Topic 2 Ecology

Standard Level

2.1.1 The biosphere is an ecological system composed of individuals, populations, communities, and ecosystems.

2.1.2 An individual organism is a member of a species.

2.1.3 Classification of organisms allows for efficient identification and prediction of characteristics.

2.1.4 Taxonomists use a variety of tools to identify an organism.

2.1.5 A population is a group of organisms of the same species living in the same area at the same time, and which are capable of interbreeding.

2.1.6 Factors that determine the distribution of a population can be abiotic or biotic.

2.1.7 Temperature, sunlight, pH, salinity, dissolved oxygen and soil texture are examples of many abiotic factors that affect species distributions in ecosystems.

2.1.8 A niche (a.k.a. ecological niche) describes the particular set of abiotic and biotic conditions and resources upon which an organism or a population depends.

2.1.9 Populations interact in ecosystems by herbivory, predation, parasitism, mutualism, disease and competition, with ecological, behavioural and evolutionary consequences.

2.1.10 Carrying capacity is the maximum size of a population determined by competition for limited resources.

2.1.11 Population size is regulated by density-dependent factors and negative feedback mechanisms.

2.1.12 Population growth can either be exponential or limited by carrying capacity.

2.1.13 Limiting factors on the growth of human populations have increasingly been eliminated, resulting in consequences for sustainability of ecosystems.

2.1.14 Carrying capacity cannot be easily assessed for human populations.

2.1.15 Population abundance can be estimated using random sampling, systematic sampling or transect sampling.

2.1.16 Random quadrat sampling can be used to estimate population size for non-mobile organisms.

2.1.17 Capture–mark–release–recapture and the Lincoln index can be used to estimate population size for mobile organisms.

2.1.18 A community is a collection of interacting populations within the ecosystem.

2.1.19 Habitat is the location in which a community, species, population or organism lives.

2.1.20 Ecosystems are open systems in which both energy and matter can enter and exit.

2.1.21 Sustainability is a natural property of ecosystems.

2.1.22 Human activity can lead to tipping points in ecosystem stability.

2.1.23 Keystone species have a role in the sustainability of ecosystems.

2.1.24 The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold.

2.1.25 To avoid critical tipping points, loss of biosphere integrity needs to be reversed.

Additional HL

2.1.26 There are advantages of using a method of classification that illustrates evolutionary relationships in a clade.

2.1.27 There are difficulties in classifying organisms into the traditional hierarchy of taxa.

2.1.28 The niche of a species can be defined as fundamental or realised.

2.1.29 Life cycles vary between species in reproductive behaviour and lifespan.

2.1.30 Knowledge of species’ classifications, niche requirements and life cycles help us to understand the extent of human impacts upon them.

Standard Level

2.2.1 Ecosystems are sustained by supplies of energy and matter.

2.2.2 The first law of thermodynamics states that as energy flows through ecosystems, it can be transformed from one form to another but cannot be created or destroyed.

2.2.3 Photosynthesis and cellular respiration transform energy and matter in ecosystems.

2.2.4 Photosynthesis is the conversion of light energy to chemical energy in the form of glucose, some of which can be stored as biomass by autotrophs.

2.2.5 Producers form the first trophic level in a food chain.

2.2.6 Cellular respiration releases energy from glucose by converting it into a chemical form that can easily be used in carrying out active processes within living cells.

2.2.7 Some of the chemical energy released during cellular respiration is transformed into heat.

2.2.8 The second law of thermodynamics states that energy transformations in ecosystems are inefficient.

2.2.9 Consumers gain chemical energy from carbon (organic) compounds obtained from other organisms. Consumers have diverse strategies for obtaining energy-containing carbon compounds.

2.2.10 Because producers in ecosystems make their own carbon compounds by photosynthesis, they are at the start of food chains. Consumers obtain carbon compounds from producers or other consumers, so form the subsequent trophic levels.

2.2.11 Carbon compounds and the energy they contain are passed from one organism to the next in a food chain. The stages in a food chain are called trophic levels.

2.2.12 There are losses of energy and organic matter as food is transferred along a food chain.

2.2.13 Gross productivity (GP) is the total gain in biomass by an organism. Net productivity (NP) is the amount remaining after losses due to cellular respiration.

2.2.14 The number of trophic levels in ecosystems is limited due to energy losses.

2.2.15 Food webs show the complexity of trophic relationships in communities.

2.2.16 Biomass of a trophic level can be measured by collecting and drying samples.

2.2.17 Ecological pyramids are used to represent relative numbers, biomass or energy of the trophic levels in an ecosystem.

2.2.18 Non-biodegradable pollutants, such as polychlorinated biphenyl (PCB), dichlorodiphenyltrichloroethane (DDT) and mercury, cause changes to ecosystems through the processes of bioaccumulation and biomagnification.

2.2.19 Non-biodegradable pollutants are absorbed within microplastics, which increases their transmission in the food chain.

2.2.20 Human activities, such as burning fossil fuels, deforestation, urbanization and agriculture, have impacts on flows of energy and transfers of matter in ecosystems.

Additional HL

2.2.21 Autotrophs synthesize carbon compounds from inorganic sources of carbon and other elements. Heterotrophs obtain carbon compounds from other organisms.

2.2.22 Photoautotrophs use light as an external energy source in photosynthesis. Chemoautotrophs use exothermic inorganic chemical reactions as an external energy source in chemosynthesis.

2.2.23 Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements.

2.2.24 Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food.

2.2.25 Net primary productivity is the basis for food chains because it is the quantity of carbon compounds sustainably available to primary consumers.

2.2.26 Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system.

2.2.27 Sustainable yields are higher for lower trophic levels.

2.2.28 Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level.

2.2.29 The second law of thermodynamics shows how the entropy of a system increases as biomass passes through ecosystems.

Standard Level

2.3.1 Biogeochemical cycles ensure chemical elements continue to be available to living organisms.

2.3.2 Biogeochemical cycles have stores, sinks and sources.

2.3.3 Organisms, crude oil and natural gas contain organic stores of carbon. Inorganic stores can be found in the atmosphere, soils and oceans.

2.3.4 Carbon flows between stores in ecosystems by photosynthesis, feeding, defecation, cellular respiration, death and decomposition.

2.3.5 Carbon sequestration is the process of capturing gaseous and atmospheric carbon dioxide and storing it in a solid or liquid form.

2.3.6 Ecosystems can act as stores, sinks or sources of carbon.

2.3.7 Fossil fuels are stores of carbon with unlimited residence times. They were formed when ecosystems acted as carbon sinks in past eras and become carbon sources when burned.

2.3.8 Agricultural systems can act as carbon stores, sources and sinks, depending on the techniques used.

2.3.9 Carbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of a solution.

2.3.10 Increases in concentrations of dissolved carbon dioxide cause ocean acidification, harming marine animals.

2.3.11 Measures are required to alleviate the effects of human activities on the carbon cycle.

Additional HL

2.3.12 The lithosphere contains carbon stores in fossil fuels and in rocks, such as limestone, that contain calcium carbonate.

2.3.13 Reef-building corals and molluscs have hard parts that contain calcium carbonate that can become fossilized in limestone.

2.3.14 In past geological eras, organic matter from partially decomposed plants became fossilized in coal, and partially decomposed marine organisms became fossilized in oil and natural gas held in porous rocks.

2.3.15 Methane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria.

2.3.16 Methane has a residence time of about 10 years in the atmosphere and is eventually oxidized to carbon dioxide.

2.3.17 The nitrogen cycle contains organic and inorganic stores.

2.3.18 Bacteria have essential roles in the nitrogen cycle.

2.3.19 Denitrification only happens in anaerobic conditions, such as soils that are waterlogged.

2.3.20 Plants cannot fix nitrogen so atmospheric dinitrogen (N2) is unavailable to them unless they form mutualistic associations with nitrogen-fixing bacteria.

2.3.21 Flows in the nitrogen cycle include mineral uptake by producers, photosynthesis, consumption, excretion, death, decomposition and ammonification.

2.3.22 Human activities such as deforestation, agriculture, aquaculture and urbanization change the nitrogen cycle.

2.3.23 The Haber process is an industrial process that produces ammonia from nitrogen and hydrogen for use as fertilizer.

2.3.24 Increases in nitrates in the biosphere from human activities have led to the planetary boundary for the nitrogen cycle being crossed, making irreversible changes to Earth systems likely.

2.3.25 Global collaboration is needed to address the uncontrolled use of nitrogen in industrial and agricultural processes and bring the nitrogen cycle back within planetary boundaries.

Standard Level

2.4.1 Climate describes atmospheric conditions over relatively long periods of time, whereas weather describes the conditions in the atmosphere over a short period of time.

2.4.2 A biome is a group of comparable ecosystems that have developed in similar climatic conditions, wherever they occur.

2.4.3 Abiotic factors are the determinants of terrestrial biome distribution.

2.4.4 Biomes can be categorized into groups that include freshwater, marine, forest, grassland, desert and tundra. Each of these groups has characteristic abiotic limiting factors, productivity and diversity. They may be further classed into many subcategories (for example, temperate forests, tropical rainforests and boreal forests).

2.4.5 The tricellular model of atmospheric circulation explains the behaviour of atmospheric systems and the distribution of precipitation and temperature at different latitudes. It also explains how these factors influence the structure and relative productivity of different terrestrial biomes.

2.4.6 The oceans absorb solar radiation and ocean currents distribute the resulting heat around the world.

2.4.7 Global warming is leading to changing climates and shifts in biomes.

Additional HL

2.4.8 There are three general patterns of climate types that are connected to biome types.

2.4.9 The biome predicted by any given temperature and rainfall pattern may not develop in an area because of secondary influences or human interventions.

2.4.10 The El Niño Southern Oscillation (ENSO) cycle is the fluctuation in wind and sea surface temperatures that characterizes conditions in the tropical Pacific Ocean. The two opposite and extreme states are El Niño and La Niña, with transitional and neutral states between the extremes.

2.4.11 El Niño is due to a weakening or reversal of the normal east–west (Walker) circulation, which increases surface stratification and decreases upwelling of cold, nutrient-rich water near the coast of north-western South America. La Niña is due to a strengthening of the Walker circulation and reversal of other effects of El Niño.

2.4.12 Tropical cyclones are rapidly circulating storm systems with a low-pressure centre that originate in the tropics and are characterized by strong winds.

2.4.13 Rises in ocean temperatures resulting from global warming are increasing the intensity and frequency of hurricanes and typhoons because warmer water and air have more energy.

Standard Level

2.5.1 Zonation refers to changes in community along an environmental gradient.

2.5.2 Transects can be used to measure biotic and abiotic factors along an environmental gradient in order to determine the variables that affect the distribution of species.

2.5.3 Succession is the replacement of one community by another in an area over time due to changes in biotic and abiotic variables.

2.5.4 Each seral community (sere) in a succession causes changes in environmental conditions that allow the next community to replace it through competition until a stable climax community is reached.

2.5.5 Primary succession happens on newly formed substratum where there is no soil or preexisting community, such as rock newly formed by volcanism, moraines revealed by retreating glaciers, wind-blown sand or waterborne silt.

2.5.6 Secondary succession happens on bare soil where there has been a preexisting community, such as a field where agriculture has ceased or a forest after an intense firestorm.

2.5.7 Energy flow, productivity, species diversity, soil depth and nutrient cycling change over time during succession.

2.5.8 An ecosystem’s capacity to tolerate disturbances and maintain equilibrium depends on its diversity and resilience.

Additional HL

2.5.9 The type of community that develops in a succession is influenced by climatic factors, the properties of the local bedrock and soil, geomorphology, together with fire and weather-related events that can occur. There can also be top-down influences from primary consumers or higher trophic levels.

2.5.10 Patterns of net productivity (NP) and gross productivity (GP) change over time in a community undergoing succession.

2.5.11 r- and K-strategist species have reproductive strategies that are better adapted to pioneer and climax communities, respectively.

2.5.12 The concept of a climax community has been challenged, and there is uncertainty over what ecosystems would develop naturally were there no human influences.

2.5.13 Human activity can divert and change the progression of succession leading to a plagioclimax.