Biodiversity — OCR A-Level Biology

Biodiversity is considered at several levels. At the habitat level, the range of habitats in an area is assessed. At the species level, species richness (the number of different species) and species evenness (how similar their abundances are) are considered. At the genetic level, variation within a species is assessed, which underpins adaptation and resilience. These levels are linked: a habitat with many niches can support more species, and genetically diverse populations are better able to survive environmental change. A single measure such as species richness alone does not capture genetic or habitat diversity, so biodiversity is described by combining levels.

Biodiversity means the variety of species in a habitat, so measuring it means estimating which species are present and how many individuals of each there are. You cannot count every organism in a field or pond, so you take samples and use them to represent the whole area. Sampling works by choosing a manageable number of small areas or transects, recording the species and their abundance in each, then scaling up or comparing the results. Random sampling avoids bias when the habitat is uniform; systematic sampling along a transect follows a change such as a gradient. The importance of sampling is that it gives representative, repeatable data without destroying the habitat, and it lets you compare biodiversity between sites or over time. Larger numbers of samples and consistent technique reduce chance and bias, making conclusions more reliable.

Field sampling turns a question about biodiversity into data. Random sampling removes investigator bias: define the area, generate coordinates with a random number generator or random number table, and place quadrats at those positions, or use a transect with quadrats at random intervals along it. Non-random sampling is chosen deliberately: a belt transect follows a gradient, a line transect records organisms touching the line, and opportunistic or stratified placement targets particular zones. In the field you must standardise quadrat size, record percentage cover or frequency consistently, and repeat enough samples to be representative. Compare random and non-random methods by asking which suits the question: random for an overall estimate of a whole area, non-random for change along a gradient or for rare, patchily distributed species.

Species richness is the number of different species present in a habitat; species evenness describes how similar the abundances of those species are. To measure richness, sample the habitat representatively, for example with random quadrats (or a transect where a gradient exists), identify every species present and count the distinct species. To measure evenness, record the number of individuals of each species, then compare the relative abundances: a community where species have similar counts is even, while one dominated by a single species is uneven. Random sampling avoids bias, and a large enough sample or several quadrats improves reliability. Richness ignores abundance, so two habitats with equal richness can differ in evenness.

Simpson's Index of Diversity (D) combines species richness and evenness into a single measure of biodiversity. Using the form D = 1 − Σ(n/N)², where n is the number of individuals of each species and N is the total number of individuals of all species, you square each species' proportion, sum these values and subtract from 1. A value close to 1 indicates high diversity (many species, fairly even abundances); a value close to 0 indicates low diversity (few species or one dominant species). To use it, sample the habitat representatively, count individuals of each species, calculate N, then apply the formula. Interpretation must consider that D depends on both richness and evenness, and that sampling bias or small samples reduce reliability.

Genetic biodiversity is the variety of alleles and genes within a species or population. It is assessed by sampling individuals and measuring variation at gene loci, for example by comparing allele frequencies or by counting polymorphic loci. A common measure is the proportion of loci that are polymorphic, and another is heterozygosity, the proportion of individuals heterozygous at a locus. Calculations matter: if 8 of 20 loci sampled are polymorphic, the proportion is 8 ÷ 20 = 0.40, or 40%. If 30 of 200 individuals are heterozygous at a locus, heterozygosity is 30 ÷ 200 = 0.15. Larger, random samples give more reliable estimates, and the same loci must be compared across populations.

Biodiversity is the variety of living organisms in an area, measured at habitat, species and genetic levels. Learners must be able to identify the factors that affect it. These include human population growth, which raises demand for land, food, water and energy, so habitats are destroyed, fragmented or polluted and species numbers fall. Climate change alters temperature and rainfall patterns, shifting the ranges of species and disrupting the timing of life cycles, so some species decline and others invade. Other factors include overexploitation, agriculture and deforestation. A question may give a scenario and ask which factor is acting, or which change would reduce biodiversity. Read the scenario carefully, name the specific factor, then explain the mechanism by which it lowers variety.

Biodiversity is maintained for three overlapping kinds of reason, and a multiple-choice question will test whether you can classify a given example correctly. Ecological reasons concern the functioning of ecosystems: diverse communities tend to be more stable and resilient, and the organisms within them provide services such as pollination, nutrient cycling, soil formation and flood regulation. Economic reasons concern direct and indirect value to people: medicines derived from wild species, food crops and their wild relatives, timber, fisheries, tourism and the insurance value of genetic diversity against future disease or climate change. Aesthetic reasons concern cultural, recreational and intrinsic value: landscapes people value, wildlife watching, and the ethical view that species have worth beyond human use.

Conservation methods are grouped as in situ (protecting organisms in their natural habitat) or ex situ (protecting them away from their natural habitat). In situ methods include nature reserves, national parks, marine protected areas and habitat restoration; they conserve the species alongside the rest of the community and its ecological interactions, but can be hard to enforce and vulnerable to a single local threat. Ex situ methods include zoos, botanic gardens, seed banks and captive breeding programmes; they protect individuals and genetic material when a habitat is lost or a population is critically small, and can support reintroduction, but are costly, hold limited genetic diversity and do not preserve the original ecosystem. A multiple-choice question will typically give a scenario and ask you to classify the method or to select an advantage or limitation of it.

Conservation agreements are formal commitments between countries or between organisations and communities that protect species and habitats. International agreements operate across national borders: CITES regulates trade in endangered species so that a species is not depleted by legal trade, and the Convention on Biological Diversity commits signatories to conserve biodiversity and use it sustainably. The Ramsar Convention protects wetlands of international importance as habitats for waterbirds and other species. Local agreements work at a smaller scale, for example a local wildlife trust managing a reserve under a management agreement with a landowner, or a community agreeing to protect a hedgerow or pond. Learn to distinguish the scale of each agreement and the mechanism it uses: trade restriction, habitat designation, or land management.