Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A species can be defined in more than one way: by the ability to interbreed to produce fertile offspring (biological), by shared physical features (morphological) or by a shared ecological niche (ecological). All cellular life is placed in three domains: Bacteria and Archaea, which are both prokaryotes but differ in their membrane lipids, ribosomal RNA and cell wall composition, and Eukarya. Eukaryotes are classified into a hierarchy of kingdom, phylum, class, order, family, genus and species, with four kingdoms: Protoctista, Fungi, Plantae and Animalia. Viruses are not in any domain; they are classified by whether their nucleic acid is DNA or RNA and whether it is single-stranded or double-stranded.
Producing offspring is not enough for the biological species concept: the offspring must be fertile. A horse and a donkey produce a mule, but the mule is infertile, so they are two species.
Archaea are not a kind of bacteria, even though both are prokaryotes. They are a separate domain with different membrane lipids, different rRNA and no peptidoglycan in their cell walls.
An ecosystem is a community of organisms and its physical environment, interacting; a niche is the role of a species in its ecosystem, including what it feeds on, where it lives and how it interacts with other organisms and the environment. Biodiversity can be assessed at three levels: the number and range of ecosystems and habitats, the number of species and their relative abundance, and the genetic variation within each species. Samples must be random to avoid bias. Frame quadrats placed at random coordinates give density, frequency or percentage cover of plants and slow-moving animals, and Simpson's index of diversity, D = 1 − Σ(n/N)2, combines richness and evenness on a scale of 0 to 1. Mobile animals are estimated by mark-release-recapture with the Lincoln index, N = (n1 × n2) ÷ m2.
Square each n/N before adding, then subtract the total from 1. Adding the n/N values first always gives 1, and forgetting the final subtraction gives a number that falls as diversity rises.
Put the recaptured marked number m2 on the bottom. If marked animals are more likely to be caught a second time (trap-happy), m2 is too large and the population is underestimated.
A line transect is a tape laid across a habitat, recording the organisms that touch it at regular intervals; a belt transect places quadrats at intervals along the line, giving abundance as well as presence. Measuring an abiotic factor (light, soil moisture, pH) or a biotic factor at each point lets you test for a correlation with a species' abundance. Spearman's rank correlation, rs = 1 − 6ΣD2 ÷ (n3 − n), works on ranks and suits data that are not normally distributed. Pearson's correlation coefficient r measures how close the points are to a straight line and needs both variables to be normally distributed. Both run from −1 to +1; the value is compared with a critical value at p = 0.05 to accept or reject the null hypothesis of no correlation. A correlation never proves cause.
A transect is systematic, not random sampling: it is used deliberately along a gradient. For estimating the overall diversity of a uniform area, use random quadrats instead.
A significant correlation does not show that one variable causes the other. Light might affect the plants, or both might depend on a third factor such as soil moisture or trampling.
Populations and species become extinct when climate change shifts conditions faster than they can move or adapt, when a competitor uses their resources more efficiently, when hunting by humans removes individuals faster than they reproduce, and when habitats are degraded or lost. Small populations also lose genetic diversity. Biodiversity is worth maintaining for ecosystem stability and services, for food, medicines and genes, for economic value such as ecotourism, and for ethical reasons. Conservation protects species in national and marine parks, and away from their habitat in zoos, botanic gardens, frozen zoos and seed banks. Rare mammals can be bred by IVF, embryo transfer and surrogacy. Invasive alien species are controlled to protect native species, and internationally the IUCN assesses threat (the Red List) while CITES regulates trade.
Do not just name the threat: explain the effect on the population. For example, habitat loss reduces the food and breeding sites available, so the carrying capacity falls and fewer individuals survive and reproduce.
Seed banks and frozen zoos do not stop a species declining in the wild on their own. They store genetic diversity; the habitat must still be protected for any reintroduced organisms to survive.