What Is Biodiversity? (OCR A-Level Biology A): Revision Notes

What Is Biodiversity?

Introduction to biodiversity

Biodiversity represents the variety of life in a given area, ranging from a small habitat to the entire planet. Despite centuries of scientific study, researchers continue to discover new species. In 2005, scientists found the wattled smoky honeyeater (Melipotes carolae) in the remote Foja mountains of New Guinea, demonstrating that Earth still holds many biological secrets.

At its simplest level, biodiversity is a catalogue of all living organisms in a particular area. However, the concept extends far beyond a simple species list to encompass the full complexity of life at multiple scales.

Defining biodiversity

The Rio Convention on Biological Diversity (1992) provides the standard definition used in conservation and ecological studies:

"Biological diversity means the variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems."

Biodiversity refers to the number of different ecosystems and habitats in an area, the number of species within those ecosystems, and the genetic variation within each species.

Most biodiversity studies focus primarily on documenting ecosystems, habitats and species. Genetic diversity is typically recorded only for species with very small populations where limited genetic variation poses conservation concerns.

Species diversity

Defining species

Biologists use at least seven different ways to define a species. The most commonly cited definition describes a biospecies:

A group of organisms that interbreed to give rise to fertile offspring and are reproductively isolated from other species. If organisms breed with other species, any offspring produced are usually sterile.

However, this definition presents practical limitations. Scientists who discover and name new species rarely observe breeding behaviour or test offspring fertility. Many species reproduce only asexually, and extinct organisms cannot be tested for reproductive compatibility.

Measuring species diversity

Species diversity refers to the variety of species within an area. This can be measured in multiple ways:

• Simple species counts record how many different species occur in a defined area
• Taxonomic diversity considers evolutionary relationships between species

Genetic diversity

Understanding genetic variation

Genetic diversity encompasses the genetic variation that exists within a species. This variation can occur between geographically separated populations or within a single population.

Consider the variation in fruit colour seen in peppers (Capsicum annuum). The different colours – red, yellow, green and purple – represent polymorphism (the existence of many forms within a species), expressing genetic diversity that exists within this single species.

Similarly, the Heliconius butterflies display remarkable genetic diversity through varied wing colours and patterns. Each individual possesses the same genes at the same positions (loci) on their chromosomes, but different versions of these genes (alleles) produce the observable differences.

Genes, alleles and the gene pool

Although different species may share many genes for basic cellular functions like respiration, each species possesses a unique combination of genes. All individuals within a species have the same genes, but different individuals carry different alleles (gene variants).

The position a gene occupies on a chromosome is its locus (plural: loci). The gene pool consists of all alleles of all genes within a species.

Factors affecting genetic diversity

Inbreeding – breeding between closely related individuals – reduces genetic diversity by increasing homozygosity (genotypes like $\text{AA}$ or $\text{aa}$). When many individuals become homozygous for numerous genes, the population's overall genetic diversity decreases. This can increase the frequency of genetic diseases caused by recessive alleles, as seen with genetic dwarfism in Californian condors and hip dysplasia in boxer dogs.

Outbreeding – breeding between unrelated individuals – maintains or increases genetic diversity. Offspring from unrelated parents are more likely to be heterozygous (genotypes like $\text{Aa}$) and show greater variation.

Assessing genetic diversity

Methods for assessment

Scientists use several approaches to measure genetic diversity within populations:

1. Proportion of polymorphic gene loci – the fraction of genes that have two or more alleles (showing genetic polymorphism)
2. Proportion of heterozygous individuals – the fraction of the population with heterozygous genotypes ($\text{Aa}$) for particular gene loci
3. Allele richness – the total number of different alleles present for certain genes

Each method requires determining whether different alleles exist for each gene locus. Some alleles produce visible differences (like pepper colour), while others require protein analysis or DNA sequencing to detect.

Calculating the proportion of polymorphic gene loci

Genetic polymorphism occurs when a gene has different alleles, where the rarest allele has a frequency exceeding either 1% or 5% (researchers choose the threshold). These percentages are arbitrary conventions rather than biologically significant values.

To calculate the proportion of polymorphic gene loci ($P$):

$P = \frac{\text{number of gene loci that are polymorphic}}{\text{total number of loci investigated}}$

A polymorphic locus is one where the most common allele has a frequency less than 95% or less than 99% (depending on the chosen threshold). Gene loci without alleles are monomorphic.

When sampling a diploid population, each individual contributes two alleles to the count (represented by two letters in genotypes: $\text{AA}$, $\text{Aa}$, or $\text{aa}$).

Case study: genetic diversity in dogs

A study of pedigree dogs (Canis familiaris) in the USA revealed that long-established breeds showed lower $P$ values than recently established breeds, demonstrating the effects of prolonged selective breeding.

Researchers examined 100 gene loci across multiple breeds:

• Total alleles observed across all breeds: 1780
• Alleles per breed ranged from 399 to 805 (average: 605)
• Breeds with smaller populations had approximately 6% fewer alleles
• Older breeds had approximately 7% fewer alleles than newer breeds

Case study: marine worm genetic diversity

The following table shows genetic variation at five gene loci in the marine horseshoe worm (Phoronopsis viridis). The table displays allele frequencies and indicates whether each locus is polymorphic at two different thresholds (0.95 and 0.99).

Gene locus	1 allele	2 alleles	3 alleles	4 alleles	5 alleles	6 alleles	Polymorphic at 0.95	Polymorphic at 0.99
Acph-1	0.995	0.005	—	—	—	—	✗	✗
Acph-2	0.882	0.066	0.024	0.014	0.009	0.005	✓	✓
Est-6	0.979	0.012	0.010	—	—	—	✗	✓
Fum	0.986	0.014	—	—	—	—	✗	✓
Lap-5	0.551	0.326	0.119	0.004	—	—	✓	✓

Beyond simple polymorphism

The genetic code is degenerate – multiple base triplets (codons) can code for the same amino acid. This creates an additional level of genetic variation that does not affect protein function. Modern biodiversity assessment increasingly focuses on DNA sequencing, examining specific sequences in nuclear DNA and mitochondrial DNA (mtDNA) to reveal the full extent of genetic variation.

Habitat diversity

Defining habitats and ecosystems

Three related terms describe where organisms live:

Biome – a region of the world characterised by a dominant type of vegetation associated with climate conditions (e.g., tropical rainforest, temperate grassland, tundra)

Ecosystem – an area supporting communities of organisms that interact with each other and their surroundings. Ecosystems can be large (like open ocean, Earth's dominant ecosystem) or small (like rock pools on a shore)

Habitat – a place where a species lives, described in one or a few words (e.g., pond, lake, river, coral reef, forest, grassland, sand dune, heathland). Habitats provide the resources organisms need: light, carbon dioxide, water and mineral ions for plants; food, water, shelter and breeding sites for animals

The niche concept

A niche describes an organism's role within an ecosystem, including its position in food webs and all interactions with environmental factors. The niche concept helps measure habitat diversity because more species indicate more available niches and therefore greater habitat complexity.

Challenges in measuring habitat diversity

Habitat diversity proves harder to quantify than species or genetic diversity because habitat boundaries are often unclear. Many species use multiple ecosystems:

• Seabirds (gannets, guillemots, puffins) feed in marine ecosystems but roost on coastal cliffs
• Migratory birds spend part of the year in one ecosystem and breed in another

Local biodiversity

Creating species lists

When investigating an area's biodiversity, the first task involves identifying and cataloguing all organisms present to create a species list. Biologists use identification keys – which may include drawings, photographs, or written descriptions – to identify specimens. Modern keys are often available as mobile applications for field use.

A timed search provides one method for generating species lists. Team members spread across the study area and record as many different species as possible within a set time (e.g., 20 minutes). The team then pools results, photographs unidentified species for later identification, and labels species that remain unidentified as "species A", "species B", etc.

Species richness

Species richness indicates how many species occur in an area. However, a single survey provides limited information:

• Surveys conducted during one season miss species active at other times
• Daytime surveys miss nocturnal species
• Surface surveys miss soil organisms
• Standard surveys may overlook bacteria and fungi, which require specialised culturing techniques