3.1 Biodiversity and evolutionIB Environmental Systems and Societies SL: Revision notes

Biodiversity is the total diversity of living systems. It exists at several levels:

• Genetic diversity: the variety of alleles within a species or population.
• Species diversity: the number of species and their relative abundance in a community.
• Habitat or ecosystem diversity: the variety of habitats and ecosystems in an area.

A Cavendish banana plantation has very low genetic diversity because the plants are clones.

The components of diversity (genetic, species and habitat diversity) contribute to the resilience of ecological systems, which is their ability to absorb disturbance and recover.

• High genetic diversity means some individuals are likely to carry alleles for resistance to a new disease or a change in conditions.
• High species diversity means that if one species is lost, others can fill its role.
• High habitat diversity gives more refuges.

Genetically uniform crops, such as Cavendish bananas, can be wiped out by one pathogen.

Biodiversity arises from evolutionary processes. Natural selection is the mechanism driving evolutionary change. It requires:

1. Variation in inherited characteristics within a population.
2. Overproduction of offspring.
3. Competition for limited resources.
4. Differences in adaptation that affect rates of survival and reproduction, so better-adapted individuals pass on more alleles.

Over generations the frequency of advantageous alleles rises. Example: after the 1977 drought on Daphne Major, deep-beaked medium ground finches survived better on large, hard seeds, and the next generation had a greater average beak depth.

Speciation is the generation of new species through evolution. Populations that use different resources or live in different conditions are selected in different directions. Over many generations differences accumulate until the populations can no longer interbreed and are separate species.

Example: the hundreds of cichlid species of Lake Victoria evolved from a few ancestral populations, differing in diet, depth and breeding colours.

Species diversity in a community is a product of richness (the number of species) and evenness (how equal the numbers of individuals in each species are).

Simpson's reciprocal index gives a quantitative measure of diversity:

D = N(N − 1) / Σn(n − 1)

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

A higher D means higher diversity. It allows different ecosystems to be compared and the change in one ecosystem to be monitored over time.

Worked example: counts 12, 11, 10, 9, 8 give N = 50, N(N − 1) = 2450, Σn(n − 1) = 460, so D = 5.3. Counts 38, 6, 3, 2, 1 have the same richness but give D = 1.7, because one species dominates.

Effective management strategies need knowledge of global and regional biodiversity: which species are present, where they are, how many there are and how this is changing. Surveys and indices such as Simpson's allow managers to identify priority species and areas, to monitor the effect of threats such as the Nile perch in Lake Victoria, and to judge whether management is working.

Knowledge is incomplete for many regions and groups of species, and strategies also depend on funding, enforcement and the needs of local people.