What is the primary difference between endotherms and ectotherms regarding energy use?

Endotherms use metabolic energy to maintain a constant internal body temperature, which is energy-expensive but allows for consistent activity. Ectotherms rely on external heat sources and behavioral changes, which is energy-efficient but limits activity based on environmental conditions.

How does sexual reproduction compare to asexual reproduction in terms of energy and fitness?

Sexual reproduction is more energy-intensive due to mate seeking and gamete production but increases genetic variation, enhancing long-term survival. Asexual reproduction requires less energy and allows for rapid population growth in stable environments but lacks the variation needed for changing conditions.

What is the difference between a detritivore and a saprotroph?

Detritivores are consumers that ingest dead organic matter and digest it internally (e.g., earthworms). Saprotrophs secrete digestive enzymes onto dead matter to break it down externally before absorbing the nutrients (e.g., fungi).

Why is it incorrect to say that energy is 'recycled' in an ecosystem?

Energy follows a one-way path and is eventually dissipated as heat, which cannot be captured back by producers. Unlike matter (carbon, nitrogen), which cycles through the system, energy must be constantly replenished by an external source like the sun.

What is a common mistake when comparing the metabolic rates of a small mammal and a large mammal?

Students often confuse total metabolic rate with metabolic rate per unit mass. While a large mammal has a higher total energy requirement, the small mammal has a much higher metabolic rate per gram of body tissue due to its higher surface-area-to-volume ratio.

Why can a pyramid of biomass be inverted, but a pyramid of energy cannot?

A biomass pyramid can be inverted if producers have a very high turnover rate (e.g., phytoplankton). However, an energy pyramid must always be upright because energy is lost at every transfer, and the total energy at one level can never exceed the energy of the level below it.

Define 'Trophic Level'.

A trophic level is the position an organism occupies in a food chain or web, determined by its main source of energy. It represents a functional step in the transfer of energy and nutrients through an ecosystem.

What is 'Reproductive Diapause'?

Reproductive diapause is a strategy where an organism temporarily suspends its reproductive activities. This occurs to conserve energy during periods of environmental stress or resource scarcity, ensuring the organism's survival until conditions improve.

What is the '10% Rule' in ecology?

The 10% rule states that only approximately $10\%$ of the energy available at one trophic level is passed on to the next. The remaining $90\%$ is lost as heat through metabolic processes, used for the organism's own life functions, or remains in undigested waste.

How does a net loss of energy affect an organism's survival?

A net loss of energy occurs when the energy expended for maintenance and activity exceeds the energy acquired from food. This leads to a loss of body mass, failure to reproduce, and eventually death if the deficit is not corrected.

Energy Flow Through Ecosystems | College Board AP Biology

Energy Flow Through Ecosystems

Summary

Energy flow is the fundamental process that sustains life within an ecosystem, governed by the laws of thermodynamics. It involves the capture of energy by autotrophs, its conversion into biomass, and its subsequent transfer through various trophic levels, where significant energy is lost as heat at each step.

1. Energy Acquisition and Metabolic Strategies

Biological Energy Use: Organisms require a constant input of energy to maintain high levels of organization, facilitate growth, and power reproduction. This energy is used to drive endergonic reactions and maintain homeostasis against the entropy of the environment.
Thermoregulation Strategies: Organisms are categorized by how they manage thermal energy. Endotherms utilize internally generated metabolic heat to maintain a stable body temperature, while ectotherms rely primarily on external environmental sources and behavioral adaptations to regulate their temperature.
Metabolic Rate and Body Size: There is a distinct inverse relationship between an organism's body mass and its metabolic rate per unit of mass. Smaller organisms generally possess higher metabolic rates per gram because their high surface-area-to-volume ratio leads to faster heat loss to the environment.
Energy Balance: An organism's survival depends on its net energy budget. A net gain in energy leads to biomass accumulation (growth) and reproductive success, whereas a net loss results in the depletion of energy stores, loss of mass, and potentially death.

2. Trophic Structures and Energy Capture

Autotrophs (Producers): These organisms serve as the foundation of all ecosystems by capturing energy from physical or chemical sources. Photoautotrophs use solar energy to synthesize organic compounds, while chemoautotrophs extract energy from small inorganic molecules in the absence of light.
Heterotrophs (Consumers): These organisms obtain energy by consuming organic matter produced by others. This group includes herbivores (primary consumers), carnivores (secondary/tertiary consumers), and specialized feeders like detritivores and saprotrophs that break down dead organic material.
Energy Transfer Models: Energy movement is visualized through food chains (linear pathways) and food webs (complex interconnected networks). Arrows in these diagrams always point in the direction of energy flow, from the organism being consumed to the consumer.

Trophic Pyramid of Energy showing the 10% rule and heat loss at each level.

3. The 10% Rule and Energy Efficiency

4. Reproductive Strategies and Energy Availability

5. Key Distinctions in Energy Processing

6. Exam Strategy & Tips