Sampling, biodiversity and Simpson's index

Last time

Before we start: last time, in Classification, the three domains and the kingdoms, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

How many, and how varied?

You cannot count every frog in a rainforest, or every plant in a meadow. So ecologists sample a small part and estimate the whole. And to compare two habitats, they need a number that captures both how many species there are and how evenly the individuals are spread. That number is Simpson's index.

Levels of biodiversity

Biodiversity is the variety of life. It is measured at three levels: ecosystem diversity, species diversity and genetic diversity. Ecosystem diversity is the range of habitats and communities in an area. Species diversity includes both the number of species, the richness, and how evenly the individuals are spread among them, the evenness. Genetic diversity is the variety of alleles within a species. It allows a population to adapt to change.

Ecosystem and niche

An ecosystem is a self contained unit of living organisms, the community, interacting with each other and with the non living environment, in an area. A niche is the role of a species in its ecosystem: what it eats, where it lives and when it is active, and how it interacts with other species. Two species cannot occupy exactly the same niche for long, because of competition.

A past-paper example

May June 2026, Paper 44, question 4 part b. Define the terms ecosystem and niche. An ecosystem is a self contained unit of the community of living organisms interacting with the non living environment. A niche is the role of a species in its ecosystem. Use the key words: community, environment, interactions.

Sampling methods

A frame quadrat is placed at random positions, found using random numbers as coordinates. In each, count the plants or estimate the percentage cover. Take many samples, and calculate a mean. A transect is a line along a gradient, such as up a shore. Place quadrats at regular intervals along it to see how species change. Mark release recapture is for mobile animals. Catch a sample, mark them, release, and later catch a second sample.

The Lincoln index

Population size equals the number marked in the first sample, times the total in the second sample, divided by the number of marked in the second sample. If 40 frogs are marked, then a second sample of 50 contains 8 marked frogs, the estimate is 40 times 50 over 8, which is 250. It assumes marking does not harm the animals, they mix with the population, and no births, deaths or migration happen between samples.

A past-paper example

May June 2025, Paper 43, question 10. One way to estimate the population of golden poison dart frogs is mark release recapture. Describe the method and calculate. Capture frogs, mark them in a harmless way, release them, and let them mix. Capture a second sample and count the marked ones. Then apply the formula for the estimate.