2.2 Energy and biomass in ecosystems

• Sustained by supplies of energy and matter • Open systems with exchanges

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• Energy transformed but not created or destroyed • Transformations: light to chemical, chemical to heat

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• Photosynthesis and cellular respiration

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• Light energy → chemical energy (glucose) • Stored as biomass by autotrophs

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• First trophic level in food chain • Plants, algae, photosynthetic bacteria

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• Releases energy from glucose • Converts to usable chemical form

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• Some energy transformed to heat during respiration • Not 100% efficient

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• 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% efficient

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• 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 decomposers

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• 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 on

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• 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 webs

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• 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 next

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• 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 activity

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• 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 energy

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• Show complexity of trophic relationships • Arrows indicate energy flow direction • Species may feed at multiple trophic levels

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• Dry mass of samples ≈ organic matter mass • Energy measured by combustion

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• Represent numbers, biomass, or energy of trophic levels • Pyramids of energy: kJ m⁻² year⁻¹

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• PCB, DDT, mercury • Bioaccumulation: increase over time in organisms • Biomagnification: increase along food chain

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• Absorb non-biodegradable pollutants • Increase transmission in food chain

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• Burning fossil fuels, deforestation, urbanization, agriculture • Reduce primary productivity, disrupt food webs

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• Autotrophs: synthesize carbon compounds from inorganic sources • Heterotrophs: obtain carbon compounds from other organisms

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• 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 webs

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• 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 time

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• 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 unabsorbed

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• 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 farmers

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• 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 systems

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• 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 relevant

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• 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 average

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• 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 respiration

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