18. Classification, Biodiversity and Conservation

A species concept is a rule for deciding which organisms should be grouped as one species. Different concepts answer different questions, so classification must match the evidence available.

Biological species concept — asks whether organisms with similar morphological and physiological features can interbreed and produce fertile offspring. Its boundary is reproductive evidence, but testing fertility can be difficult and time-consuming; it is also limited when organisms cannot be tested directly.

Morphological species concept — asks whether organisms share physical features that distinguish them from other groups. It is practical when morphology is observable, but appearance alone may not reveal reproductive or ecological differences.

Ecological species concept — groups similar organisms living in the same area at the same time. Its boundary is ecological context, so it should not be treated as a fertility test or as a purely visual definition.

Use the evidence in this order:

1. Identify whether the question gives reproductive, morphological or ecological evidence.
2. Match that evidence to the corresponding concept.
3. State the conclusion with its boundary: a concept supports a classification decision, but no single concept works universally.
4. Do not infer a new species from one visible difference alone when the relevant reproductive or ecological evidence is missing.

Phylogenetic definitions are not added here because the matched SME page supports biological, morphological and ecological concepts, not a separate phylogenetic treatment. Do not move from this card into domains, kingdoms or viruses.

The three domains are Bacteria, Archaea and Eukarya. Domain is the highest taxonomic rank, and grouping organisms into domains helps represent deep evolutionary divisions.

Bacteria and Archaea contain prokaryotic cells: they lack a nucleus. Eukarya contains eukaryotic cells: genetic material is enclosed in a nucleus and the cell has compartmentalised structures, including membrane-bound organelles.

Cell type is an important first boundary, but it is not enough to classify all prokaryotes. Molecular evidence, especially comparisons of RNA genes, supports separating the two prokaryotic domains and recognising their deeper relationships.

Use the classification in this order:

1. Ask whether the cell is prokaryotic or eukaryotic.
2. If prokaryotic, keep Bacteria and Archaea as separate possible domains rather than treating “prokaryote” as a domain.
3. Use molecular evidence alongside cell organisation before assigning the domain.
4. State the result as Bacteria, Archaea or Eukarya; detailed Archaea–Bacteria differences and lower Eukaryotic kingdoms are separate questions.

A simple cell shape cannot by itself identify the domain. “Prokaryote” describes cell organisation, whereas Bacteria and Archaea are separate domains; Eukarya is the eukaryotic domain. Viruses are outside this cellular three-domain classification.

Archaea and Bacteria are both domains of unicellular prokaryotes, so both lack a nucleus. They are nevertheless separate domains because molecular and structural evidence shows important differences.

Shared boundary — both have prokaryotic cells and no nucleus; a bacterial-looking shape or a similar size does not identify the domain.
Membrane evidence — archaeal membrane lipids are distinctive, whereas bacterial membrane lipids have a different structure.
Cell-wall evidence — bacterial cell walls contain peptidoglycan; archaeal cell walls do not contain peptidoglycan.
Genetic evidence — both have 70S ribosomes, but archaeal ribosomal RNA and ribosomal structure show greater similarity to Eukarya than to Bacteria.

Use the evidence in this order:

1. Confirm the organism is prokaryotic; this places it in the Bacteria-or-Archaea branch, not automatically in Bacteria.
2. Compare membrane-lipid, cell-wall and ribosomal-RNA evidence.
3. Assign the domain from the combined evidence rather than from habitat, shape or the fact that the cell lacks a nucleus.
4. Treat extreme-environment survival as a possible observation, not as the definition of Archaea; not all Archaea are extremophiles.

“Both lack a nucleus” does not mean Archaea and Bacteria are the same domain. This card stops at their comparison; Eukarya nested taxonomy, kingdoms and viruses belong to later cards.

Taxonomy places similar organisms into nested ranks. Within Eukarya, the broad domain contains kingdoms, and each lower rank contains a smaller, more similar group.

Read the hierarchy from broad to specific:
Domain: Eukarya
→ Kingdom
→ Phylum
→ Class
→ Order
→ Family
→ Genus
→ Species

At a higher rank, the group contains more organisms with less similarity between them. Moving down the nested hierarchy applies more shared classification evidence, so the group contains fewer organisms with greater similarity. Species is the lowest rank in this sequence.

Use the hierarchy in this order:

1. Confirm the organism belongs to Eukarya.
2. Place it in a kingdom, then progressively narrower phylum, class, order, family, genus and species groups.
3. Treat each lower group as nested within the preceding one, not as an unrelated label.
4. Use the evidence appropriate to the rank; do not substitute a kingdom name for a species identification.

Domain is the highest rank, while species is the lowest in this hierarchy. The card establishes nested classification; detailed kingdom features and virus classification are separate cards.

The main Eukarya kingdoms are Protoctista, Fungi, Plantae and Animalia. Kingdom classification combines cell structure, body organisation and nutrition; one visible feature is rarely enough.

Protoctista — a broad group of eukaryotic organisms outside the other three kingdoms; members vary widely in cell structure, feeding and body form.
Fungi — eukaryotic heterotrophs with non-cellulose cell walls; they obtain nutrients by extracellular digestion and absorption, including from decaying material or living hosts.
Plantae — multicellular eukaryotes with cellulose cell walls, large vacuoles and usually chloroplasts; they are autotrophs and form complex body systems.
Animalia — multicellular eukaryotes with no cell walls; they are heterotrophs with varied feeding mechanisms and specialised cells that can form tissues and organs.

Classify from combined evidence:

1. Confirm the organism is eukaryotic.
2. Record cell-wall and organelle features, body organisation and the way nutrition is obtained.
3. Compare the whole feature pattern with the four kingdom profiles.
4. State the best-supported kingdom while keeping exceptions and the broad Protoctista boundary in view; do not infer a kingdom from multicellularity or one cell feature alone.

Plants and fungi are not defined by being multicellular, and fungi are not autotrophs. The kingdom profiles are evidence-based teaching groupings; this card does not extend into virus nucleic-acid classification.

Viruses are acellular and have no metabolism of their own, so they depend on host cells for replication. They are therefore outside the cellular three-domain and Eukaryotic-kingdom classifications.

Classify a virus by two genome features:
Nucleic-acid type — DNA or RNA.
Strand form — single-stranded or double-stranded.
These give four broad combinations: single-stranded DNA, double-stranded DNA, single-stranded RNA and double-stranded RNA.

Use the classification in this order:

1. Identify whether the viral genome is DNA or RNA.
2. Determine whether that genome is single-stranded or double-stranded.
3. Report the combination as the virus classification; do not use the host organism to replace the genome evidence.
4. Keep the boundary clear: this is a nucleic-acid classification of an acellular, host-dependent entity, not a domain or kingdom assignment.

A virus is not placed in Bacteria, Archaea, Eukarya or one of the four Eukarya kingdoms simply because of the cell it infects. This card stops at genome-based classification and does not add named virus examples or replication-cycle detail.

An ecosystem is the interacting system of a community of organisms and the environment they live in. It includes biotic components, abiotic conditions, energy flow and nutrient cycling. A habitat is where a species lives; its niche is the role it plays there.

Ecosystem — a system scale: populations interact with one another and with physical and chemical conditions.
Habitat — a place boundary: where the organism is found.
Niche — a role-and-resource boundary: how the organism obtains energy, uses conditions and interacts with other species and the physical environment.

Niche overlap means two species use some of the same resources or conditions. The more their roles and resource use overlap, the greater the potential for competition; distinct resource use can allow different species to fit into the same ecosystem. This is a relationship-level interpretation, not a biodiversity count or an automatic proof of competition.

Use the terms in this order:

1. Name the ecosystem as the interacting living-and-non-living system.
2. Identify the habitat as the place occupied by a species.
3. Describe the niche through energy use, physical conditions and interactions.
4. Compare niches when explaining overlap, possible competition or how multiple species can occupy an ecosystem; keep measurement of biodiversity for a separate assessment.

A habitat is not a niche: location alone does not describe a species’ role. Ecosystem, habitat and niche are concepts rather than sampling methods; this card does not introduce quadrats, correlation statistics or Simpson’s index.

Biodiversity describes the range and variety of genes, species and habitats within a region. It can be assessed at three linked levels.

• Ecosystem/habitat diversity — the number and range of different ecosystems or habitats in the area.
• Species diversity — the number of different species and how evenly individuals are distributed among them.
• Genetic diversity — the variety of genes and alleles within each species, including differences between populations of the same species.

Species richness is only the number of species. Species diversity also considers evenness: an area can contain many species but still have lower species diversity if most individuals belong to one or two species. Keep these terms separate when interpreting evidence.

Biodiversity can support ecosystem resilience because variation provides more ways for populations and ecosystems to respond to environmental change. This is a general ecological relationship, not a guarantee that every diverse ecosystem resists every disturbance.

Biodiversity is broader than a species count: ecosystem/habitat, species and genetic levels answer different questions. This card defines the assessment levels; random sampling, field methods, correlation tests and Simpson’s index are separate cards.

Random sampling chooses sampling points by chance so the investigator does not deliberately favour particular parts of the study area. It is most suitable when the area is reasonably uniform and has no clear distribution pattern.

1. Define the whole study area and the population or community to be sampled.
2. Generate random coordinates or point locations across that area.
3. Place the same sampling unit, such as a quadrat where appropriate, at each selected point.
4. Use the same sampling rules and record the observations from every point.
5. Repeat across enough independent points, then judge whether the sample represents the whole area.

Because the locations are selected by chance, random sampling reduces selection bias from the person carrying out the investigation. Standardising the sampling unit and recording rule makes comparisons between points fairer; repeated points improve confidence in the estimate.

Systematic sampling places points by a planned pattern chosen by the investigator, so the chosen pattern or starting point can miss parts of the area or introduce bias. Do not treat random sampling as automatically best for a strongly patterned or clearly non-uniform area.

Random sampling is a method for choosing representative sample locations. It does not by itself prove that a sample is large enough or replace later analysis of abundance, distribution, correlation or Simpson’s index.

Choose the field method that matches the evidence needed: a transect shows how distribution changes across a gradient, while quadrats record what is present within a standard area.

1. Define the area, species and physical gradient or comparison being investigated.
2. For distribution along a gradient, lay a line transect across the area and record organisms touching the line at fixed intervals; this gives qualitative presence data.
3. For abundance within areas, place the same-sized quadrat at random or at regular transect intervals, then record each species in every quadrat.
4. Use frequency for how often a species occurs in the quadrats; use density for individuals per unit area, or percentage cover when individuals are difficult to count.
5. Repeat across multiple points, keep quadrat size, interval, observer rule and recording time consistent, and present the data against position or sample area.

A belt transect combines a line across the gradient with quadrats at regular intervals, so it can show how abundance changes quantitatively. A line transect alone records which organisms meet the line and does not provide the same abundance measure.

Randomly choosing quadrat locations reduces selection bias when estimating an area-wide value. A planned interval is useful for a clear environmental gradient, but the starting point and interval should be stated. Use the same area and sampling rules so differences are not caused by inconsistent effort.

Method choice and recording units come before statistical interpretation. Do not treat a single transect or quadrat as the whole ecosystem, and do not move this card into Pearson’s or Spearman’s correlation calculations or Simpson’s index.

Pearson’s linear correlation tests whether two quantitative variables show a linear relationship. The coefficient r ranges from -1 to +1: a value near +1 indicates a strong positive relationship, a value near -1 a strong negative relationship, and a value near 0 little or no linear correlation.

1. Pair each reading of variable x with the reading of variable y from the same sample or quadrat.
2. Plot the paired values on a scatter graph and check that a linear pattern is plausible.
3. State the null hypothesis that there is no linear correlation between the variables.
4. Check the data assumptions: both variables are quantitative, the relationship is approximately linear, and the data show a normal distribution.
5. Calculate the means, products and standard deviations required by the supplied Pearson equation, then substitute the values to obtain r.
6. Compare the result with the appropriate significance criterion or critical value for the sample, then state the strength and direction of the correlation and whether the null hypothesis is rejected.

Interpret the statistic with the scatter graph and the sample size, not from a single point. A statistically significant correlation supports an association in the sampled data under the test assumptions; it does not show that one variable causes the other.

Correlation is not causation: a third factor, sampling pattern or coincidence may explain an association. Pearson’s method is for the stated quantitative, approximately linear and normally distributed case; Spearman’s rank correlation and Simpson’s index are separate cards.

Spearman’s rank correlation tests for association between two variables using their ranks. It is useful when the data are not quantitative, are not normally distributed, or show a non-linear but monotonic relationship.

1. Pair the two observations from each sample, then state the null hypothesis that there is no correlation.
2. Rank each variable separately, using the course convention for tied values when ties occur.
3. For every pair, calculate the difference in rank, d, then calculate d²; add the d² values to obtain Σd².
4. Substitute Σd² and the number of samples n into the supplied Spearman equation to calculate rₛ.
5. Determine the direction from the sign and the strength from how close rₛ is to +1 or -1; values near 0 indicate little rank association.
6. Compare the calculated value with the critical value for n and the stated probability level, then reject or retain the null hypothesis with a conclusion in context.

The test evaluates a monotonic rank relationship: as one variable increases in rank, the other tends to increase or decrease in rank. It does not require a straight-line relationship, but a non-monotonic pattern is not captured reliably by one coefficient.

A significant rank correlation is evidence of association in the sampled data, not proof that one variable causes the other. A third factor, sampling pattern or coincidence may explain the relationship. This is distinct from Pearson’s linear correlation and Simpson’s index.

Extinction is the end of a species. Environmental change and human activity can reduce survival and reproduction until a population becomes unable to persist; several pressures may act together.

• Climate or environmental change — changed temperature, sea level, ocean conditions or ice cover can make a habitat unsuitable and reduce access to food.
• Competition — limited food, water, habitat or mates can reduce population size; competition may be within or between species.
• Hunting and overexploitation — removing too many individuals, especially from a vulnerable population, can leave too few to survive.
• Habitat degradation or loss — destruction, pollution or reduction of the habitat removes the resources and conditions on which the species depends.
• Introduced invasive species — a non-native species may lack natural predators, competitors or pathogens, increase rapidly, and then reduce native populations through competition or disease.

Use the causal chain: direct pressure → reduced resources, survival or reproduction → smaller population → increased extinction risk. Habitat loss can also force individuals into the remaining area, increasing competition. A small population may then experience more inbreeding and genetic drift, reducing genetic variation and fitness; this feedback can increase mortality and reduce reproduction further.

Do not treat one pressure as necessary in every extinction event, or confuse a falling population with immediate extinction. This card explains causes and the small-population vulnerability feedback; conservation methods, IUCN status and assisted reproduction are separate cards.

Maintaining biodiversity protects ecological relationships and preserves options for people and other species. The reasons overlap: ecosystems, species and genes support one another, so loss at one level can reduce benefits at others.

• Ecological stability and resilience — more diverse ecosystems may contain species able to tolerate environmental change, while loss of a keystone species can disrupt many other organisms.
• Environmental services — plants, fungi, bacteria and other organisms contribute to carbon dioxide absorption, the water cycle, decomposition, nutrient cycling and food webs.
• Genetic resources and future value — wild relatives can provide alleles that help crops resist disease or other disasters, and organisms may contain medicines or useful materials that have not yet been discovered.
• Economic value — biodiversity supports medicines, ecotourism, employment, science and technology.
• Social, cultural and aesthetic value — natural environments provide recreation, inspiration and opportunities for people to connect and learn.
• Moral and ethical value — people may regard humans as responsible for reducing human-caused loss and valuing other species.

These reasons are interdependent rather than isolated: genetic and species diversity can support ecosystem resilience; functioning ecosystems provide services and resources; and those benefits affect human wellbeing and future choices. The evidence supports a general reason to maintain biodiversity, not a guarantee that every diverse ecosystem resists every disturbance.

Do not reduce conservation value to economic usefulness alone, and do not assume that an ecosystem service or future genetic resource is certain or unlimited. This card explains why biodiversity matters; conservation methods, assisted reproduction, invasive-species control and IUCN/CITES tools are separate cards.

In situ conservation protects a species in its natural habitat. Ex situ conservation protects organisms or their genetic material outside the original habitat when in situ protection is not sufficient or possible.

• Protected areas (in situ) — national and marine parks protect habitats and species through controls on access, development, hunting, overfishing or pollution; the natural support systems remain available.
• Zoos and botanic gardens (ex situ) — captive populations support conservation, research and education, and may provide material for future reintroduction; suitable conditions and genetic diversity must be maintained.
• Frozen zoos and gene banks (ex situ) — animal eggs, sperm or tissues can be stored at very low temperature to preserve genetic material from different individuals.
• Seed banks (ex situ) — dried seeds are stored under controlled conditions to conserve plant genetic diversity and allow future regrowth; some plants require continued growth or tissue culture instead of freezing.

Choose the method to match the threat and biological need. In situ protection maintains the whole habitat but depends on effective management and community acceptance. Ex situ storage or captivity can protect a small or isolated population, but it costs resources, may not reproduce the natural habitat, and small captive populations can lose genetic diversity. Long-term success also depends on a suitable habitat or use for the conserved material.

In situ and ex situ methods are complementary, not interchangeable. This card compares conservation settings and genetic-resource storage; assisted-reproduction procedures, invasive-species control and IUCN/CITES roles are separate cards.

Endangered mammals may have small, isolated populations, inbreeding and too few suitable reproductive mates. Assisted reproduction can move gametes or embryos between animals and give reproduction more controlled support.

1. Obtain eggs and sperm from suitable individuals while protecting the reproductive female; the source animals retain the species’ genetic identity.
2. In IVF, extract eggs, keep them briefly in culture, mix them with semen outside the body, and allow zygotes to develop into embryos.
3. Transfer an embryo to the mother or a suitable surrogate; embryo transfer can reduce pregnancy risk for a vulnerable female.
4. In surrogacy, a female carries an embryo to full term after her uterus has been prepared; artificial insemination or IVF can be used to create the embryo.
5. Offspring may be managed in captivity and, if the habitat and population conditions are suitable, used in a later reintroduction programme.

IVF confirms fertilisation outside the body. Embryo transfer moves an embryo after fertilisation, and surrogacy focuses on the female that carries it; a surrogate may be the same or a closely related species when compatibility permits.

Assisted reproduction can increase reproductive opportunities, but it does not by itself restore habitat, remove inbreeding risk or guarantee adaptation after release. Cost, welfare, genetic diversity, suitable habitat and reintroduction success remain constraints. General conservation settings, invasive-species control and IUCN/CITES roles are separate cards.

An invasive species is a species that has moved into an ecosystem where it was previously unknown and then disrupts the balance of the ecosystem. Most recorded introductions are linked to human transport, trade or deliberate release.

1. Identify the introduced species, the route by which it arrived and the native species or ecosystem processes affected.
2. Monitor its spread and population increase, together with competition, predation, disease and changes in biodiversity or productivity.
3. Prevent further introduction or spread by controlling the human pathways that transport species between ecosystems; deliberate biological-control introductions also require evidence because a non-native species may become invasive.
4. If control is considered, judge the intervention against reduction of the invasive population and recovery of native species, while checking non-target effects, ecosystem disruption, cost and ethical acceptability.
5. Continue long-term monitoring: a short-term fall in numbers is not the same as lasting recovery of the native community.

The risk chain is: few natural predators, competitors or pathogens → rapid population increase → competition for prey, nutrients, light or space, successful predation or introduction of disease → native-population decline and reduced biodiversity/productivity. Human health, travel and economic costs can also be affected.

Do not call every non-native species invasive, and do not assume that one control action is automatically effective or harmless. The matched SME page does not specify a physical, chemical or biological control protocol, so no named treatment, dose or non-target claim is added here. Assisted reproduction and IUCN/CITES roles are separate cards.

The IUCN assesses the conservation status of animal and plant species worldwide. Its classification uses evidence about population numbers, threats and risks to those populations; scientists use data and modelling to estimate the appropriate category, which is made public through the IUCN Red List.

Use the assessment chain:

1. Gather evidence about population size or trend, distribution, threats and risks, noting whether the evidence is sufficient.
2. Use the available data and modelling to assess the species against the IUCN classification system.
3. Record the resulting conservation-status category and communicate it through the public Red List.
4. Use the status assessment to identify species needing conservation attention and to support coordinated decisions about safeguarding them.
5. Continue collecting evidence and update the assessment as populations, threats or data quality change.

More severe population decline, stronger threats or greater risk can support a more urgent conservation-status assessment. A status category is therefore an evidence summary used to guide priorities; it is not itself a recovery programme or a guarantee that a species will be protected.

Do not invent a numerical threshold or treat one observation as a complete assessment. The SME page explicitly allows insufficient-data outcomes and states that scientists continually review and update species status, so scale, evidence quality and uncertainty matter. IUCN status assessment is distinct from CITES trade controls and from the habitat, invasive-species and assisted-reproduction methods in neighbouring cards.