Populations and communities

A population is a group of organisms of the same species living in the same area at the same time. Its members normally interbreed. One population is separated from another of the same species by reproductive isolation, for example by distance or a barrier.

Counting every individual is usually impossible, so we count a sample and scale up to an estimate. The sample must be random, so that every part of the area has an equal chance of being sampled and the result is not biased.

For sessile organisms, place quadrats at random coordinates, count the individuals in each and find the mean. Population size = mean per quadrat × total area ÷ area of one quadrat. The standard deviation of the counts shows how much they vary, and so how evenly the organism is spread.

For motile organisms, capture and mark a sample (M), release them, then capture a second sample (N) and count the marked ones (R). The Lincoln index gives population size = (M × N) ÷ R. It assumes that marks are not lost, marked animals mix fully, and there are no births, deaths or migration between samples.

The carrying capacity is the maximum population size that an environment can support. It is set by limited resources, such as food, water, territory and nesting sites for animals, and light, water and minerals for plants.

Competition, predation and the spread of pathogens and pests all increase as a population becomes denser. Above the carrying capacity they raise deaths and reduce births, and below it they relax. This negative feedback keeps the population near the carrying capacity. Density-independent factors, such as floods or fires, can change numbers suddenly but do not do this.

In the exponential phase, resources are plentiful and limiting factors are weak. In the transition phase, density-dependent factors increase and growth slows. At the plateau, births and deaths balance at the carrying capacity. The return of the bearded vulture to the Swiss Alps is a growing population.

Watch out: a lag phase is not expected as part of this curve.

Count the fronds of duckweed (Lemna) in a container every day or two until the surface is covered. Plot the counts on a log scale against time on a normal scale: exponential growth gives a straight line, and the line curves away as growth slows.

A community is all the interacting populations in an area, including plants, animals, fungi and bacteria. A community and its abiotic environment form an ecosystem.

Members of a species have the same needs, so they compete for food, light, territory and mates, as when wildebeest males fight over territory. They also cooperate: hyenas hunt in groups, and leafcutter ant workers gather leaves to grow the fungus that feeds the colony.

Herbivory (bison and grass), predation (lynx and hare), interspecific competition (lions and leopards), mutualism (Rhizobium and clover), parasitism (ticks and deer) and pathogenicity (chytrid fungus in the evergreen robber frog).

In legume root nodules, Rhizobium fixes nitrogen for the plant and receives sugars. Orchid seeds have no food store, so mycorrhizal fungi supply them with sugars and minerals; adult orchids later supply carbon compounds. In hard corals, zooxanthellae supply products of photosynthesis and receive shelter in sunlit water, carbon dioxide and nutrients.

An invasive species, introduced by humans, often lacks its natural predators and pathogens, and may take a shared resource more effectively. Grey squirrels outcompete red squirrels for seeds in Britain. Local examples include the nutria, Himalayan balsam and Japanese knotweed.

Approaches include laboratory experiments, field observations by random sampling, and field manipulation by removing one species, as Connell did with barnacles. If a species does better without the other, competition is indicated but not proven.

Record the presence or absence of two species in many random quadrats. Expected value = row total × column total ÷ grand total. Chi-squared is the sum of (O − E)² ÷ E. For a 2 × 2 table there is 1 degree of freedom, and the critical value at p = 0.05 is 3.84. A larger value means the species are associated.

When snowshoe hares are plentiful, lynx have more food and increase. More lynx eat more hares, so hares fall, and then lynx fall too. The two populations cycle, with the predator lagging behind: predation is density-dependent control.

In top-down control, a higher trophic level limits the one below, by predation or herbivory. In bottom-up control, the supply of nutrients or producers limits the levels above. Both are possible, but one usually dominates in a community.

In allelopathy, a plant such as bracken or a walnut tree releases chemicals that stop other plants germinating or growing. Fungi such as Penicillium secrete antibiotics that kill bacteria. In both, a chemical released into the environment deters competitors.