Biodiversity

Biodiversity is the variety of living organisms present in an area (a community or ecosystem).

It can be measured at three levels: habitat (ecosystem) diversity, species diversity and genetic diversity.

Species richness is the number of different species present in a community. It does not take into account how many individuals of each species there are.

Species evenness describes how equally the individuals are distributed between the different species — whether the numbers are similar (high evenness) or one species dominates (low evenness).

Species diversity depends on both because a community with many species but dominated by one (low evenness) is less diverse than one with the same number of species spread evenly. Only by combining richness and evenness does a measure such as the index of diversity reflect the true diversity.

An endemic species is one that is found naturally in only one geographical area (one location) and nowhere else in the world.

Endemic species are particularly vulnerable to extinction because if their single habitat is destroyed or changed, there is no other population elsewhere to maintain the species. They often also have low genetic diversity and small population sizes, so they are less able to adapt to environmental change such as disease or climate change.

Step 1 — find N (total of all individuals):

N = 7 + 5 + 4 + 2 + 1 = 19

Step 2 — find n(n−1) for each species:

Step 3 — sum them: Σn(n−1) = 42 + 20 + 12 + 2 + 0 = 76

Step 4 — substitute into the formula:

$D = \frac{N(N-1)}{\sum n(n-1)} = \frac{19 \times 18}{76} = \frac{342}{76}$

Step 5 — evaluate: D = 4.50 (3 s.f.). The index of diversity has no units.

(Note that the single centipede contributes 1 × 0 = 0 to the sum — a species represented by one individual cannot form a pair.)

High biodiversity is important for both ecological and human reasons.

Ecosystem stability. A community with high biodiversity contains more species and more food chains, so the loss of any one species has less impact on the ecosystem. This makes the ecosystem more stable and better able to withstand environmental change such as disease, drought or climate change. High genetic diversity within each species also means it is more likely to contain individuals that can survive such changes, reducing the risk of extinction.

A genetic resource (gene pool). Wild species are a reservoir of alleles that can be used in selective breeding and genetic engineering — for example, to introduce disease resistance or higher yield into crop plants and livestock. Once a species is lost, its alleles are lost forever.

Direct human uses. Many medicines are derived from wild organisms, and undiscovered species may be sources of future drugs; biodiversity also provides food, fibres and other materials.

Ecosystem services. Diverse ecosystems carry out vital processes such as pollination of crops, nutrient cycling, water purification and climate regulation (e.g. carbon storage by forests), on which agriculture and human life depend.

Ethical, aesthetic and economic value. Many people believe species have a right to exist, and biodiversity has aesthetic value and supports ecotourism, which provides income and an incentive to conserve.

For these reasons, maintaining biodiversity safeguards both the stability of ecosystems and a wide range of present and future resources for humans.

The heterozygosity index is the proportion of individuals that are heterozygous:

$\text{heterozygosity index} = \frac{\text{number of heterozygous individuals}}{\text{total number of individuals}} = \frac{28}{80} = 0.35$

So the heterozygosity index for this locus is 0.35 (a proportion between 0 and 1; it has no units).

A high heterozygosity index indicates that a large proportion of individuals carry two different alleles, so the population has a wide variety of alleles — that is, high genetic diversity. This is important because a genetically diverse population is more likely to contain individuals that can survive environmental change (such as a new disease), making the population more able to adapt and less likely to become extinct.

Human activities reduce biodiversity by reducing the variety and abundance of species and the variety of habitats.

Habitat destruction and fragmentation. Clearing land for agriculture, building and roads, and deforestation, removes habitats so the species that lived there lose their food and shelter and their populations fall. Fragmentation divides populations into small isolated groups, which reduces gene flow and lowers genetic diversity, making local extinction more likely.

Over-exploitation. Overfishing, overhunting and over-collecting remove individuals faster than the population can replace them, reducing population size and, in the worst cases, causing extinction; this also lowers genetic diversity.

Pollution. Chemicals such as pesticides and fertilisers (causing eutrophication), as well as plastic and oil, kill organisms or change habitats so that fewer species can survive there.

Climate change. Burning fossil fuels raises temperatures and changes rainfall, altering habitats so that some species can no longer survive in an area and may not be able to migrate fast enough.

Introduced (invasive) species. Species moved by humans may out-compete, prey on or bring disease to native species, especially endemic species, reducing native biodiversity.

Through these mechanisms human activity reduces habitat, species and genetic diversity, threatening the stability of ecosystems.

Site 1. N = 40 + 6 + 3 + 1 = 50.

Σn(n−1) = (40 × 39) + (6 × 5) + (3 × 2) + (1 × 0) = 1560 + 30 + 6 + 0 = 1596

$D_1 = \frac{50 \times 49}{1596} = \frac{2450}{1596} = 1.5\ \text{(2 s.f.)}$

Site 2. N = 14 + 13 + 12 + 11 = 50.

Σn(n−1) = (14 × 13) + (13 × 12) + (12 × 11) + (11 × 10) = 182 + 156 + 132 + 110 = 580

$D_2 = \frac{50 \times 49}{580} = \frac{2450}{580} = 4.2\ \text{(2 s.f.)}$

Comparison. Both sites have the same species richness (4 species) and the same total number of organisms (N = 50), yet their indices of diversity are very different: Site 2 (D = 4.2) is far more diverse than Site 1 (D = 1.5).

The difference is species evenness. Site 1 is heavily dominated by grass (40 of the 50 individuals), giving low evenness and a large Σn(n−1), so a low D. In Site 2 the four species are present in roughly equal numbers, giving high evenness and a much smaller Σn(n−1), so a high D.

Site 2 is therefore more biodiverse and more likely to be a stable ecosystem, because it has more food chains and greater interdependence: no single species is critical, so the loss of one species would have less impact and the community is better able to withstand environmental change.

In situ conservation protects a species in its natural habitat, for example by establishing nature reserves, national parks or marine protected areas and giving the habitat legal protection.

• Advantage: it conserves the whole ecosystem, allows the species to behave naturally and to continue evolving, and usually maintains a larger, more genetically diverse population.
• Disadvantage: it only works if the habitat can be protected; it cannot easily guard against threats that cross reserve boundaries, such as climate change, poaching or disease.

Ex situ conservation protects a species outside its natural habitat, for example by captive breeding in zoos, botanic gardens or storing gametes and seeds in gene/seed banks.

• Advantage: it safeguards the species even if the wild habitat is lost and allows captive breeding to increase numbers for later reintroduction.
• Disadvantage: captive populations are small, risking inbreeding and a loss of genetic diversity (heterozygosity); it is expensive, animals may lose natural behaviours, and reintroduction is difficult.

Conclusion. For a critically endangered mammal whose habitat is shrinking, the most effective approach is to combine both methods: use ex situ captive breeding to prevent immediate extinction and maintain numbers, while working to protect and restore the habitat (in situ) so that captive-bred animals can eventually be reintroduced into a viable wild population. Neither method alone is sufficient.

The heterozygosity index is a measure of genetic diversity — the proportion of individuals that are heterozygous, and so the variety of alleles in the gene pool. A fall from 0.42 to 0.15 means the population has lost genetic diversity, even though the number of birds (300) has increased.

This loss happens because the programme started from only 8 founders, so it began with a limited set of alleles (a founder effect). In a small population, inbreeding between closely related birds is unavoidable, and alleles are lost by chance (genetic drift), so the proportion of heterozygotes falls.

Low genetic diversity is a concern because the population now contains fewer alleles, so it is less likely to contain individuals able to survive a change such as a new disease or a change in conditions — the population is less able to adapt and more vulnerable to extinction. Inbreeding can also increase the chance of harmful recessive alleles being expressed (inbreeding depression), reducing fertility and survival.

To reduce the problem, conservationists could:

• Exchange individuals (or gametes) with other captive populations or zoos to introduce new alleles and increase gene flow.
• Use studbooks to plan breeding so that unrelated birds are paired, avoiding inbreeding.
• Where possible, add wild-caught individuals to the breeding population.
• Maintain as large a breeding population as possible to slow genetic drift.

In this way the heterozygosity (genetic diversity) can be raised again, improving the population's long-term chance of survival.