2.2 Energy and biomass in ecosystems

Objective notes

29 learning objectives

2.2.1Ecosystem sustenance

• Sustained by supplies of energy and matter

• Open systems with exchanges

2.2.2First law of thermodynamics

• Energy transformed but not created or destroyed

• Transformations: light to chemical, chemical to heat

2.2.3Energy and matter transformation

• Photosynthesis and cellular respiration

2.2.4Photosynthesis

• Light energy → chemical energy (glucose)

• Stored as biomass by autotrophs

2.2.5Producers

• First trophic level in food chain

• Plants, algae, photosynthetic bacteria

2.2.6Cellular respiration

• Releases energy from glucose

• Converts to usable chemical form

2.2.7Heat generation

• Some energy transformed to heat during respiration

• Not 100% efficient

2.2.8Second 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% efficient

2.2.9Consumer 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 decomposers

2.2.10Producers 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 on

2.2.11Food 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 webs

2.2.12Food-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 next

2.2.13Gross 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 activity

2.2.14Limits 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 energy

2.2.15Food webs

• Show complexity of trophic relationships

• Arrows indicate energy flow direction

• Species may feed at multiple trophic levels

2.2.16Biomass measurement

• Dry mass of samples ≈ organic matter mass

• Energy measured by combustion

2.2.17Ecological pyramids

• Represent numbers, biomass, or energy of trophic levels

• Pyramids of energy: kJ m⁻² year⁻¹

2.2.18Non-biodegradable pollutants

• PCB, DDT, mercury

• Bioaccumulation: increase over time in organisms

• Biomagnification: increase along food chain

2.2.19Microplastics

• Absorb non-biodegradable pollutants

• Increase transmission in food chain

2.2.20Human activity impacts

• Burning fossil fuels, deforestation, urbanization, agriculture

• Reduce primary productivity, disrupt food webs

2.2.21(HL)—Autotrophs vs. heterotrophs

• Autotrophs: synthesize carbon compounds from inorganic sources

• Heterotrophs: obtain carbon compounds from other organisms

2.2.22(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 webs

2.2.23(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 time

2.2.24(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 unabsorbed

2.2.25(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 farmers

2.2.26(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 systems

2.2.27(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 relevant

2.2.28(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 average

2.2.29(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 respiration