Scivra
Pro 🔒~15 min

Ecosystems & Food Webs

Energy flow, matter cycling, and ecosystem balance

How it works

An ecosystem is a community of organisms interacting with each other and their physical environment. Food webs describe who eats whom, showing the flow of energy and matter. Energy flows one-way through a food web (sun → plants → herbivores → carnivores), with 90% lost as heat at each level. Matter cycles continuously: carbon (photosynthesis, respiration, decomposition), nitrogen (fixation, nitrification, denitrification), water (evaporation, precipitation). Populations in ecosystems are interdependent: removing predators causes herbivore populations to explode (trophic cascade). The Lotka-Volterra equations describe predator-prey cycles: when prey are plentiful, predators increase; then prey decline; then predators decline, letting prey recover. Biodiversity increases ecosystem stability — more species means more alternative pathways if one disappears.

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Step-by-step

  1. Set initial populations and press Play.
  2. Watch population graphs evolve — observe the classic predator-prey oscillations.
  3. Try removing foxes (carnivores) — what happens to rabbits?
  4. Then to plants?
  5. Add an invasive species (Pro) to see how it disrupts the food web.
  6. Track the carbon cycle overlay to see how carbon moves between organisms.

Key formulas

  • dNdt=rN(1−NK)(logistic growth)\frac{dN}{dt} = rN\left(1-\frac{N}{K}\right) \quad (\text{logistic growth})Population growth limited by carrying capacity K
  • 10% rule: energyn+1=0.1×energyn\text{10\% rule: energy}_{n+1} = 0.1 \times \text{energy}_nOnly 10% of energy passes to the next trophic level

Frequently asked questions

In a food web: grass → grasshoppers → frogs → snakes → hawks. If frogs are removed, what likely happens to grasshoppers? To snakes?
Without frogs, grasshoppers (prey) have no predator → population explodes. Snakes lose a food source → snake population decreases. This is a trophic cascade — removing one species ripples through the web.
Why is it impossible for an ecosystem to have more carnivores than herbivores?
Energy decreases at each trophic level (10% rule). There's not enough energy to support more carnivores than herbivores. Carnivores need many prey animals to get enough energy — a fox needs to eat many rabbits per month.
How does deforestation affect the carbon cycle?
Trees store carbon in their wood (carbon sink). Cutting them down releases stored CO₂ (combustion or decomposition). Fewer trees also means less photosynthesis to remove atmospheric CO₂. Both effects increase atmospheric CO₂, contributing to climate change.
Explain the Lotka-Volterra predator-prey cycle. Why do populations oscillate rather than reach a steady state?
When prey (rabbits) are abundant, predators (foxes) thrive and increase. More foxes eat more rabbits → rabbit population falls. Fewer rabbits → foxes starve and decline. Fewer foxes → rabbit population recovers. This creates cycles. A true steady state requires perfect balance, which is unstable to small disturbances.