Biodiversity, classification and sampling: biodiversity at the genetic, species and habitat levels, species richness and evenness, calculating and interpreting a diversity index, the hierarchical classification of organisms and binomial nomenclature, the use of quadrats, transects and mark-release-recapture (the Lincoln index) to sample organisms, and the structural, physiological and behavioural adaptations of organisms to their habitats.

What this dot point is asking

CCEA wants you to define biodiversity at the genetic, species and habitat levels, explain species richness and evenness and calculate a diversity index, describe the hierarchical classification of organisms and binomial nomenclature, describe how quadrats, transects and mark-release-recapture (the Lincoln index) are used to sample organisms, and explain the structural, physiological and behavioural adaptations of organisms to their habitats.

Levels of biodiversity

Genetic diversity within a species is the raw material for natural selection and gives a population the flexibility to adapt to change; a species with low genetic diversity is more vulnerable to disease and environmental change. Species diversity describes a community, and habitat diversity describes a wider area: a region with many different habitats (such as woodland, grassland, freshwater and peat bog) usually supports more species overall.

Species richness, evenness and the diversity index

Species richness is simply the number of different species present. Species evenness is how equally the individuals are distributed among those species. A community where one species dominates has low evenness, even if many species are present.

A higher diversity index is usually linked to greater stability: more species and more feeding relationships mean the community is less dependent on any single species and better able to resist environmental change.

Classification and naming

Organisms are placed in a hierarchical classification in which smaller groups are nested inside larger ones. From the largest and most inclusive group to the smallest, the levels are: domain, kingdom, phylum, class, order, family, genus, species. The deeper two organisms share groups, the more closely related they are, so members of the same genus are more closely related than members of the same family.

The binomial system gives every species a two-part Latin name. The first word is the genus (capital letter) and the second is the species name (lower case), with the whole name italicised in print or underlined when handwritten, for example Canis lupus for the grey wolf. Modern classification is phylogenetic: it aims to group organisms by their evolutionary relationships, increasingly using molecular evidence such as DNA and protein sequence comparisons, rather than by superficial similarity alone.

Sampling organisms

Because it is impossible to count every organism, ecologists sample and use the results to estimate the whole population. To avoid bias, plant and slow-moving samples are taken at random (for example using coordinates from a random number generator) and the sample size is kept large.

• Quadrats (frames of known area) are placed at random to estimate density (individuals per unit area), frequency (the proportion of quadrats a species appears in) or percentage cover.
• Belt or line transects are used to study how a community changes across an environmental gradient, such as up a sea shore; quadrats are placed at intervals along a line.
• Mark-release-recapture estimates the population of mobile animals using the Lincoln index:

where $n_1$ is the number marked and released, $n_2$ is the number caught in the second sample, and $m$ is the number of marked animals recaptured. The method assumes that marked individuals mix randomly with the population, the mark does not affect survival or recapture, and there is no significant migration, birth or death between samples.

Adaptation

An adaptation is a feature that increases an organism's chance of survival and reproduction in its habitat. Adaptations are of three kinds: structural (a physical feature, such as a thick coat or a streamlined body), physiological (a feature of how the body works, such as producing antifreeze proteins or concentrating urine), and behavioural (an action, such as migration or basking to warm up). Xerophytes (with sunken stomata and rolled leaves) and desert mammals (with long loops of Henle) are examples of structural and physiological adaptations to dry habitats.

Examples in context

Example 1. A rocky sea shore transect. Moving from the low-water mark up the shore, the species change with exposure to air and wave action. A belt transect with quadrats at fixed intervals records this zonation, showing how abiotic gradients shape which species can survive at each height, a classic CCEA fieldwork example.

Example 2. Conserving genetic diversity in crops. Modern crop varieties are often genetically uniform, which makes a whole field vulnerable to a single new disease. Seed banks store the wide genetic diversity of old varieties and wild relatives, preserving alleles that might be needed to breed in resistance, an application of why genetic biodiversity matters.

Try this

Q1. State the three levels at which biodiversity is measured. [3 marks]

• Cue. Genetic diversity (within a species), species diversity (within a community) and habitat diversity (within an area).

Q2. Explain why random sampling is used when placing quadrats. [2 marks]

• Cue. To avoid bias so the sample fairly represents the whole area, making any estimate of abundance more valid.

Q3. Write the taxonomic groups in order from largest to smallest. [2 marks]

• Cue. Domain, kingdom, phylum, class, order, family, genus, species.