are adapted Oxygen availability --> in aquatic conditions its beneficial to have fast-flowing water as it has high concentrations of oxygen, in waterlogged soil the air spaces between soil particles are filled with water so reduces the oxygen available to the plants Edaphic (soil) factors  clay soil is easily waterlogged and forms clumps when wet loam soil retains water but doesn't become waterlogged sandy soil is free draining so doesn't retain water and is easily eroded
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Trophic levels Each stage of a food chain is a trophic level First trophic level = producers aka plants The subsequent trophic levels are consumers  Decomposers are also important in food webs as they break down dead material Measuring biomass Biomass = mass of living material present in an area or organism To calculate biomass at each trophic level, biomass present in each organism * total number of organisms Easy way to measure biomass is to weigh fresh material but the water content is too variable for this to be accurate so the material is burned and then the ashed are weighed --> may not be representative of the population as a whole Measured in grams per cubic metre for water or grams per square metre for land areas Efficiency of biomass and energy transfer between trophic levels Biomass of each trophic level is generally less than the trophic level below it This is because it consists of all the cells and tissues including the carbohydrates and other carbon compounds that the organisms contain As carbon compounds contain energy biomass = energy content Small proportion of the food eaten by animals is converted to new tissue which is all that is passed onto the next trophic level Energy available for each trophic level is measured in kilojoules per metre squared per year  As biomass is transferred between trophic levels the energy contained is passed on The efficiency with which the biomass or energy is transferred = ecological efficiency Efficiency at producer level Producers convert 1-3% of sunlight to chemical energy aka biomass not all solar energy available is used for photosynthesis (90% is reflected) other factors limit photosynthesis  a proportion of the energy is "lost" Plants 20-50% of chemical energy in respiration so the rest is converted to biomass Energy available to next trophic level = gross production - respiratory losses gross production = total energy they convert to chemical energy Efficiency at consumer level They at most convert 10% of consumed biomass to their own tissue because not all of the biomass is eaten some energy is transferred to environment as heat due to movement or respiration some parts that are eaten are indigestible and are egested as faeces some energy is lost from the animal in excretory materials like urine Only 0.001% of total energy present in sunlight is embidies as biomass in a tertiary consumer Efficiency of energy transfer = ecological efficiency ecological efficiency = (energy or biomass available after the transfer / energy or biomass available before the transfer) * 100 Human activities can manipulate biomass through ecosystems In agriculture, plants and animals are provided with abiotic conditions they need to thrive like water and warmth and competition from other species is removed as well as the threat of predators Agriculture creates simple food chains where only 3 trophic levels are present (farming) producers --> animal feed primary consumers --> livestock secondary consumers --> humans This means minimum energy is lost along the way since there are fewer trophic levels Highest energy and biomass transfer possible to humans
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Decomposition Chemical process in which a compound is broken down into smaller molecules or constituent elements Decomposer = an organism that feeds on and breaks down dead plant and animal matter thus turning organic compounds back to nutrients to be used again Mostly microscopic fungi and bacteria They are also called saprotrophs because they get their energy from dead material They digest food externally by excreting enzymes and then absorb the products Almost as a byproduct, they release inorganic compounds and elements back to the environment Detritivores are another class of organism that are involved --> they help speed up the decaying process as they feed on dead and decaying matter  They break it down into smaller pieces which increases the surface area for the decomposers They perform internal digestion Recycling nitrogen Essential element for amino acids and nucleic acids in both plants and animals Abundant in the atmosphere as nitrogen gas but this cannot be taken up by plants so bacteria or lightning have to convert it first Nitrogen fixation Free-living bacteria like Azotobacter use nitrogenase to fix atmospheric nitrogen with hydrogen to produce ammonium ions Mutualistic bacteria like Rhizobium use nitrogenase to fix atmospheric nitrogen with hydrogen to produce ammonium ions --> symbiotic relationship with plants, live in the root nodules and exchange the ammonium ions for carbohydrates made by the plant Lightning and the Haber process also fix nitrogen into ammonium ions Nitrification Ammonium compounds in the soil are converted to nitrogen-containing molecules that plants can use Free living nitrifying bacteria like Nitrosomonas oxidise ammonium ions into nitrites Nitrobacter oxidise nitrites to nitrates which are highly soluble so can be taken up by the plant Denitrification Denitrifying bacteria in the soil convert nitrates back to nitrogen gas when there's an absence of oxygen --> nitrates are used by the bacteria as a source of energy Ammonification The process by which decomposers convert nitrogen-containing compounds to ammonium ions Recycling carbon Oceans act as carbon stores Photosynthesis removes carbon from the atmosphere  Producers and consumers release carbon through respiration Carbon passes from producers to consumers through feeding After both die, their stored carbon is released by decomposers The burning of fossil fuels releases more carbon into the atmosphere Levels fluctuate throughout the day as plants only photosynthesise when it's light but respire at almost all times (unless its a cactus) --> carbon dioxide levels are lower in the summer as its light for longer so more photosynthesis occurs Over the past 200 years atmospheric carbon dioxide has increased significantly because the combustion of fossil fuels rocketed in the Industrial Revolution and has remained high since then deforestation has removed many of the plants that take the carbon dioxide out of the air Increased carbon dioxide traps more thermal energy in the atmosphere thereny causing global warming and climate change The amount of carbon dioxide dissolved in the oceans is affected by the temperature of the water (higher temperature = less gas) so global warming decreases the ability of the oceans to act as carbon banks
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Succession is a way in which ecosystems are changing over time --> occurs as a result of changes to the environment causes plant and animal species present to change Two types primary succession --> area of land is newly formed or exposed so there is no soil or organic matter to begin with when volcanoes erupt, depositing lava  sand is blown by the wind or deposited in the sea to make new sand dunes mud is deposited in river estuaries  glaciers retreat depositing rubble and exposing rock secondary succession --> areas of land where soil is present but contains no plant or animal species Stages of succession Steps = seral stages At each seral stage key species can be identified than change the abiotic factors, especially the soil, to make it more suitable for the next species barren land pioneer community with colonisers intermediate community with secondary colonisers, tertiary colonisers and scrubland climax community with dominant species (which arises from secondary succession or primary succession) Pioneer community begins by the colonisation of inhospitable environment by a pioneer species  they often arrive as spores or seeds that have been carried on the wind from other land masses pionner species need a number of adaptations to colonise the environment: ability to produce large quantities of seeds and spores that can be carried on the wind seeds that germinate rapidly ability to photosynthesise tolerance to extreme conditions ability to fix nitrogen from the atmosphere Intermediate community over time, weathering of the bare rock produces particles that when mixed with the dead matter of the pioneer species forms soil the organic component of soil is humus the soil becomes able to support the growth of a new species of plant aka secondary colonisers --> arrive as spores or seeds as the conditions improve further new plant species arrive as ferns aka tertiary colonisers --> have waxy cuticle to protect them from water loss at each stage the rock continues to be eroded so the soil gets better until it can support flowering plants such as grasses, then shrubs, then small trees SO it has multiple seral stages and at each seral stage the plant and animal species are better adapted to the current conditions of the ecosystem --> they outcompete the species previously present and become the dominant species Climax community final seral stage community is in stable state there are normally a few dominant plant and animal species --> which ones depends on the climate although biodiversity increases as succession takes place, the climax community isn't also the most biodiverse biodiversity tends to reach its peak mid-succession and then decreases as the dominant species outcompete the pioneer communities Animal succession primary consumers (insects) are the first to colonise as they consumer and shelter in the mosses and lichens secondary consumers arrive when a suitable food source has been established and the existing plant cover will provide them with suitable habitats eventually larger organisms like mammals and reptiles colonise the area when the biotic conditions are favourable Deflected succession human activities can halt natual succession when succession is stopped artificially its called plagioclimax agriculture is the main reason delfected succession occurs  grazing and trampling of vegetation by animals removing existing vegetation to plant crops burning as a means of forest clearance
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Distribution of organisms distribution = where the individual organisms are found within an ecosystem it is usually uneven throughout an ecosystem as organisms are generally found where the biotic and abiotic factors suit them most measuring it: a line or belt transect is used --> forms of systematic, non-random sampling systematic allow scientists to see how different abiotic factors affect the distribution of organisms Abundance of organisms abundance = number of individuals of a species present in an area at a given time may fluctuate because: immigration and births increase the numbers of individuals emigration and deaths decrease the numbers of individuals measuring it: populations are estimated through sampling techniques --> never entirely representative of the whole population to increase accuracy, use large sample sizes measuring plant abundance: quadrat placed randomly in the area estimated number in population = number of individuals counted in sample / area of sample measuring animal abundance: capture-mark-release-recapture method capture as many individuals as possible in sample area mark or tag each individual  release the marked individuals back into the sample area and allow time for them to redistribute themselves recapture as many as possible record the number of marked individuals and unmarked individuals use the Lincoln index to estimate population size estimated population size = (number of individuals in first sample * number in second sample) / number of marked recaptured individuals  once the abundance is determined, scientists can work out the biodiversity by using Simpson's Index of Biodiversity
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