What you’ll learn29 learning objectivesChoose one objective for a focused lesson, or study the complete topic.2.2.1Ecosystem sustenance• Sustained by supplies of energy and matter• Open systems with exchangesSyllabus objective2.2.2First law of thermodynamics• Energy transformed but not created or destroyed• Transformations: light to chemical, chemical to heatSyllabus objective2.2.3Energy and matter transformation• Photosynthesis and cellular respirationSyllabus objective2.2.4Photosynthesis• Light energy → chemical energy (glucose)• Stored as biomass by autotrophsSyllabus objective2.2.5Producers• First trophic level in food chain• Plants, algae, photosynthetic bacteriaSyllabus objective2.2.6Cellular respiration• Releases energy from glucose• Converts to usable chemical formSyllabus objective2.2.7Heat generation• Some energy transformed to heat during respiration• Not 100% efficientSyllabus objective2.2.8Second law and energy loss• The second law of thermodynamics states that energy transformations in ecosystems are inefficient• The second law of thermodynamics relates to the quality of energy, and that when energy is transformed, some must be degraded into a less useful form, such as heat• In ecosystems, the biggest losses occur during cellular respiration• The second law of thermodynamics explains why energy transfers are never 100% efficientSyllabus objective2.2.9Consumer energy sources• Consumers gain chemical energy from carbon (organic) compounds obtained from other organisms• Consumers have diverse strategies for obtaining energy-containing carbon compounds• Include, with examples, herbivores, detritivores, predators, parasites, saprotrophs, scavengers and decomposersSyllabus objective2.2.10Producers and food chains• 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• In a food chain, organic matter flows from primary producers to primary consumers to secondary consumers, and so onSyllabus objective2.2.11Food chains and trophic levels• 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• Traditionally, decomposers are not included in food chains as they typically gain carbon compounds from a variety of sources• However, consider the role of decomposers in energy transformations in food websSyllabus objective2.2.12Food-chain energy losses• There are losses of energy and organic matter as food is transferred along a food chain• Not all the food available to a given trophic level is harvested: of what is harvested, not all is consumed; of what is consumed, not all is absorbed; of what is absorbed• There is, therefore, never 100% transference of organic matter from one trophic level to the nextSyllabus objective2.2.13Gross and net productivity• 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• Consider: values of both GP and NP from given data• Losses due to cellular respiration are typically greater in consumers than in producers due to more energy-requiring activitySyllabus objective2.2.14Limits on trophic levels• The number of trophic levels in ecosystems is limited due to energy losses• Energy released by cellular respiration and lost as heat by organisms is unavailable to organisms in higher trophic levels• Because of this and other energy losses, typically 10% or less of the energy flowing to a trophic level is available to the next level, limiting the length of food chains• Avoid the common misconception that organisms at higher trophic levels must eat more food to get enough energySyllabus objective2.2.15Food webs• Show complexity of trophic relationships• Arrows indicate energy flow direction• Species may feed at multiple trophic levelsSyllabus objective2.2.16Biomass measurement• Dry mass of samples ≈ organic matter mass• Energy measured by combustionSyllabus objective2.2.17Ecological pyramids• Represent numbers, biomass, or energy of trophic levels• Pyramids of energy: kJ m⁻² year⁻¹Syllabus objective2.2.18Non-biodegradable pollutants• PCB, DDT, mercury• Bioaccumulation: increase over time in organisms• Biomagnification: increase along food chainSyllabus objective2.2.19Microplastics• Absorb non-biodegradable pollutants• Increase transmission in food chainSyllabus objective2.2.20Human activity impacts• Burning fossil fuels, deforestation, urbanization, agriculture• Reduce primary productivity, disrupt food websSyllabus objective2.2.21(HL)—Autotrophs vs. heterotrophs• Autotrophs: synthesize carbon compounds from inorganic sources• Heterotrophs: obtain carbon compounds from other organismsSyllabus objective2.2.22(HL)—Photoautotrophs and chemoautotrophs• Photoautotrophs use light as an external energy source in photosynthesis• Chemoautotrophs use exothermic inorganic chemical reactions as an external energy source in chemosynthesis• Chemoautotrophs exist in a variety of ecosystems, especially those where there is little or no light• In such ecosystems, chemoautotrophs are the principal source of energy to sustain food websSyllabus objective2.2.23(HL)—Primary productivity• Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements• The units usually used for productivity are kg carbon m-2 year-1 (kilograms of carbon per square metre of ecosystem per year)• Consider: protocols for determining primary productivity in ecosystems• Estimates can be based on photosynthesizing samples within a laboratory or, in the field, measuring change in biomass of samples (such as grassland) over timeSyllabus objective2.2.24(HL)—Secondary productivity• Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food• Secondary productivity is ingested food minus faecal waste• The units are the same as those for primary productivity• Faecal matter is not included as it is material that has remained undigested and unabsorbedSyllabus objective2.2.25(HL)—NPP and food chains• Net primary productivity is the basis for food chains because it is the quantity of carbon compounds sustainably available to primary consumers• Net primary production can be thought of as the plant growth that is sustainably harvestable by primary consumers in natural ecosystems or by farmersSyllabus objective2.2.26(HL)—Maximum sustainable yield• Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system• Consider: the MSYs in natural ecosystems and in agricultural or silvicultural systemsSyllabus objective2.2.27(HL)—Lower trophic level yields• Sustainable yields are higher at lower trophic levels• Food production is easier to sustain when humans consume lower trophic levels• Plant-based foods are especially relevantSyllabus objective2.2.28(HL)—Ecological efficiency• Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level• The percentage varies between ecosystems, trophic levels and species• Work out the efficiency of the transfer of energy between trophic levels with given data• The percentage of energy transferred from one trophic level to the next is very variable, and the value of 10% is neither a fixed amount nor a true averageSyllabus objective2.2.29(HL)—Entropy in ecosystems• The second law of thermodynamics shows how the entropy of a system increases as biomass passes through ecosystems• Entropy refers to the amount of disorder within a system• Living systems are able to maintain a high degree of organization and low entropy through the net increase in entropy resulting largely from cellular respirationSyllabus objective
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