2.1 Individuals, populations, communities and ecosystems

All life on Earth exists within the biosphere, the thin zone stretching from the deepest ocean trenches to the highest altitudes where organisms can survive. The biosphere is not a single place but a network of interacting systems: the atmosphere (gases surrounding Earth), the hydrosphere (all water bodies), and the lithosphere (rocks and soils). Life depends on all three.

Within the biosphere, ecologists recognise four nested levels of organisation:

1. Individual: a single organism, such as one baobab tree or one wolf.
2. Population: a group of individuals of the same species living in the same area at the same time and capable of interbreeding.
3. Community: all the interacting populations of different species in a given area.
4. Ecosystem: a community together with the physical (abiotic) environment it interacts with.

Each level includes everything below it. A coral reef ecosystem contains communities of fish, algae, and invertebrates; those communities are made up of populations; and each population consists of individual organisms.

The biosphere functions as an open system. Solar energy enters, drives photosynthesis, passes through food chains, and eventually leaves as heat. Matter, by contrast, cycles: carbon, nitrogen, phosphorus, and water move continuously between living organisms and the abiotic environment. Both energy flow and nutrient cycling are needed to keep the biosphere operational.

An individual organism belongs to a species. According to the biological species concept (BSC), a species is a group of organisms that can interbreed under natural conditions and produce fertile offspring. Fertility of offspring is the key test: members of the same species share a gene pool and can exchange genetic material across generations.

Three criteria follow from the BSC:

• Members of the same species can reproduce with one another (interbreeding).
• Their offspring must themselves be capable of reproduction (fertile offspring).
• Different species are reproductively isolated and cannot naturally produce viable, fertile offspring together.

The BSC has practical limits. It cannot be applied to asexual organisms such as bacteria, which reproduce without interbreeding. It cannot be tested for extinct species. Geographically isolated populations may diverge over time yet remain the same species, and some species hybridise under unusual conditions. For these reasons, taxonomists often combine the BSC with other lines of evidence, including genetic analysis, when classifying organisms.

With millions of described species and many more yet unknown, a universal system for naming and organising organisms is essential. Classification allows scientists to communicate precisely across languages and regions, and it allows predictions: if two species share a genus, they are likely to share traits, ecology, and evolutionary history.

The system in widest use today descends from the work of Carl Linnaeus in the 18th century. It arranges organisms into a hierarchy of taxa, from the broadest grouping down to the most specific:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

Each species receives a binomial name consisting of two parts:

• The genus name, which is capitalised (e.g., Panthera).
• The species name, which is lower case (e.g., leo).

Both names are italicised when typed or underlined when written by hand. This rule is not optional: examiners expect it.

Species in the same genus share more recent common ancestry and more traits than species in different genera. Panthera leo (lion) and Panthera tigris (tiger) are more closely related to each other than either is to Acinonyx jubatus (cheetah).

Scientific names offer three advantages over common names. They are unique to each species worldwide. They are universal, used identically in every language. And they carry information about relationships, since the genus name signals which other species are close relatives.