Classification, biodiversity and conservation

• Three domains, not five kingdoms; sample at random; every formula is given, but the choice of test and the meaning of the answer are yours.
• Three domains from rRNA; fertile offspring; niche is a role, habitat is a place; random removes bias; N = n1n2/m2; size of the coefficient against the critical value, and correlation is not causation; higher D, higher diversity.

• 18.1.1 I can discuss — discuss the meaning of the term species, limited to the biological species concept, morphological species concept and ecological species concept
• 18.1.2 I can describe — describe the classification of organisms into three domains: Archaea, Bacteria and Eukarya
• 18.1.3 I can state — state that Archaea and Bacteria are prokaryotes and that there are differences between them, limited to differences in membrane lipids, ribosomal RNA and composition of cell walls
• 18.1.4 I can describe — describe the classification of organisms in the Eukarya domain into the taxonomic hierarchy of kingdom, phylum, class, order, family, genus and species
• 18.1.5 I can outline — outline the characteristic features of the kingdoms Protoctista, Fungi, Plantae and Animalia
• 18.1.6 I can outline — outline how viruses are classified, limited to the type of nucleic acid (RNA or DNA) and whether this is single stranded or double stranded
• 18.2.1 I can define — define the terms ecosystem and niche
• 18.2.2 I can explain — explain that biodiversity can be assessed at different levels, including: • the number and range of different ecosystems and habitats • the number of species and their relative abundance • the genetic variation within each species
• 18.2.3 I can explain — explain the importance of random sampling in determining the biodiversity of an area
• 18.2.4 I can describe — describe and use suitable methods to assess the distribution and abundance of organisms in an area, limited to frame quadrats, line transects, belt transects and mark-release-recapture using the Lincoln index (the formula for the Lincoln index will be provided, as shown in the Mathematical requirements)
• 18.2.5 I can use — use Spearman's rank correlation and Pearson's linear correlation to analyse the relationships between two variables, including how biotic and abiotic factors affect the distribution and abundance of species (the formulae for these correlations will be provided, as shown in the Mathematical requirements)
• 18.2.6 I can use — use Simpson's index of diversity (D) to calculate the biodiversity of an area, and state the significance of different values of D (the formula for Simpson's index of diversity will be provided, as shown in the Mathematical requirements)
• 18.3.1 I can explain — explain why populations and species can become extinct as a result of: • climate change • competition • hunting by humans • degradation and loss of habitats
• 18.3.2 I can outline — outline reasons for the need to maintain biodiversity
• 18.3.3 I can outline — outline the roles of zoos, botanic gardens, conserved areas (including national parks and marine parks), 'frozen zoos' and seed banks, in the conservation of endangered species
• 18.3.4 I can describe — describe methods of assisted reproduction used in the conservation of endangered mammals, limited to IVF, embryo transfer and surrogacy
• 18.3.5 I can explain — explain reasons for controlling invasive alien species
• 18.3.6 I can outline — outline the role in conservation of the International Union for Conservation of Nature (IUCN) and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)

The northern white rhino. The last two northern white rhinos are females unable to carry a pregnancy. Eggs collected from them have been fertilised by IVF with sperm from the frozen store of males now dead, and the embryos are intended for southern white rhino surrogate mothers — a closely related, more common subspecies. All three methods meet in one programme.

• “Archaea are a kind of bacteria, and all prokaryotes form one kingdom.” Correct Archaea are not bacteria: there are three domains, not five kingdoms. Prokaryotes form two domains, Archaea and Bacteria, which differ in membrane lipids, rRNA and cell walls; rRNA shows that Archaea are more closely related to Eukarya than to Bacteria.
• “Archaea have peptidoglycan cell walls, like all prokaryotes.” Correct Only Bacteria have peptidoglycan. Archaean walls contain no peptidoglycan; they are built of other polysaccharides or proteins.
• “A niche is where an organism lives.” Correct That is the habitat. The niche is the role of a species in its ecosystem: where it lives, what it feeds on, what feeds on it, and all its interactions with other organisms and abiotic factors.
• “Biodiversity is the number of species in an area.” Correct Number of species is richness, one part of one level. Biodiversity also includes their relative abundance, the range of ecosystems and habitats, and the genetic variation within each species.
• “A Simpson’s index of 0.2 means high diversity, because a low index means few species dominate.” Correct In the syllabus form D = 1 − Σ(n/N)², a higher D means higher diversity. D near 0: one or a few species dominate. D near 1: many species, evenly represented.
• “The correlation is significant, so light intensity causes the grass to grow better.” Correct Correlation does not show causation. A significant correlation means the relationship is unlikely to be due to chance; both variables may depend on a third factor, such as soil moisture.
• “I put my quadrats where the plants were, to get good data.” Correct That sample is biased. Place quadrats at random coordinates so every point has an equal chance of being sampled; the statistical tests assume it.
• “Viruses belong to the domain Bacteria because they are microscopic and cause disease.” Correct Viruses are non-cellular and sit outside the three domains. They are classified only by the type of nucleic acid (DNA or RNA) and whether it is single or double stranded.
• “A species is a group of organisms that look the same.” Repair That is only the morphological concept. Give the concept the question asks for; the biological one depends on interbreeding to produce fertile offspring and reproductive isolation.
• “Horses and donkeys are the same species because they can breed together.” Repair Their offspring, mules, are infertile. The biological concept needs fertile offspring, so they are separate species.
• “Archaea are a kind of bacteria that live in extreme places.” Repair Archaea are a separate domain, differing in membrane lipids, rRNA and cell walls, and more closely related to Eukarya. Many live in soil, oceans and guts; habitat is not what defines them.
• “Archaean membranes have ester bonds; bacterial ones have ether bonds.” Repair The other way round: Archaea ether (branched chains), Bacteria ester (fatty acids), as in eukaryotes.
• “Fungi are plants that cannot photosynthesise.” Repair Fungi are a separate kingdom: chitin walls, hyphae, heterotrophic with external digestion, glycogen storage.
• “Kingdom, phylum, order, class, family, genus, species.” Repair Class comes before order: kingdom, phylum, class, order, family, genus, species.
• “HIV is a DNA virus because it inserts DNA into the host.” Repair Classify by the nucleic acid in the virus particle: HIV contains single-stranded RNA.
• “Biodiversity is the number of species.” Repair It also includes their relative abundance, the range of ecosystems and habitats, and the genetic variation within species.
• “Random sampling makes the results more accurate.” Repair Be precise: random sampling removes bias, so the sample is representative of the area, and it is assumed by the statistical tests. How many quadrats you use is a separate question.
• “In the Lincoln index, n2 is the number of unmarked animals in the second sample.” Repair n2 is the whole second sample, marked and unmarked; m2 is the number of marked ones in it.
• “Use Pearson’s for percentage-cover estimates.” Repair Pearson’s needs normally distributed continuous data and a possibly linear relationship; ranked or non-normal data need Spearman’s.
• “rs = −0.81 is below the critical value of 0.648, so there is no correlation.” Repair Compare the size, ignoring the sign: 0.81 ≥ 0.648, so there is a significant negative correlation.
• “The correlation shows light causes the grass to grow.” Repair Correlation does not show causation; another factor that changes along the transect may be responsible.
• “A Simpson’s D of 0.2 is high diversity.” Repair D near 1 is high diversity; near 0, one species dominates.
• “Seed banks store seeds in liquid nitrogen at −196 °C.” Repair That is the frozen zoo (sperm, eggs, embryos, tissue). Seed banks dry seeds and store them at about −20 °C, testing viability by germinating samples.
• “In surrogacy the embryo comes from the surrogate’s own eggs.” Repair The surrogate is not the genetic mother; she only carries the embryo.
• “CITES bans all trade in endangered species.” Repair Appendix I bans commercial international trade; Appendix II allows trade with permits.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss and assess want both sides and a judgement. Suggest means the question is set in an unfamiliar context and any sound biological reasoning is creditable.
• The conclusion is part of the answer. Each concept fails for some organisms, so no single one can be used alone. Biologists combine them, and increasingly use DNA sequence data (17.3.2) to decide whether two populations are separate species. A discuss answer that lists three definitions and stops has not discussed.
• Explain, not list. The outcome verb is explain: for each level say what is counted and why it counts as diversity. “Genetic diversity” alone is a name; “the number of different alleles in the gene pool, which gives variation for selection to act on” is an explanation.
• Interleave with the chapters that use this one. Topic 19 (19.3.1) asks how genetically modified crops affect biodiversity: re-answer “what are the three levels of biodiversity?” when you reach it. Looking back, topic 17’s bottlenecks and speciation are the reasons genetic diversity and reproductive isolation matter here, and the t-test sequence of topic 17 is the one both correlation tests follow. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 18 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• There is no multiple-choice paper on A Level content. A structured question on topic 18 asks you to define ecosystem or niche, discuss the species concepts, state how Archaea and Bacteria differ, place an organism in a domain or kingdom from its features, explain a cause of extinction or a reason to control an invasive species, and outline what a zoo, a seed bank, the IUCN or CITES does.
• Tables of quadrat counts, capture numbers, transect readings and scatter graphs. The Lincoln index, Simpson’s index, Spearman’s and Pearson’s formulae are printed in the question, with the critical values; you choose the test, finish a partly completed table, compare with the critical value, and say what the result means. Density and percentage frequency you must recall.
• A belt transect across a gradient (independent variable: distance, giving a range of an abiotic factor; dependent variable: percentage cover or density) or random quadrats in two areas compared by Simpson’s D. Standardise quadrat size, time, weather and observer; replicate transects or quadrats; the main errors are the estimate of cover and conditions changing during the survey.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss and assess want both sides and a judgement. Suggest means the question is set in an unfamiliar context and any sound biological reasoning is creditable.