Biodiversity and Species Richness

Biodiversity is a measure of the variety of life on Earth. It encompasses the diversity within species, between species, and of ecosystems. Understanding biodiversity — what it is, why it matters, and how it varies — is central to ecology and conservation biology.

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What Is Biodiversity?

Biodiversity (biological diversity) can be defined at three levels:

Level	Definition	Example
Genetic diversity	The variety of alleles (gene variants) within a species or population	Different alleles for coat colour in a population of mice
Species diversity	The variety of species in an area, encompassing both the number of species and their relative abundance	A tropical rainforest containing thousands of plant, animal, fungal and microbial species
Ecosystem diversity	The variety of different ecosystems (habitats) in a region	A landscape containing forests, wetlands, grasslands and rivers

All three levels are interconnected. High genetic diversity within species contributes to species diversity, and diverse ecosystems support greater species diversity.

Exam Tip: When defining biodiversity, always mention all three levels — genetic, species and ecosystem. Many students lose marks by only referring to "the number of species," which is an incomplete definition.

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Species Richness

Definition

Species richness is simply the number of different species present in an area. It is the most straightforward component of biodiversity but does not take into account how many individuals of each species are present.

Species Richness vs Species Diversity

It is crucial to distinguish between species richness and species diversity:

Measure	What It Counts	Includes Abundance?	Example
Species richness	Number of different species	No	An area has 20 species of bird
Species diversity	Number of species AND relative abundance (evenness)	Yes	An area has 20 species, each represented by roughly equal numbers (high diversity) vs one species dominating 95% of individuals (low diversity despite same richness)

Consider two woodland plots, each with 100 individual birds and 5 species:

Plot A:

Species	Number of Individuals
Robin	20
Blackbird	20
Blue tit	20
Wren	20
Chaffinch	20

Plot B:

Species	Number of Individuals
Robin	92
Blackbird	3
Blue tit	2
Wren	2
Chaffinch	1

Both plots have the same species richness (5 species). However, Plot A has much higher species diversity because the individuals are evenly distributed among species. Plot B is heavily dominated by one species, making it less diverse despite having the same richness.

Exam Tip: Examiners frequently test whether you understand the difference between species richness and species diversity. Always state that diversity includes BOTH the number of species AND the evenness of their abundance.

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Genetic Diversity

What Is Genetic Diversity?

Genetic diversity refers to the total number of genetic characteristics (alleles) in the genetic make-up of a species. Within a population, it is the variety of alleles present at each gene locus.

Why Genetic Diversity Matters

High genetic diversity is essential for the long-term survival of a species because:

1. Adaptation to environmental change — A genetically diverse population is more likely to contain individuals with alleles that confer resistance to new diseases, tolerance to changing climate conditions, or ability to exploit new food sources.

2. Resistance to disease — If all individuals are genetically identical, a single pathogen could wipe out the entire population. Genetic variation means some individuals are likely to be resistant.

3. Avoidance of inbreeding depression — In small populations with low genetic diversity, closely related individuals are forced to breed together. This increases the frequency of homozygous recessive genotypes, often resulting in reduced fitness (lower fertility, higher mortality, increased susceptibility to disease).

Factors That Reduce Genetic Diversity

Factor	Mechanism	Example
Genetic bottleneck	A dramatic reduction in population size (e.g., due to disease, habitat destruction, natural disaster) eliminates many alleles from the gene pool	Northern elephant seals were hunted to ~20 individuals in the 1890s; the current population of ~200,000 has very low genetic diversity
Founder effect	A small number of individuals colonise a new area; the new population has only a fraction of the original population's genetic diversity	Amish communities in the USA descend from a small number of founders, leading to high frequency of certain rare genetic conditions
Selective breeding	Humans select organisms with desirable traits, reducing the variety of alleles in the population	Modern crop varieties and pedigree dog breeds have much lower genetic diversity than their wild ancestors
Inbreeding	Mating between closely related individuals increases homozygosity	Captive cheetah populations have extremely low genetic diversity due to historical bottlenecks and inbreeding

Exam Tip: Genetic bottlenecks and the founder effect are frequently tested. Be prepared to explain how each reduces genetic diversity and give a named example.

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Factors Affecting Biodiversity

Biodiversity is not evenly distributed across the Earth. Many factors influence biodiversity levels:

Abiotic Factors

Factor	Effect on Biodiversity
Temperature	Higher temperatures generally support more species (more energy for photosynthesis, faster metabolic rates). Tropical regions have the highest biodiversity.
Rainfall/water availability	Water is essential for all life. Areas with reliable, abundant rainfall support more species. Deserts have low biodiversity.
Light intensity	Affects photosynthesis rates, which underpins food webs. Light availability varies with latitude, altitude and depth in water.
Soil type and nutrient availability	Nutrient-rich soils support more plant species, which in turn support more animal species.
Altitude	Biodiversity generally decreases with altitude due to lower temperatures, stronger winds, and thinner soils.
Latitude	Biodiversity is highest near the equator (tropics) and decreases towards the poles — this is the latitudinal diversity gradient.

Biotic Factors

Factor	Effect on Biodiversity
Competition	Intense competition can exclude weaker competitors, reducing species richness. However, niche partitioning allows coexistence.
Predation	Predators can increase biodiversity by preventing any single prey species from dominating (the "predator-mediated coexistence" hypothesis).
Mutualism	Mutualistic relationships (e.g., pollinators and flowering plants) can increase biodiversity by supporting the persistence of both partners.
Habitat complexity	More complex habitats (e.g., coral reefs, tropical forests) offer more niches, supporting more species.
Succession	Biodiversity typically increases during ecological succession as more species colonise and establish.

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The Latitudinal Diversity Gradient

One of the most striking patterns in ecology is that biodiversity is highest in the tropics and decreases towards the poles. This pattern holds for most groups of organisms (plants, animals, fungi, microbes) across terrestrial and marine environments.

Hypotheses Explaining the Gradient

Hypothesis	Explanation
Energy availability	Tropical regions receive more solar energy year-round, supporting higher productivity and more species
Climate stability	Tropical climates are relatively stable; species in stable environments have had more time to specialise and diversify
Area	Tropical biomes cover a larger total area than temperate or polar biomes, supporting more species (species-area relationship)
Evolutionary time	Tropical regions were less affected by ice ages, so species have had longer uninterrupted periods to evolve and diversify
Speciation rates	Higher temperatures may increase mutation rates and reduce generation times, accelerating speciation

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Endemism

An endemic species is one that is found naturally in only one specific geographic area and nowhere else on Earth. Islands, isolated mountains, and unique habitats often have high levels of endemism.

Examples of endemic species:

• Lemurs — found only in Madagascar
• Giant tortoises — Chelonoidis nigra is endemic to the Galapagos Islands
• Scottish crossbill (Loxia scotica) — the only bird species endemic to the UK

Endemic species are particularly vulnerable to extinction because:

• They have a restricted range — any threat to their habitat affects the entire species
• They often have small population sizes
• They may have low genetic diversity

Exam Tip: Endemism is an important concept linking biodiversity to conservation. If asked about conservation priorities, areas with high endemism (biodiversity hotspots) are often prioritised.

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Biodiversity Hotspots

A biodiversity hotspot is a region that has:

1. At least 1,500 endemic plant species (0.5% of the world total)
2. Lost at least 70% of its original habitat

There are currently 36 recognised biodiversity hotspots, covering just 2.5% of Earth's land surface but supporting more than 50% of all plant species and 43% of all vertebrate species as endemics.

Examples include:

• Tropical Andes — the richest biodiversity hotspot, with ~30,000 plant species (half endemic)
• Madagascar and Indian Ocean Islands — 90% of species are endemic
• Sundaland (Southeast Asia) — covering Borneo, Sumatra and surrounding islands
• Mediterranean Basin — rich in endemic plants despite extensive human modification

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Summary

Key Concept	Detail
Biodiversity	Variety of life at genetic, species and ecosystem levels
Species richness	Number of different species in an area
Species diversity	Richness plus evenness of abundance
Genetic diversity	Variety of alleles within a species
Bottleneck	Population crash reducing genetic diversity
Founder effect	Small colonising group has limited diversity
Latitudinal gradient	Biodiversity highest in tropics, decreasing towards poles
Endemism	Species found in only one geographic area
Biodiversity hotspot	Region with high endemism that has lost >70% of habitat

Exam Tip: Biodiversity questions often require extended answers. Structure your response around the three levels of biodiversity, explain why genetic diversity is important for species survival, and link human activities to changes in biodiversity.

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A-Level Deep Dive: Biodiversity and Species Richness

Spec mapping

The Edexcel 9BI0 specification places biodiversity within Topic 4: Biodiversity and Natural Resources, with substantial synoptic overlap into the previous lesson on phylogenetics and cladistics (the species concept used to count species is itself a phylogenetic question — most modern microbial diversity surveys use a sequence-similarity threshold for "species" rather than a morphological one), the previous lessons on classification and taxonomy and the five-kingdom-to-three-domain reorganisation (taxonomy supplies the unit — species — that biodiversity counts; the three-domain framework structures the count, with the bacterial and archaeal domains contributing the overwhelming majority of unrecognised diversity), the next lesson on measuring biodiversity (Simpson's index and the related indices operationalise the three-level definition into numbers), Topic 5: On the Wild Side (ecosystem function and the biodiversity–stability relationship link species and genetic diversity to net primary productivity, nutrient cycling and resilience to perturbation) and Topic 8: Genetics, Populations, Evolution and Ecosystems (within-species genetic diversity is quantified by allele and genotype frequencies under Hardy–Weinberg, and population-genetic methods supply the molecular tools — heterozygosity, $F_{ST}$FST​ — that turn the qualitative "genetic diversity" of this lesson into measurable quantities). The relevant statements concern: defining biodiversity at the genetic, species and ecosystem levels; distinguishing species richness (count) from species diversity (count plus evenness) and from abundance (number of individuals per species); explaining the importance of genetic diversity for adaptation, disease resistance and avoidance of inbreeding depression; and outlining factors that affect biodiversity (latitudinal gradient, climate, succession, endemism, biodiversity hotspots) (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

A field ecologist surveys two adjacent grassland plots, each $100\,\text{m}^2$100m2, for plant species. The number of individuals of each species recorded in each plot is shown below.

Species	Plot A (individuals)	Plot B (individuals)
Common bent	50	90
Yorkshire fog	30	5
Ribwort plantain	20	5
Total	100	100

(a) State the species richness of each plot, and explain why richness alone is an inadequate measure of biodiversity. (2)

(b) Calculate Simpson's index of diversity $D = 1 - \sum (n/N)^2$D=1−∑(n/N)2 for each plot, showing your working, and interpret the contrast between the two values. (4)

(c) Discuss two further dimensions of biodiversity that Simpson's index fails to capture, and outline how each could be measured. (2)

Solution with mark scheme:

(a) Step 1. Both plots have the same species richness — three species (common bent, Yorkshire fog, ribwort plantain).

M1 (AO1.1) — states richness $= 3$=3 for both plots.

M1 (AO3.1a) — explains the inadequacy: richness counts species but ignores their relative abundance (evenness). Plot A has individuals distributed $50/30/20$50/30/20 across the three species, which is much more even than Plot B's $90/5/5$90/5/5 skew toward common bent. A measure that is identical for both plots is missing a real biological difference — the dominance structure of the community. Examiner reward language: "richness ignores evenness, so two communities with very different dominance structures appear equivalent".

(b) Step 1 — apply Simpson's formula plot by plot.

Plot A: $\sum (n/N)^2 = (50/100)^2 + (30/100)^2 + (20/100)^2 = 0.25 + 0.09 + 0.04 = 0.38$∑(n/N)2=(50/100)2+(30/100)2+(20/100)2=0.25+0.09+0.04=0.38, so $D_A = 1 - 0.38 = 0.62$DA​=1−0.38=0.62.

Plot B: $\sum (n/N)^2 = (90/100)^2 + (5/100)^2 + (5/100)^2 = 0.81 + 0.0025 + 0.0025 = 0.815$∑(n/N)2=(90/100)2+(5/100)2+(5/100)2=0.81+0.0025+0.0025=0.815, so $D_B = 1 - 0.815 = 0.185$DB​=1−0.815=0.185.

M1 (AO2.1) — Plot A working and answer to two decimal places ($D_A = 0.62$DA​=0.62).

M1 (AO2.1) — Plot B working and answer to two decimal places ($D_B = 0.19$DB​=0.19, accept $0.185$0.185).

M1 (AO3.1a) — interprets the contrast: a higher $D$D indicates higher diversity. Plot A's $D = 0.62$D=0.62 is more than three times Plot B's $D = 0.19$D=0.19, despite identical richness, because Plot A's evenness is much greater. Simpson's index is dominated by the common species — a single species at $90\%$90% contributes $0.81$0.81 to $\sum (n/N)^2$∑(n/N)2, which alone almost forces a low $D$D regardless of how the rest of the community is structured.

A1 (AO3.2a) — concludes by linking the calculation to the ecological meaning: $D$D is the probability that two individuals drawn at random belong to different species. In Plot A this is about $62\%$62%; in Plot B it is about $19\%$19%. The single number captures both richness and evenness in a way that richness alone cannot.

(c) Step 1 — name two further dimensions and one measurement each.

M1 (AO3.1a) — first dimension: genetic diversity within species. Simpson's index treats every individual of "common bent" as equivalent, but the population of common bent in Plot A may carry many more alleles at most loci than the population in Plot B. Measurement: expected heterozygosity $H_e = 1 - \sum p_i^2$He​=1−∑pi2​ at SNP loci across the genome, computed from population-level allele frequencies, gives the within-species analogue of Simpson's index.

M1 (AO3.1a) — second dimension: phylogenetic diversity (evolutionary distinctness). Simpson treats common bent and Yorkshire fog as equivalent "species" units, but both are grasses (family Poaceae); ribwort plantain (family Plantaginaceae) is far more distantly related. A community of three closely related grasses has lower phylogenetic diversity than a community of three species spanning three plant families. Measurement: Faith's phylogenetic diversity (PD) — the total branch length of a phylogenetic tree connecting all species in the sample — weights species by how much unique evolutionary history they represent.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Define biodiversity and explain why a single species count is an incomplete description of it. Use a worked numerical contrast to support your answer.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	Defines biodiversity at three levels — genetic (variety of alleles within a species), species (variety of species in a community, including richness and evenness), ecosystem (variety of habitats and ecological processes in a region).
2	AO1.2	States that species richness is the count of species in a sample and species diversity combines richness with evenness — the relative abundance of each species.
3	AO2.1	Applies a contrast: two communities of three species each ($50/30/20$50/30/20 vs $90/5/5$90/5/5) have identical richness but different evenness; Simpson's index $D = 1 - \sum (n/N)^2$D=1−∑(n/N)2 separates them ($D = 0.62$D=0.62 vs $D = 0.19$D=0.19).
4	AO2.1	Applies the level distinction: a single-species count says nothing about within-species genetic diversity (allele richness), nor about the variety of habitats that supports the species pool.
5	AO3.1a	Evaluates: a single number always loses information; richness loses evenness, Simpson loses phylogenetic distinctness, all species-level metrics lose ecosystem-level variety. The "right" measure depends on the question being asked.
6	AO3.2a	Concludes that biodiversity is a multidimensional construct — at minimum genetic, species and ecosystem levels — and that conservation prioritisation often weights phylogenetic distinctness or endemism in addition to raw richness.

Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. This is a typical Section B "define and evaluate" question — Edexcel rewards candidates who use a numerical contrast to justify the definitional point (AO2 + AO3) rather than merely listing the three levels (AO1).

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Synoptic links

1. Lesson 1 — Principles of Classification and Taxonomy supplies the unit of the count: the species. Without an operational species concept, "number of species" is undefined. The biological species concept (interbreeding populations producing fertile offspring) works for vertebrates but not for asexual microbes; the morphospecies concept fails for cryptic species; the phylogenetic species concept (smallest monophyletic group with diagnostic synapomorphies) is the workhorse for modern microbial diversity surveys.
2. Lesson 2 — The Five Kingdoms and Three Domains structures the count by domain. Macroscopic biodiversity (animals, plants, fungi) sits in the eukaryotic domain; bacterial and archaeal diversity is enormously larger and almost entirely uncharacterised — most current estimates put bacterial "species" (here defined by 97–99% 16S rRNA similarity) in the millions to billions, dwarfing the ~1.8 million described eukaryotic species.
3. Lesson 3 — Phylogenetics and Cladistics supplies the species concept used in microbial biodiversity (sequence-similarity thresholds) and the framework for phylogenetic diversity (Faith's PD), which weights species by branch length on a phylogenetic tree rather than treating each species as equivalent.
4. Lesson 5 — Measuring Biodiversity quantifies the qualitative ideas of this lesson. Simpson's index of diversity $D = 1 - \sum (n/N)^2$D=1−∑(n/N)2 combines richness and evenness into a single number; Shannon's index $H' = -\sum p_i \ln p_i$H′=−∑pi​lnpi​ is its information-theoretic relative; Pielou's evenness $J = H' / \ln S$J=H′/lnS extracts the evenness component alone.
5. Topic 5 — On the Wild Side develops the biodiversity–ecosystem-function relationship: more diverse communities tend to have higher productivity, more efficient nutrient use and greater resilience to perturbation. The "insurance hypothesis" — that diverse communities buffer ecosystem function against disturbance — is the mechanistic content of the conservation case.
6. Topic 6 — Immunity, Infection and Forensics connects through microbial biodiversity. The human gut microbiome contains $10^{13}$1013–$10^{14}$1014 bacterial cells from hundreds of species; reduced gut microbial diversity is associated with inflammatory bowel disease, obesity and Clostridioides difficile infection.
7. Topic 8 — Genetics, Populations, Evolution and Ecosystems supplies the population-genetic machinery for measuring within-species genetic diversity. Hardy–Weinberg equilibrium gives expected allele and genotype frequencies under no selection, no mutation, no migration and random mating; expected heterozygosity $H_e = 1 - \sum p_i^2$He​=1−∑pi2​ is the within-species analogue of Simpson's index; $F_{ST}$FST​ partitions genetic diversity into within- and between-population components, the genetic-diversity analogue of beta diversity.

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Mark-scheme literacy

AO	Typical share on this topic	Earned by
AO1 (knowledge)	35–45%	Defining biodiversity at three levels (genetic, species, ecosystem); distinguishing richness, abundance and evenness; recalling causes of low genetic diversity (bottleneck, founder effect, inbreeding); naming the latitudinal gradient and biodiversity hotspots
AO2 (application)	35–45%	Calculating Simpson's index from a frequency table; identifying which of two communities is more diverse and justifying with the calculation; placing a named species into the endemic / hotspot framework; predicting the effect of habitat fragmentation on genetic diversity
AO3 (analysis / evaluation)	15–25%	Evaluating richness as a single measure; distinguishing evenness from richness in worked cases; explaining why phylogenetic and genetic diversity supplement species diversity; discussing why mass extinction events and current human pressures drive biodiversity loss