Ecosystems, Biomes and Ecological Interactions

An ecosystem is a community of living organisms interacting with each other and with their physical environment. Its two component groups are:

• Biotic components — all the living organisms, sorted by how they get food: producers (green plants, algae — make their own food), consumers (animals that eat others), and decomposers (bacteria, fungi — break down dead matter).
• Abiotic components — the non-living physical and chemical factors: sunlight, temperature, water, air, soil, and minerals.

Two things flow through every ecosystem: energy (one-way, from the Sun through producers to consumers) and nutrients (recycled in cycles by decomposers).

A food chain is a linear sequence showing who eats whom, with energy passing from one feeding level to the next. The trophic levels are:

• Producers (1st level) — green plants and algae; trap solar energy to make food.
• Primary consumers (2nd level) — herbivores that eat producers (caterpillar, goat, grasshopper, deer).
• Secondary consumers (3rd level) — small carnivores/omnivores that eat herbivores (frog, small fish).
• Tertiary / top consumers (4th level) — top carnivores that eat secondary consumers (snake, hawk).

The arrow in a food chain points from the eaten to the eater (toward the energy receiver). By the 10% law (Lindeman), only about 10% of the energy at one level is passed to the next — the rest is lost as heat — so a typical chain has only 4–5 links.

Three nutrition modes, classified by carbon and energy source:

• Autotrophs — 'self-feeders'; build their own food from carbon dioxide using light (photosynthesis) or chemical energy (chemosynthesis). They are the producers.
• Heterotrophs — 'other-feeders'; cannot fix CO₂, so they obtain carbon by eating other organisms. All animals are heterotrophs.
• Decomposers (saprotrophs) — heterotrophs that feed on dead organic matter, recycling nutrients back to the soil (bacteria, fungi).

Interactions between two species, named by their effect on each partner (+ benefit, − harm, 0 no effect):

• Mutualism (+ / +) — both partners benefit (bee and flower; lichen = alga + fungus; nitrogen-fixing bacteria in legume roots).
• Commensalism (+ / 0) — one benefits, the other is unaffected (orchid growing on a tree; remora fish on a shark).
• Parasitism (+ / −) — one (parasite) benefits, the host is harmed (tapeworm, Plasmodium, Cuscuta).
• Predation (+ / −) — the predator kills and eats the prey (lion and deer).
• Competition (− / −) — both compete for the same limited resource.

Major land (terrestrial) biomes, identified by climate and vegetation:

• Tropical rainforest — hot, very high rainfall year-round; dense evergreen trees; the richest biodiversity.
• Temperate (deciduous) forest — high rainfall, cold-to-mild seasons; deciduous trees (maple, oak, hickory, beech) that shed leaves; raccoons, squirrels, deer.
• Taiga (boreal forest) — cold; coniferous evergreens (pine, spruce).
• Grassland / Savanna — moderate rainfall; grasses, grazing herbivores.
• Desert — very low rainfall; cacti, xerophytes.
• Tundra — coldest; no trees, mosses and lichens.

In aquatic biomes (oceans, lakes), the main producers are tiny floating phytoplankton, which carry out most of the ocean's photosynthesis and produce most of its organic carbon.