What the exam asks
- Section I (Multiple Choice): Stimulus sets often feature oxygen-consumption graphs comparing mammals and reptiles across ambient temperatures, or marine food webs where you must predict the short-term and long-term population effects of removing a prey species or adding a toxin.
- Section II (Free Response): Frequently appears in Q4 (Conceptual Analysis), Q5 (Analyze Model or Visual Representation), or Q6 (Analyze Data). You may be asked to explain how an environmental shift alters net energy availability, predict changes to trophic pyramid biomass, or justify why food chains rarely exceed four or five levels.
Core ideas
1. Metabolic strategies and thermoregulation
Organisms use distinct strategies to regulate body temperature and metabolism:
| Feature | Endotherms (e.g., mammals, birds) | Ectotherms (e.g., reptiles, amphibians, fish, invertebrates) |
|---|---|---|
| Primary heat source | Internal thermal energy generated by metabolism | External environment |
| Adaptations | Shivering, panting, vasoconstriction/dilation, insulation (fat, feathers) | Behavioral (basking in sun, seeking shade, burrowing) |
| Metabolic rate vs. ambient temperature | Rises as ambient temperature drops below the thermoneutral zone (to generate heat) | Rises as ambient temperature increases (kinetic molecular effects increase enzyme activity up to an optimum) |
| Dietary energy requirement | High (must fuel constant thermoregulation) | Low (can survive longer periods between meals) |
2. Energy budgets and reproductive strategies
All organisms require a continuous input of energy to maintain organization, grow, and reproduce:
- Net energy gain (energy intake metabolic expenditure): Excess energy is stored as glycogen or lipids, or allocated to growth and reproduction.
- Net energy loss (energy intake metabolic expenditure): Results in loss of mass, halted reproduction, and eventually death.
Organisms evolve life-history strategies tied to energy availability. In stable environments with high competition, organisms often invest heavy energy into few offspring with high parental care. In fluctuating environments with abundant temporary energy, organisms often produce many small offspring with little parental care. Many species only reproduce during seasons when food resources peak, minimizing the risk of net energy loss during gestation or lactation.