AP Syllabus focus:
‘Net energy gain supports storage, growth, and reproduction, whereas net loss reduces mass and reproductive output.’
Energy balance determines whether organisms can build biomass, produce offspring, and survive challenging conditions. This page focuses on how energy intake and expenditure set limits on growth, maintenance, and reproductive output across changing environments.
Core idea: energy balance drives performance
Organisms acquire chemical energy in food (or stored reserves) and allocate it among competing demands.
Energy-flow diagram for a consumer population showing how ingested material (I) is partitioned into feces (F), respiration (R), and production (P). It visually reinforces the idea that production is the energy/mass available for growth and reproduction after respiratory costs are paid, consistent with (with assimilation as the absorbed fraction of ingestion). Source
The outcome is an energy budget that determines changes in body mass and fitness-related traits.
Energy balance: The net difference between energy gained (intake/assimilation) and energy spent (maintenance, activity, and other losses), determining whether biomass increases, stays constant, or decreases.
A positive energy balance supports increased biomass and investment in offspring, whereas a negative energy balance forces organisms to use stored reserves and reduce costly processes.
Where energy goes: maintenance vs production
Maintenance costs (must be paid first)
Maintenance includes processes required to stay alive and functional:
Basal metabolism (basic cellular work, ion gradients, protein turnover)
Homeostasis (osmotic balance, pH regulation)
Repair and immune function (tissue repair, pathogen defence)
Activity costs (movement, foraging, ventilation)
These demands are often prioritized; if energy intake drops, organisms commonly reduce growth and reproduction before maintenance fails.
Production (what’s left after maintenance)
When sufficient energy remains, organisms can allocate to:
Growth (new biomass: cell division, tissue expansion)
Storage (energy reserves such as fat or glycogen)
Reproduction (gamete production, parental investment, mating behaviours)
Net energy gain therefore supports storage, growth, and reproduction, directly matching the syllabus statement.
A pyramid of energy (productivity) summarizes how much energy is converted into new biomass at each trophic level versus lost (primarily as heat through respiration). It provides a visual link between individual energy budgets (assimilation vs. respiration) and ecosystem-level patterns of declining available production up the food chain. Source
A simple quantitative model for net gain
Energy available for building biomass is often expressed as energy assimilated minus energy used in respiration.
= energy converted to new biomass and/or reproductive output (e.g., kJ day)
= energy absorbed from food after undigested loss (e.g., kJ day)
= energy expended in cellular respiration for maintenance and activity (e.g., kJ day)
If is high, organisms can grow quickly and/or reproduce more. If falls to near zero, growth and reproduction stall even if the organism is still alive.
Net energy gain: effects on growth and reproduction
Positive net energy gain (surplus)
Mass increases via tissue growth or stored reserves.
Reproductive output increases (more gametes, larger offspring, greater parental care).
Survival often improves because reserves buffer future shortages.
Net energy loss (deficit)
When , organisms must compensate by using reserves:
Mass decreases (catabolism of glycogen, then lipids, and potentially proteins)
Reduced reproductive output (delayed breeding, fewer offspring, reduced investment)
Higher mortality risk if deficits persist and critical tissues are consumed or maintenance fails
This directly reflects the syllabus requirement that net loss reduces mass and reproductive output.
Trade-offs and survival under changing conditions
Because energy is finite, organisms face allocation trade-offs:
Investing energy in reproduction now can reduce survival later if reserves become too low.
Prioritizing maintenance and storage can increase survival but reduce immediate reproductive success.
Environmental changes (food availability, habitat quality, stressors) shift the balance by altering:
Energy intake/assimilation (feeding success, diet quality, digestion efficiency)
Energy expenditure (increased activity to find food, costs of coping with stress)
These shifts explain why the same organism may grow rapidly in one season but lose mass and stop reproducing in another.
FAQ
Lipids store more energy per gram and support longer-term deficits.
Glycogen is mobilised quickly but is limited and often used early in fasting.
Differences in assimilation efficiency (digestion/absorption) change $A$.
Differences in activity level, stress, or body size change $R$.
Low $P$ can delay reaching the body size/condition needed for maturation.
It can also lengthen intervals between breeding attempts by slowing recovery of reserves.
Downregulation of protein synthesis and reduced ion pumping can lower ATP demand.
Some tissues may decrease growth-related signalling, reallocating energy to essential maintenance.
Common measures include number of offspring, offspring mass, or total reproductive biomass.
In some studies, energetic cost is estimated from the change in adult energy stores during reproduction.
Practice Questions
State two biological outcomes of a sustained negative energy balance in an animal. (2 marks)
Mass decreases due to use of energy stores / catabolism of tissues (1)
Reduced reproductive output (e.g., fewer offspring, delayed breeding, reduced gamete production) (1)
An ecologist finds that a population shows reduced average body mass and fewer offspring per adult after a reduction in food availability. Using energy balance and the relationship , explain these observations. (5 marks)
Food reduction decreases energy intake and/or assimilation, so decreases (1)
Maintenance and activity require respiration; continues and may not fall proportionally (1)
Therefore decreases and may become near zero or negative (1)
Lower/negative leads to reduced growth or loss of stored reserves, decreasing body mass (1)
Lower reduces energy available for reproduction, decreasing offspring number/investment (1)
