Learning objective
2.2.1—Ecosystem sustenance
• Sustained by supplies of energy and matter • Open systems with exchanges
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Course review
2.2 Energy and biomass in ecosystems
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Learning objective
2.2.2—First law of thermodynamics
• Energy transformed but not created or destroyed • Transformations: light to chemical, chemical to heat
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Learning objective
2.2.3—Energy and matter transformation
• Photosynthesis and cellular respiration
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Learning objective
2.2.4—Photosynthesis
• Light energy → chemical energy (glucose) • Stored as biomass by autotrophs
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Learning objective
2.2.5—Producers
• First trophic level in food chain • Plants, algae, photosynthetic bacteria
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Learning objective
2.2.6—Cellular respiration
• Releases energy from glucose • Converts to usable chemical form
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Learning objective
2.2.7—Heat generation
• Some energy transformed to heat during respiration • Not 100% efficient
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Learning objective
2.2.8—Second 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
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Learning objective
2.2.9—Consumer 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
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Learning objective
2.2.10—Producers 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
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Learning objective
2.2.11—Food 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
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Learning objective
2.2.12—Food-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
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Learning objective
2.2.13—Gross 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
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Learning objective
2.2.14—Limits 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
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Learning objective
2.2.15—Food webs
• Show complexity of trophic relationships • Arrows indicate energy flow direction • Species may feed at multiple trophic levels
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Learning objective
2.2.16—Biomass measurement
• Dry mass of samples ≈ organic matter mass • Energy measured by combustion
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Learning objective
2.2.17—Ecological pyramids
• Represent numbers, biomass, or energy of trophic levels • Pyramids of energy: kJ m⁻² year⁻¹
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Learning objective
2.2.18—Non-biodegradable pollutants
• PCB, DDT, mercury • Bioaccumulation: increase over time in organisms • Biomagnification: increase along food chain
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Learning objective
2.2.19—Microplastics
• Absorb non-biodegradable pollutants • Increase transmission in food chain
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Learning objective
2.2.20—Human activity impacts
• Burning fossil fuels, deforestation, urbanization, agriculture • Reduce primary productivity, disrupt food webs
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