BiodiversityIB MYP Biology: Revision notes

Biodiversity is the variety of living things in an area. It includes the number of different species, the genetic variety within each species, and the variety of ecosystems.

Species richness is the number of different species in an area. A plot with 12 species has greater species richness than a plot with 5 species, even if the second has many more individuals.

For ecosystems: a biodiverse ecosystem is more stable. Food webs have more links, so if one species declines, others can take its place. Different species pollinate plants, recycle nutrients and control pests.

For humans: biodiversity gives us food, wood, fibres and medicines. Ecosystems provide services such as clean water, flood control, carbon storage and pollination of crops. Wild places also bring tourism income and are enjoyable.

Scientists sample an area using quadrats for plants and pitfall traps or nets for animals. Sampling should be random, repeated and use the same effort in each area.

A simple diversity index (Simpson's index) combines the number of species and how evenly the individuals are spread:

$D = \dfrac{N(N-1)}{\sum n(n-1)}$

where $N$ is the total number of individuals and $n$ is the number of individuals of each species.

Worked example: a sample has 10 individuals of species X and 5 of species Y. Then N = 15 and Σn(n − 1) = 10 × 9 + 5 × 4 = 110, so D = 15 × 14 ÷ 110 = 1.9.

A higher D means higher diversity.

Most threats come from human activity:

• Habitat loss: land cleared for farming, building and mining
• Climate change: changes in temperature and rainfall faster than species can adapt
• Pollution: pesticides, plastics and fertiliser run-off
• Introduced species: new predators or competitors
• Over-hunting and over-harvesting

Small, isolated populations lose genetic variety and are more at risk. Biodiversity can be protected with protected areas, laws against hunting, breeding programmes and reducing pollution.