Notes · Environmental ScienceUK · A-Levels

Conservation of biodiversity

This chapter examines what biodiversity is, how it is measured, why it matters and how it can be conserved. It covers species richness, evenness and genetic diversity and the calculation of Simpson's index, the ecosystem services that give biodiversity its value, the human activities driving its loss, the estimation of population size by quadrats and the Lincoln index, and the range of in-situ and ex-situ conservation strategies together with their evaluation.

5 sections~17 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 2

T·0333 / 16

Exam profile

AO1 · Describe biodiversity, its value, the threats to it and the methods used to conserve itAO2 · Calculate Simpson's index of diversity and Lincoln-index population estimates from sample dataAO3 · Analyse biodiversity data and evaluate the effectiveness and trade-offs of conservation strategies

Operators:describeexplaincalculateanalyseevaluatejustify

basic level

AS-Level expects you to describe biodiversity and its importance, the main threats, and the main in-situ and ex-situ conservation methods, and to use simple sampling.

higher level

The full A-Level requires calculation and interpretation of Simpson's and Lincoln indices and the reasoned evaluation of competing conservation strategies and their trade-offs.

Depth

Text

Contents · 5 sections

Conservation of biodiversity

What biodiversity is and how it is measured#

StandardAQA 7447 3.1.2

Comparing the diversity of two habitats

Fig. 1Simpson's index compared for two habitats (illustrative values): the semi-natural woodland is more diverse.

Biodiversity is the variety of life, and it is measured at three levels. Genetic diversity is the variety of alleles within a species, which gives a population the raw material to adapt to change and resist disease. Species diversity is the variety of species in a community, and habitat (or ecosystem) diversity is the variety of habitats in an area. A full assessment considers all three, because a region with many habitats will usually support many species, and species with high genetic diversity are more resilient.

Species diversity itself has two components. Species richness is simply the number of different species present, whereas species evenness describes how evenly the individuals are distributed among those species. A community with a few dominant species and many rare ones is less even, and generally regarded as less diverse, than one where the species are equally abundant, even if the two have the same richness. A good diversity index takes both richness and evenness into account, which is why a simple species count is an incomplete measure.

Simpson's index of diversity does exactly this. It is calculated as

D = 1 - \sum \left( \dfrac{n}{N} \right)^2

, where

n

is the number of individuals of each species and

N

is the total number of individuals of all species. The value lies between 0 and almost 1: a value near 0 means low diversity (one species dominates), and a value near 1 means high diversity (many species, evenly spread). Because it uses the proportion of each species, it responds to both richness and evenness, and it allows two communities to be compared objectively.

A high diversity index is generally associated with a stable, mature ecosystem, because a complex food web with many interacting species is more able to withstand and recover from disturbance; the loss of one species is buffered by others in similar roles. A low index often indicates a stressed, polluted or heavily managed environment, such as an intensively farmed field. Interpreting an index, rather than just calculating it, is what the higher marks reward: a number means little until it is related to the ecology of the site.

D = 1 - \sum \left( \frac{n}{N} \right)^2

Simpson's index of diversity

$n$ is the number of individuals of each species and $N$ the total of all individuals; $D$ ranges from 0 (low diversity) to near 1 (high diversity) and reflects both richness and evenness.

Worked example

Calculating Simpson's index

A quadrat contains three plant species: 10 individuals of species A, 8 of species B and 2 of species C. Calculate Simpson's index of diversity.

Find N
Total individuals $N = 10 + 8 + 2 = 20$ .
Find each n/N and square it
$(10/20)^2 = 0.25$ , $(8/20)^2 = 0.16$ , $(2/20)^2 = 0.01$ .
Sum and subtract from 1
$\sum (n/N)^2 = 0.25 + 0.16 + 0.01 = 0.42$ ; $D = 1 - 0.42$ .
$D = 1 - 0.42 = 0.58$
Interpret
A value of 0.58 indicates moderate diversity: three species are present but one is fairly dominant, lowering the evenness.

Result: Simpson's index of diversity D = 0.58, indicating moderate diversity.

Typical mistakes

Forgetting the final step of subtracting the sum from 1, or squaring after dividing incorrectly.
Equating diversity with richness alone and ignoring evenness, or treating a bare index value as meaningful without relating it to the habitat.

Active revision

A pond sample contains 12 individuals of species A, 9 of species B and 4 of species C. Calculate Simpson's index of diversity and comment on what it suggests about the pond.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Why biodiversity matters: ecosystem services and values#

FoundationAQA 7447 3.1.2

Ecosystem services provided by biodiversity

Fig. 2The four categories of ecosystem service, each underpinned by biodiversity.

The reasons for conserving biodiversity are often grouped into ecological, economic, aesthetic and ethical arguments. Ecologically, diverse ecosystems are more stable and resilient and continue to provide the processes on which life depends. Economically, biodiversity is a source of food, timber, fibres, medicines and genetic material, and underpins industries such as tourism and fishing. Aesthetically and culturally, wild places and species have recreational, spiritual and inspirational value; and ethically, many argue that species have an intrinsic right to exist and that the present generation has a duty of stewardship to the future.

A useful framework is the idea of ecosystem services, the benefits that people obtain from ecosystems. These are conventionally divided into provisioning services (goods such as food, fresh water, timber and medicines), regulating services (processes such as climate regulation, flood control, water purification and pollination), supporting services (the underlying processes such as nutrient cycling, soil formation and photosynthesis that make the others possible) and cultural services (recreation, education and spiritual value). This framework makes clear that biodiversity is not a luxury but the basis of processes with enormous, if often hidden, value.

Many of these services depend directly on biodiversity. Pollination of crops by insects, natural pest control by predators, the purification of water by wetlands and the regulation of the carbon cycle by forests and oceans all rely on functioning, diverse communities. The genetic diversity within crop species and their wild relatives is an insurance against pests, disease and climate change, and wild species remain a source of new pharmaceuticals. Losing biodiversity therefore erodes services that would be extremely costly or impossible to replace by technology.

The argument for conservation is strengthened by recognising that these services are frequently undervalued because they are not traded in markets and their loss is gradual. Placing an economic value on ecosystem services, though difficult and controversial, has helped to make the case for conservation in decisions that would otherwise weigh only the short-term profit from exploitation. Being able to marshal ecological, economic and ethical arguments, and to weigh them, is exactly what extended-response questions on conservation demand.

Worked example

Valuing a wetland

A wetland is threatened by drainage for farmland. Outline the ecosystem services that would be lost and use them to argue for its conservation.

Provisioning and regulating
The wetland stores and purifies water, reduces flooding downstream, and can store carbon, all regulating services with real economic value.
Supporting and cultural
It supports nutrient cycling and provides habitat for diverse species, and offers recreation and education.
Weigh the decision
The short-term gain in farmland must be set against the loss of flood protection, water purification and biodiversity, which would be costly to replace, so conservation is justified.

Result: The wetland's regulating, supporting and cultural services outweigh the short-term value of drainage, supporting its conservation.

Typical mistakes

Giving only economic reasons and neglecting the ecological and ethical arguments.
Confusing supporting services (underlying processes such as nutrient cycling) with the provisioning services that depend on them.

Active revision

Explain, using the idea of ecosystem services, why the loss of insect pollinators would have serious economic as well as ecological consequences.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Threats to biodiversity and human impact#

StandardAQA 7447 3.1.2

Drivers of biodiversity loss

Fig. 3The main drivers of biodiversity loss interact and reinforce one another.

The main drivers of biodiversity loss are human activities, often summarised by the memorable acronym HIPPO: Habitat destruction and fragmentation, Invasive species, Pollution, human Population growth and Over-exploitation. Habitat destruction, through deforestation, drainage, urbanisation and intensive agriculture, is the single largest cause, because it removes the places species need to live. Fragmentation, in which a habitat is broken into isolated patches, is especially damaging because small, isolated populations lose genetic diversity, are more affected by edge effects and are more likely to go extinct.

Over-exploitation, taking species faster than they can reproduce, has driven many fisheries, forests and hunted animals into decline; K-selected species with slow reproduction are most vulnerable. Invasive non-native species, introduced deliberately or accidentally, can out-compete, prey on or bring disease to native species that have no defences against them, sometimes causing rapid extinctions on islands. Pollution degrades habitats and poisons organisms, and climate change is an increasingly important driver, shifting the ranges of species and outpacing the ability of specialists to adapt or move.

These drivers interact and reinforce one another. A fragmented population weakened by habitat loss is more easily finished off by an invasive predator, a disease or a run of bad weather; climate change forces species to move just as habitat loss removes the corridors they would move through. This is why biodiversity loss is accelerating and why the current wave of extinctions is often described as being far above the natural background rate, though precise global figures are uncertain and should be treated with care.

The consequences of extinction are permanent and, because of the interconnectedness of ecosystems, unpredictable. The loss of a keystone species, one whose role is disproportionate to its abundance, can cause a cascade of further losses; the loss of genetic diversity reduces the ability of species and crops to adapt. Recognising which species and habitats are most at risk, using tools such as the IUCN Red List categories from least concern through vulnerable and endangered to critically endangered and extinct, is the first step in setting conservation priorities.

Worked example

Analysing an island extinction

After rats were accidentally introduced to a remote island, several ground-nesting bird species became extinct within decades. Explain why the impact was so severe.

No coevolved defences
The native birds evolved without ground predators, so they had no behavioural or other defences against rats eating their eggs and chicks.
Small, isolated populations
Island populations are small and cannot be rescued by immigration, so losses cannot be replaced.
Conclude
An invasive predator meeting defenceless, isolated prey caused rapid over-predation and extinction, illustrating why invasive species are so damaging on islands.

Result: Defenceless, isolated island species were driven extinct by an introduced predator, showing the severity of invasive-species impacts.

Typical mistakes

Treating the drivers as independent rather than interacting and reinforcing one another.
Quoting precise global extinction figures as if they were certain; the rate is elevated but exact numbers are uncertain.

Active revision

Explain why a small, isolated population left after habitat fragmentation is more likely to become extinct than the same number of individuals in a single continuous habitat.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Estimating populations: quadrats and the Lincoln index#

AdvancedAQA 7447 3.1.2AQA 7447 3.7

The mark-release-recapture method

Fig. 4The Lincoln index estimates population size from the proportion of marked animals recaptured.

Conservation depends on knowing how many organisms are present and whether numbers are changing, but counting every individual is rarely possible, so populations are estimated from samples. For plants and slow-moving animals, quadrats are used: a quadrat of known area is placed at random positions and the organisms within it counted, and the mean density is scaled up to the whole area. Random placement, using coordinates from a random-number generator, is essential to avoid bias, and enough quadrats must be taken for the estimate to be reliable.

For mobile animals, the mark-release-recapture method is used, and the population is estimated with the Lincoln index. A first sample is captured, marked harmlessly and released; after time to mix back into the population, a second sample is captured and the number of marked individuals recaptured is recorded. The population is then estimated as

N = \dfrac{M \times n}{m}

, where

M

is the number marked and released in the first sample,

n

is the total caught in the second sample, and

m

is the number of marked individuals in the second sample. The logic is that the proportion of marked animals in the second sample equals the proportion of the whole population that was marked.

The method relies on several assumptions, and being able to state and evaluate them is important. It assumes that the marked individuals mix randomly back into the population, that marking does not affect survival or behaviour (for example by making animals more visible to predators), that marks are not lost, that there is no significant birth, death, immigration or emigration between the two samples, and that marked and unmarked animals are equally likely to be caught. If any assumption is violated the estimate is biased, so a good answer identifies which assumption is at risk in a given context.

Repeated estimates over time show whether a population is stable, growing or declining, which is the information conservation managers need. Combined with the diversity indices from earlier in the chapter, population estimates allow the health of an ecosystem to be monitored and the effect of a management intervention, such as removing an invasive species or restoring a habitat, to be assessed against real data rather than impression.

N = \frac{M \times n}{m}

Lincoln index

$N$ is the estimated total population, $M$ the number marked in the first sample, $n$ the total in the second sample and $m$ the number of marked individuals recaptured.

Worked example

Applying the Lincoln index

To estimate a population of beetles, 60 were captured, marked and released. Later, 80 beetles were captured, of which 15 carried the mark. Estimate the total population.

Identify the values
$M = 60$ (marked and released), $n = 80$ (second sample), $m = 15$ (marked recaptures).
Apply the Lincoln index
$N = \dfrac{M \times n}{m} = \dfrac{60 \times 80}{15}$ .
$N = \frac{60 \times 80}{15} = \frac{4800}{15} = 320$
State the answer
The estimated population is 320 beetles, assuming random mixing and no births, deaths or migration between the two samples.

Result: The estimated population is 320 beetles.

Typical mistakes

Putting the numbers in the wrong positions in the Lincoln index, for example dividing by the total second sample instead of the recaptures.
Failing to state or evaluate the assumptions, such as random mixing and no births or deaths between samples.

Active revision

In a study of woodlice, 48 were captured, marked and released. Two days later 60 were captured, of which 16 were marked. Estimate the population and state one assumption that might not hold.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Conservation strategies and their evaluation#

AdvancedAQA 7447 3.1.2

In-situ and ex-situ conservation strategies

Fig. 5Conservation strategies fall into in-situ (in the wild) and ex-situ (outside the natural habitat) methods.

Conservation strategies are broadly divided into in-situ methods, which conserve species in their natural habitat, and ex-situ methods, which conserve them outside it. In-situ conservation includes protected areas such as national parks and nature reserves, active habitat management (grazing, coppicing, controlling invasive species), the creation of wildlife corridors to reconnect fragmented habitats, and rewilding, which aims to restore natural processes and sometimes reintroduce lost species. In-situ conservation is generally preferred because it protects whole ecosystems and the interactions within them and allows populations to continue evolving in the wild.

Ex-situ conservation includes captive breeding in zoos and aquaria, botanic gardens, and gene and seed banks that store genetic material at low temperature for the long term. These methods are a vital safety net for species that are critically endangered or extinct in the wild, they support research and public education, and captive-bred individuals can be used for reintroduction. However, ex-situ methods are expensive, can maintain only a small number of species and small populations that risk losing genetic diversity, cannot easily recreate the natural environment, and do not by themselves address the reasons a species declined.

Conservation also depends on legislation and international cooperation. National laws protect species and designate protected sites; internationally, CITES (the Convention on International Trade in Endangered Species) restricts trade in threatened species and their products, and biodiversity agreements set shared targets. Legislation is only as good as its enforcement, and its success varies, so its effectiveness is a legitimate subject for evaluation. The most durable conservation usually combines protection with the involvement and economic benefit of local communities, for example through sustainable tourism.

Evaluating conservation strategies means weighing effectiveness, cost, feasibility and trade-offs rather than simply listing methods. In-situ conservation protects more, and more cheaply per species, but requires large areas and cannot save a species whose habitat is already gone; ex-situ conservation can rescue individual species but at high cost and with limited capacity. The strongest answers recognise that the two are complementary, that resources are limited so priorities must be set, and that lasting success requires tackling the underlying drivers of loss identified earlier in the chapter.

Worked example

Choosing a conservation strategy

A species of orchid survives only in a few fragments of an ancient grassland that is being lost to development. Recommend and justify a conservation strategy.

Assess in-situ options
Protecting and managing the remaining grassland fragments in-situ conserves the orchid with its pollinators and soil fungi, but the fragments are small and threatened by development.
Add an ex-situ safety net
Storing seed in a seed bank and propagating plants in a botanic garden protects the genetic material against the loss of a fragment.
Combine and justify
The best strategy protects the habitat in-situ, connects fragments where possible, and keeps an ex-situ seed store as insurance, because neither approach alone is secure.

Result: A combined strategy of in-situ habitat protection plus an ex-situ seed bank is justified, as the approaches are complementary.

Typical mistakes

Listing methods without evaluating their effectiveness, cost and trade-offs.
Presenting ex-situ conservation as an alternative to protecting habitat rather than as a complementary safety net that does not address the causes of decline.

Active revision

Evaluate the use of captive breeding in zoos as a strategy for conserving a critically endangered mammal, considering its strengths and limitations.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Conservation of biodiversity

5 sections~17 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 2

Conservation of biodiversity

Key points

Calculating Simpson's index

Key points

Valuing a wetland

Key points

Analysing an island extinction

Key points

Applying the Lincoln index

Key points

Choosing a conservation strategy

Sources

Conservation of biodiversity

Key points

Calculating Simpson's index

Key points

Valuing a wetland

Key points

Analysing an island extinction

Key points

Applying the Lincoln index

Key points

Choosing a conservation strategy

Sources