Species Communities and Ecosystems

Species. A group of organisms that can interbreed to produce fertile offspring.

The infertility of hybrids (e.g. mule = horse × donkey) shows the two parents are different species.

Population. All the individuals of ONE species living in a defined area at the same time.

Community. All the populations of all the species living together in an area, interacting.

Ecosystem. A community plus its abiotic (non-living) environment — soil, water, climate. Ecosystems are largely self-sustaining: nutrients cycle, but energy flows through and is lost.

Habitat vs niche.

• Habitat = the place an organism lives (where).
• Niche = its role / how it uses resources (what it does).

Two species cannot occupy exactly the same niche in the same habitat (competitive exclusion principle).

Autotrophs (producers). Synthesise organic compounds from inorganic (CO₂, H₂O, ions). Include:

• Photoautotrophs — use light energy (plants, algae, cyanobacteria).
• Chemoautotrophs — use chemical energy from inorganic reactions (some bacteria at hydrothermal vents).

Heterotrophs (consumers). Get organic carbon by feeding on other organisms.

• Herbivores (primary consumers) — eat plants.
• Carnivores (secondary, tertiary consumers) — eat animals.
• Omnivores — eat both.

Detritivores — feed on dead organic matter or waste (e.g. earthworms, woodlice).

Saprotrophs — secrete digestive enzymes onto dead matter, then absorb digested products (e.g. fungi, bacteria). They are the principal decomposers in ecosystems.

Food chain. Linear pathway showing energy flow. Arrows point from food TO consumer (showing energy transfer direction):

$\text{Grass} \rightarrow \text{Rabbit} \rightarrow \text{Fox}.$

Food web. Network of interconnected food chains, more realistic representation.

Trophic level = position in the food chain.

• T1: producer (grass).
• T2: primary consumer (rabbit).
• T3: secondary consumer (fox).
• T4: tertiary consumer (wolf eating fox), and so on.

Most ecosystems have ≤4 trophic levels because of energy loss between levels (covered in next subtopic).