Energy Flow Through Ecosystems (College Board AP® Biology): Study Guide

Energy use in organisms

• Organisms use energy to organize, grow, reproduce, and maintain homeostasis.

• Balancing energy use and energy loss is an important feature with a direct impact on survival and reproductive success

• Organisms use different strategies to achieve this balance

Body temperature regulation strategies

• Different strategies exist to regulate body temperature and metabolism:

  • Endotherms:

    • Endotherms are organisms that use thermal energy generated by metabolism to maintain body temperature within restricted limits

    • This is called thermoregulation and is an example of a homeostasis

    • Mechanisms for thermoregulation include shivering, sweating, vasodilation, and vasoconstriction

  • Ectotherms:

    • Ectotherms rely on behavioral adaptations within their external environment to regulate temperature, such as basking in the sun or seeking shade

    • Ectotherms might huddle with others to share body heat or avoid extreme environments

Reproductive strategies

• Organisms adjust reproductive strategies based on energy availability

• Some organisms alternate between asexual and sexual reproduction depending on conditions

  • Asexual reproduction is less energy-intensive and allows for rapid population growth

  • Sexual reproduction requires more energy but increases genetic variation, improving survival in changing environments

• Examples of reproductive strategies:

  • Seasonal reproduction ensures offspring are born during favorable conditions, such as abundant food or suitable temperatures

  • Photoperiodism is a biological response to day length that regulates growth and reproduction in plants and animals

    • For example: Plants flower and fruit based on changes in daylight duration

  • Biennial plants complete their life cycles over two years, growing in the first year and reproducing in the second:

    • For example: Sugar beet stores energy in the first year for reproduction in the second

  • Reproductive diapause involves the suspension of reproductive activity to conserve energy for survival

    • For example: Monarch butterflies delay reproduction during migration to focus on flight

Metabolic rates and organism size

• Metabolic rate is the amount of energy expended by an organism in a given time

• Metabolic rate per unit body mass is inversely related to body size

  • Smaller organisms have higher metabolic rates

  • Larger organisms have lower metabolic rates, requiring less energy per unit body mass

Energy balance and survival

• A net gain in energy (more energy acquired than lost) results in growth, energy storage, and increased reproductive output

• A net loss of energy (more energy lost than acquired) leads to loss of mass, decreased reproductive output, and, eventually, death

Energy flow & trophic levels

The impact of energy availability

• Energy availability affects population sizes and ecosystem dynamics

• Ecosystem disruptions occur due to changes in energy resources:

  • Decreased energy resources reduce population sizes at all higher trophic levels

  • For example:

    • A reduction in sunlight availability will affect the number and size of all trophic levels

    • A reduction in the energy available from the producer level will impact the size and number of consumers at higher trophic levels

  • Increased energy resources promote population growth within ecosystems

    • Increased energy from sunlight will increase stored chemical energy in producers

    • Increased energy available from the producer level will support the size and number of consumers at higher trophic levels

Energy flow through food chains

• Energy transfer is represented through food chains, food webs, and trophic pyramids

• In a food chain, arrows are used show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

• Trophic pyramids show the energy stored in the biomass of organisms at each trophic level

  • Energy decreases as it moves up the trophic levels, with only ~10% transferred to the next level due to losses like respiration, incomplete digestion, and excretion

  • This explains why a pyramid of energy always appears pyramid-shaped

Pyramid of Energy Diagram

The role of autotrophs & heterotrophs

Energy capture by autotrophs

• Autotrophs produce organic molecules from simple inorganic substances

• They act as producers in food chains and include:

  • Photosynthetic organisms (e.g. plants, algae) use sunlight to convert carbon dioxide into carbohydrates

  • Chemosynthetic organisms oxidize small inorganic molecules to generate energy in the absence of sunlight or oxygen

Energy capture by heterotrophs

• Heterotrophs rely on organic molecules produced by other organisms

  • They consume and metabolize carbohydrates, lipids, and proteins for energy via hydrolysis (breaking down macromolecules into smaller units)

  • This organic mater is then incorporated into their own tissues

• Heterotrophs include:

  • Consumers, who ingest tissues of living or recently dead organisms for energy

    • Primary consumers eat producers (e.g. herbivores)

    • Secondary consumers eat primary consumers (e.g. carnivores)

    • Higher-level consumers (e.g. tertiary and quaternary) feed on other consumers

  • Scavengers, which feed on dead animals (e.g., vultures)

  • Detritivores ingest dead organic matter (e.g. earthworms, woodlice)

  • Saprotrophs externally digest dead matter using enzymes, absorbing the resulting nutrients (e.g. fungi, bacteria)