C4.1 Populations and communitiesIB Biology HL: Revision notes

A population is a group of organisms of the same species living in an area that normally interbreed. Reproductive isolation distinguishes one population from another.

Counting every individual is usually impossible, so population size is estimated from random samples. Random sampling avoids bias, but any estimate differs from the true size: this difference is sampling error.

• Sessile organisms (plants, barnacles): random quadrat sampling. The standard deviation of the mean number per quadrat shows how variable the counts are and how evenly the population is spread.
• Motile organisms: capture–mark–release–recapture, using the Lincoln index:

Population size = $M \times N \div R$, where M = number first caught and marked, N = total caught in the second sample, R = number of marked individuals recaptured.

Assumptions: marks are not lost and do not affect survival; marked individuals mix randomly; no births, deaths or migration between samples; all individuals equally likely to be caught.

Carrying capacity is the maximum population size an environment can support sustainably. Limiting resources include food, water, light, space and nest sites.

Density-independent factors (weather, fire) cause fluctuations whatever the density. Density-dependent factors act more strongly as density rises — competition for resources, increased predation, and faster spread of pathogens and pests — and push the population back towards carrying capacity by negative feedback.

Sigmoid growth: an exponential phase (plentiful resources, every individual reproduces), a transitional phase as density-dependent factors act, then a plateau at carrying capacity. A lag phase is not expected. Exponential growth gives a straight line when log population is plotted against time. Yeast and duckweed are good organisms for collecting data.

NOS: the curve is an idealised model — a simplification of a complex system.

Members of the same species compete for limited resources such as food, territories, nest sites and mates — for example, male red deer compete for females, and trees in a dense stand compete for light.

They also cooperate: honeybees divide labour within the hive, wolves hunt in packs, and emperor penguins huddle to reduce heat loss.

A community is all the interacting populations in an area: plants, animals, fungi and bacteria.

• Herbivory: animal feeds on a plant (e.g. rabbit eats grass).
• Predation: animal kills and eats another animal (e.g. lynx and snowshoe hare).
• Interspecific competition: two species use the same limited resource (e.g. red and grey squirrels).
• Mutualism: both benefit.
• Parasitism: parasite benefits by living on or in a host that is harmed (e.g. ticks on deer).
• Pathogenicity: a pathogen causes disease in its host (e.g. chytrid fungus in frogs).

Mutualism examples:

• Root nodules in Fabaceae (legumes): Rhizobium bacteria fix nitrogen for the plant; the plant gives them sugars and a protected habitat.
• Mycorrhizae in Orchidaceae: fungi supply minerals, water and (in seedlings) carbon compounds; the adult orchid supplies sugars.
• Zooxanthellae in hard corals: algae give carbon compounds from photosynthesis; the coral gives shelter, CO₂ and nitrogenous waste.

An invasive species is an introduced species that spreads because it has a competitive advantage over endemic species in obtaining resources — e.g. the grey squirrel in the UK out-competes the native red squirrel for food because it can digest acorns more efficiently.

Interspecific competition is indicated but not proven if one species is more successful when the other is absent. Approaches: laboratory experiments, field observations by random sampling, and field manipulation (removing one species).

The chi-squared test on presence/absence of two species in many quadrats tests for an association. Expected = (row total × column total) ÷ grand total; $\chi^2 = \sum \frac{(O - E)^2}{E}$; for a 2 × 2 table, degrees of freedom = 1. If $\chi^2$ is above the critical value, the association is significant.

NOS: hypotheses can be tested by both experiments (variables manipulated) and observations (no manipulation).

Predator–prey relationships are density-dependent control: e.g. in the Canadian boreal forest, lynx numbers rise after snowshoe hare numbers rise, then hares fall as predation increases, and lynx fall in turn.

Top-down control: a population is limited by the level above it (predators). Bottom-up control: limited by the level below it (resources, nutrients, producers). Both are possible, but one usually dominates in a community.

Allelopathy: a plant releases chemicals that inhibit competitors — e.g. black walnut (Juglans nigra) releases juglone, which inhibits nearby plants. Antibiotic secretion: a microorganism releases a chemical that kills competitors — e.g. the fungus Penicillium secretes penicillin, killing bacteria. Both deter potential competitors.