Discover how living organisms interact with each other and their environment — from the grass on the savanna to the lion at the top of the food chain.
An ecosystem is a community of living organisms interacting with one another and with their non-living environment in a particular area. Ecosystems range in size from a small rock pool to an entire ocean.
Ecosystem components are divided into two categories:
A food chain shows a single linear pathway of energy transfer through an ecosystem. Arrows show the direction of energy flow (from eaten to eater):
A food web shows all the feeding relationships in an ecosystem — multiple interconnected food chains. Food webs are a more realistic representation because most animals eat several different species.
Each step in a food chain is called a trophic level:
| Trophic Level | Role | SA Savanna Example |
|---|---|---|
| 1st — Producers | Make own food (photosynthesis) | Grass, Acacia trees, Buffalo thorn |
| 2nd — Primary consumers | Herbivores — eat producers | Zebra, Wildebeest, Impala, Giraffe |
| 3rd — Secondary consumers | Carnivores/omnivores — eat primary consumers | Cheetah, Wild dog, Jackal |
| 4th — Tertiary consumers | Top predators — eat secondary consumers | Lion, Leopard, Crocodile |
| — Decomposers | Break down dead organic matter | Bacteria, fungi, dung beetles, vultures |
Energy is lost at each trophic level — used for the organism's own respiration, movement, growth, and heat. Only about 10% of the energy at one trophic level is available to the next level. This gives us the energy pyramid: a wide base of producers, narrowing toward the top predators.
This explains why top predators are always rarer than herbivores — the energy supply is too small to support large numbers of lions.
Carbon cycle: Carbon moves through ecosystems via photosynthesis (CO₂ → glucose), respiration (glucose → CO₂), decomposition, and combustion. Plants absorb CO₂; all organisms release it through respiration.
Nitrogen cycle: Nitrogen makes up 78% of air but most organisms cannot use it directly. Nitrogen-fixing bacteria in the soil (and in root nodules of legumes) convert N₂ into nitrates (NO₃⁻) that plants can absorb. Animals get nitrogen by eating plants. Decomposers return nitrogen to the soil when organisms die.
When prey numbers increase, predator numbers also increase (more food). Increased predation then reduces prey numbers, which in turn causes predator numbers to decline. This creates a natural oscillating cycle that keeps populations in balance.
Ecosystems provide humans with essential services:
Click any organism to highlight its connections. Drag nodes to rearrange. Use "Remove species" to see cascade effects.
| Year | Wild dog population | Impala population (estimate) |
|---|---|---|
| 2015 | 320 | 15 000 |
| 2016 | 210 | 15 800 |
| 2017 | 90 | 17 200 |
| 2018 | 60 | 19 500 |
| 2019 | 140 | 18 000 |
| 2020 | 260 | 16 200 |