General Ecology · Organismal Ecology
Physiological Ecology: Energy and Nutrient Acquisition
On this page 7 sections
In 30 seconds
Every organism must acquire energy and nutrients to build, maintain, and reproduce. Autotrophs make their own food — photoautotrophs using light (Photosynthesis Sugar from CO₂ and water using light Full entry →) and chemoautotrophs using inorganic chemicals (Chemosynthesis Sugar from inorganic chemical energy Full entry →) — while heterotrophs consume other organisms or their remains. Optimal foraging theory Feeding as maximizing energy gain per time Full entry → models feeding as maximizing energy gain per unit time — a simplified picture of real behavior.
Why this matters
Energy and Nutrient acquisition Obtaining essential elements and molecules Full entry → explain productivity, food-web length, and conservation. Because assimilating plant material is inefficient, food chains are short and top predators are rare and vulnerable. Nutrient limitation Growth capped by a scarce element Full entry → underlies fertilizer use and eutrophication when excess nitrogen and phosphorus run off. Optimal foraging theory informs wildlife-habitat management and human-wildlife conflict: animals "choose" crops or garbage with high profitability and low search cost. Any management response is governed by permits, wildlife and chemical-safety rules, land-management rules, and Indigenous land and data sovereignty.
The college version
1. Making Food: Autotrophy
An Autotroph Builds its own food from inorganic sources Full entry → builds organic molecules from inorganic sources. A Photoautotroph Autotroph powered by light Full entry → uses light energy in photosynthesis, converting CO₂ and water into sugars using chlorophyll (plants, algae, cyanobacteria). A Chemoautotroph Autotroph powered by inorganic reactions Full entry → uses energy from oxidizing inorganic chemicals such as hydrogen sulfide or ammonia in chemosynthesis — the base of deep-sea vent and cave ecosystems.
2. Eating Food: Heterotrophy
A Heterotroph Consumes organic matter for energy Full entry → obtains energy and carbon from organic matter. A Herbivore Consumer of plants or algae Full entry → eats plants or algae, a Carnivore Consumer of other animals Full entry → eats other animals, and an Omnivore Consumer of plants and animals Full entry → eats both. A detritivore consumes dead organic particles (earthworms, dung beetles); a decomposer — mainly fungi and bacteria — breaks down dead matter externally and absorbs the products.
3. Digestion and Assimilation
Digestion breaks food into absorbable molecules; assimilation is their uptake into tissues. Only the assimilated fraction is usable; the rest is egested. Assimilation efficiency is high for animal tissue and low for fibrous plants, which is why herbivores need specialized guts.
4. Nutritional Ecology and Limitation
Nutritional ecology studies how organisms meet their nutrient needs. Growth is often limited not by total energy but by a scarce element — nutrient limitation. Nitrogen and phosphorus commonly limit systems; animals balance energy against protein, salt, or vitamins.
5. Optimal Foraging Theory
Optimal foraging theory (OFT) treats feeding as optimization: maximize net energy gain per unit time, weighing costs against benefits. Two predictions follow: prey choice — specialize on high-profitability items when common, broaden the diet when scarce — and patch choice — leave a patch when its intake rate drops to the habitat average. Risk-sensitive foraging adds that when reserves are low, foragers may prefer reliable, low-variance food over a gamble.
How it works
- Autotrophs capture energy via photosynthesis or chemosynthesis.
- Heterotrophs consume that matter — herbivore, carnivore, omnivore, detritivore, or decomposer.
- Food is digested, then assimilated into tissues; the rest is egested.
- Foragers rank items by profitability (energy per unit time).
- When a patch's return drops below the habitat average, the forager leaves.
- Nutrient needs and risk make energy models only approximate.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Autotroph | Heterotroph | Makes vs. consumes organic matter |
| Photoautotroph | Chemoautotroph | Light- vs. chemical-driven |
| Photosynthesis | Chemosynthesis | Light vs. chemical energy |
| Detritivore | Decomposer | Eats particles vs. digests externally |
| Herbivore | Omnivore | Plants only vs. both |
| Digestion | Assimilation | Break down vs. absorb into tissues |
| Energy | Nutrient | Fuel vs. building blocks |
Memory aids
The energy chain as "Producers Power Everything": Photo/chemoautotrophs → Primary consumers (herbivores) → Everything else → Detritivores and decomposers recycle it all. For foraging, "High Profit, Low Hassle": high energy (E), low time cost (h+s).
Quick review
Topic Recap
- All organisms must acquire energy and nutrients.
- Autotrophs make food; heterotrophs consume it.
- Photosynthesis and chemosynthesis are the two capture pathways.
- Heterotrophs are herbivores, carnivores, omnivores, detritivores, or decomposers.
- Digestion breaks food down; assimilation absorbs it; nutrient limitation caps growth.
- Optimal foraging theory models feeding as energy-per-time optimization.
Knowledge Check
- What key difference separates a photoautotroph from a chemoautotroph?
- Name the five heterotroph diet categories and each food source.
- Distinguish digestion from assimilation.
- In optimal foraging theory, what makes a food item "profitable"?
- How does risk-sensitive foraging modify the simple energy-maximizing model?
Answers and Rationales
- A photoautotroph uses light energy (photosynthesis); a chemoautotroph uses energy from oxidizing inorganic chemicals (chemosynthesis).
- Herbivore (plants/algae), carnivore (animals), omnivore (both), detritivore (dead particles), decomposer (dead matter, external breakdown).
- Digestion breaks food into absorbable molecules; assimilation is their uptake into tissues — only assimilated material is usable.
- Profitability is net energy per unit time — energy content divided by handling plus search time (P = E/(h+s)).
- When reserves are low, prefer reliable, low-variance food over a risky high-variance option.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Every living thing must "charge its battery and buy groceries" — take in energy and raw materials. Plants and algae have built-in solar panels: photosynthesis turns sunlight, water, and CO₂ into sugar. A few microbes run on chemical fuel — chemosynthesis makes sugar from rocks and vents. Everything else must eat: grazers eat plants, hunters eat animals, and detritivores and decomposers recycle dead material.
The comparison stops being exact because food is more than energy. Animals also need protein, vitamins, and minerals that energy alone can't provide, and eating is risky. A real forager balances energy against nutrients and danger. This matters because who eats what, and how efficiently, shapes food webs and nutrient flow.
Simple Example
A shorebird probing mudflat patches for worms moves on once a patch is picked thin and the next promises a faster catch — the "leave when the going gets slow" rule optimal foraging theory predicts.
Worked example
The prey-choice model ranks food items by profitability:
P = Eh + s
- Define variables. P = profitability (energy per unit time, J s⁻¹). E = net energy content (J). h = handling time (s). s = search time (s).
- Units. Energy per time; larger E and smaller h or s raise P.
- Assumptions. Maximizes long-term net energy intake; complete information; energy the sole currency; no risk, nutrient, or learning costs.
- Output versus evidence. Predicts a diet and patch-leaving rule; real animals match qualitatively but vary due to nutrients, toxins, and risk.
- Limits. Normative (what a perfect economist should do), not cognitive — the core limits of optimality models.
Key takeaways
- High yield: Autotrophs make food — photoautotrophs use light, chemoautotrophs use chemicals (photosynthesis vs. chemosynthesis).
- High yield: Heterotrophs split by diet: herbivore (plants), carnivore (animals), omnivore (both), detritivore (dead particles), decomposer (external breakdown).
- High yield: Assimilation is the fraction of ingested food absorbed; only assimilated energy is usable.
- High yield: Nutrient limitation means a single scarce element, not total energy, caps growth.
- High yield: Optimal foraging theory predicts diet breadth and patch-leaving time from energy profitability.
- High yield: Risk-sensitive foraging explains preference for reliable food when starvation looms.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Distinguish autotrophs from heterotrophs, and photoautotrophs from chemoautotrophs.
- Compare photosynthesis and chemosynthesis as pathways of primary energy capture.
- Classify herbivores, carnivores, omnivores, detritivores, and decomposers by food source.
- Explain optimal foraging theory, its assumptions, and limits.
Key vocabulary
- Energy acquisition
- Obtaining energy for metabolism and growth
- Nutrient acquisition
- Obtaining essential elements and molecules
- Autotroph
- Builds its own food from inorganic sources
- Photoautotroph
- Autotroph powered by light
- Chemoautotroph
- Autotroph powered by inorganic reactions
- Photosynthesis
- Sugar from CO₂ and water using light
- Chemosynthesis
- Sugar from inorganic chemical energy
- Heterotroph
- Consumes organic matter for energy
- Herbivore
- Consumer of plants or algae
- Carnivore
- Consumer of other animals
- Omnivore
- Consumer of plants and animals
- Detritivore
- Consumer of dead organic particles
- Decomposer
- Breaks down dead matter externally
- Assimilation
- Uptake of digested molecules into tissues
- Digestion
- Breaking food into absorbable molecules
- Nutritional ecology
- Meeting nutrient needs in the wild
- Nutrient limitation
- Growth capped by a scarce element
- Optimal foraging theory
- Feeding as maximizing energy gain per time
- Costs and benefits
- Trade-offs (energy, time, risk) of a choice
- Patch choice
- Deciding when to leave a food patch
- Risk-sensitive foraging
- Preferring reliable food when reserves are low
- Limits of optimality models
- Constraints from imperfect info, nutrients, risk
- Ecological roles
- An organism's function in its community
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.