2.2 Energy and biomass in ecosystems

• Sustained by supplies of energy and matter

• Open systems with exchanges

• Energy transformed but not created or destroyed

• Transformations: light to chemical, chemical to heat

• Photosynthesis and cellular respiration

• Light energy → chemical energy (glucose)

• Stored as biomass by autotrophs

• First trophic level in food chain

• Plants, algae, photosynthetic bacteria

• Releases energy from glucose

• Converts to usable chemical form

• Some energy transformed to heat during respiration

• Not 100% efficient

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Show complexity of trophic relationships

• Arrows indicate energy flow direction

• Species may feed at multiple trophic levels

• Dry mass of samples ≈ organic matter mass

• Energy measured by combustion

• Represent numbers, biomass, or energy of trophic levels

• Pyramids of energy: kJ m⁻² year⁻¹

• PCB, DDT, mercury

• Bioaccumulation: increase over time in organisms

• Biomagnification: increase along food chain

• Absorb non-biodegradable pollutants

• Increase transmission in food chain

• Burning fossil fuels, deforestation, urbanization, agriculture

• Reduce primary productivity, disrupt food webs