Scivra
Pro 🔒~15 min

Ecosystems & Food Webs

Energy flow, matter cycling, and ecosystem balance

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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.

Step-by-step

  1. Set the initial prey population and the predator birth rate, then press Play and watch the population graph evolve — the classic predator-prey oscillation emerges by itself.
  2. Push the sliders to the extremes: a tiny prey base or a fast-breeding predator can crash the cycle, while balanced settings keep both populations swinging steadily.

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.