Measuring populations and biodiversity

Populations, communities and biodiversity

• A population is all the members of one species living in an area at the same time, such as all the koalas in a patch of forest.
• A community is all the populations of different species living and interacting in that area.
• Population size is how many individuals there are; population density is how many per unit area, such as 4 koalas per hectare. Distribution is where they are found.
• Biodiversity is the variety of life in an area. It includes the number of different species, the genetic variety within each species, and the variety of ecosystems.

Australia is one of about 17 "megadiverse" countries. Most of its mammals, reptiles, frogs and flowering plants are found nowhere else. Biodiversity matters because more diverse ecosystems are usually more stable: if one species declines, others can fill its role. It also gives us food, medicines and clean water, and it is central to the identity and law of First Nations peoples, whose kinship systems link people to particular plants and animals.

Sampling

Counting every individual is usually impossible, so ecologists count a sample and scale it up. A good sample is random (so you don't only sample the interesting spots) and large enough (more samples give a more reliable average).

Quadrats

A quadrat is a square frame, often 1 m × 1 m, placed at random positions in the study area. You count the individuals inside each one, find the average per quadrat, then multiply up to the whole area. Quadrats work for plants and slow-moving animals such as snails and barnacles.

Transects

A transect is a line (a long tape measure) laid across an area where conditions change, such as from the low-tide mark up a rocky shore, or from a creek bank into dry woodland. You record what is touching the line, or place quadrats at regular intervals along it. Transects show how distribution changes with an abiotic factor such as height above water, shade or soil moisture.

Capture–recapture

Animals that move around can't be counted in quadrats. Instead:

1. Trap a sample, mark each animal harmlessly (a dot of paint, an ear tag or a microchip) and release it.
2. Wait for the marked animals to mix back into the population.
3. Trap a second sample and count how many are marked.

The fraction marked in the second sample should equal the fraction marked in the whole population:

population size N = (number marked first time × number caught second time) ÷ number marked in second catch

This only works if marks don't fall off or make animals easier for predators to see, the marked animals mix evenly, and few animals are born, die or move away between samples. Some animals become "trap happy" (coming back for the bait) or "trap shy", which skews the result.

Newer methods

Ecologists now also use motion-sensing camera traps, which photograph animals day and night without disturbing them; drones with heat cameras, which can spot koalas in treetops at night; acoustic recorders that identify frogs and birds by their calls; and environmental DNA (eDNA), where a water sample is tested for tiny traces of DNA shed by animals. eDNA has been used to find platypus in creeks where nobody had seen one for years. Citizen scientists help too, by logging sightings in apps such as iNaturalist and FrogID.

Indigenous rangers combine these tools with knowledge of tracks, scats, seasons and where animals shelter. Ranger groups across northern and central Australia run many of the country's surveys of threatened species such as the bilby and the night parrot.