Biodiversity

Biodiversity is not simply a count of species. It is measured at three levels at once, and a place can look rich at one level while being dangerously poor at another — which is exactly what exam questions are built around.

What biodiversity means

The words “specified area” matter. You can talk about global biodiversity, or about the biodiversity of one pond, one field or one coral reef. Whenever you answer a biodiversity question, be clear which area you are talking about.

The reason biologists care so much is resilience. The more biodiverse an ecosystem is, the more stable and resistant to change it will be. A diverse ecosystem has spare parts: if one species is knocked out, others can often do a similar job. An ecosystem with only a handful of species has no backup.

The three levels

Ecosystem diversity

This is the range of different ecosystems, or habitats, within a particular area.

• If there are many different habitats in an area, that area has high biodiversity. A coral reef is the classic example: reefs are structurally complex, with many microhabitats and niches to be exploited.
• If there are only one or two habitats, biodiversity is low. Large sandy deserts are the usual example, because conditions are very similar across the whole area.

Species diversity

Species diversity is measured in two ways, and you need both.

For an ecosystem to have high species diversity it must have high richness and high evenness. Two examples show why one alone is not enough.

• An ecosystem with 1000 species, but only a few individuals of 500 of them, has high richness but low evenness. Those rare species are one bad year away from disappearing.
• An ecosystem with 10 species and thousands of individuals of each has high evenness but low richness. It is crowded, but it is not varied.

This links straight back to stability. Ecosystems with high species diversity are usually more stable because they are more resilient to environmental change. In the pine forests of Florida, for example, the ecosystem is dominated by one or two tree species. If a pathogen arrives that targets one of those dominant trees, the whole population could be wiped out and the ecosystem it supports could collapse.

Genetic diversity

Genetic diversity is the number of different alleles of genes that are present. It can be measured across an entire species, or within one local population.

• Two things contribute to it: the proportion of genes that have more than one allele, and the number of different alleles each gene has.
• There can be genetic differences between populations of the same species. Populations living in different areas are subject to slightly different selection pressures, which affect allele frequencies.
• There can also be genetic differences between individuals within a single population.

Genetic diversity is important because it helps a population adapt to, and survive, changes in the environment. A population where every individual is genetically almost identical has no variation for selection to work on, so a new disease or a hotter summer can take all of them at once.

How much biodiversity is there?

• Current estimates vary, but recent ones suggest there could be around 8.7 million species on Earth.
• Only about 1.2 million have been recorded and classified, so if the estimates are right there are still many more species to be discovered.
• The number of species has not always been the same. The fossil record shows many species that once existed are no longer present, because of extinction.
• It also shows there are many more species alive today than have been alive in the past.

Speciation increases the number of species. When speciation occurs at a higher rate than extinction, global biodiversity increases. Despite the many extinctions of recent years, past periods of speciation mean global biodiversity levels are still higher than ever before.

NOS: classification is pattern recognition

The estimate of how many species exist depends entirely on the process of classification, and classification is not as objective as it looks.

• If species are classified correctly, estimates will be more accurate.
• If species are incorrectly classified together, estimates will be too small.
• If species are incorrectly separated, estimates will be too large.

Classification can be subjective. It is not always easy to decide the point at which one species has become two, or whether similar organisms belong to the same species or two different ones.

Worked examples