C4.1 Populations and communities

No organism lives alone. Every individual belongs to a population, and populations of different species share a habitat as a community, competing for the same limited resources, eating one another and sometimes helping one another along. C4.1 is really about counting and explaining: how do ecologists estimate how many individuals are out there, why do populations grow in an S-shaped curve rather than forever, and what kinds of relationships hold a community together? Master the logic of limiting factors and the few named species interactions and most of this topic falls into place.

Populations, communities and estimating numbers

A population is all the organisms of the same species living in the same area at the same time and able to interbreed. A community is all the populations of different species living and interacting in an area. Because counting every individual is rarely possible, ecologists estimate population size by sampling.

For sessile (non-motile) or slow-moving organisms such as plants, quadrats are used. A quadrat is a frame of known area placed randomly (to avoid bias) many times; the mean number per quadrat is scaled up to the whole habitat. Random sampling matters because choosing nice-looking spots would distort the estimate.

For motile animals, the capture–mark–release–recapture method (the Lincoln index) is used. A first sample is captured, marked harmlessly and released; after time to remix with the population a second sample is captured. The population is estimated as:

N = (M × n) ÷ m, where M is the number first marked, n the total in the second sample and m the number of marked individuals recaptured. The method assumes marks do not harm or make animals conspicuous, there is no significant migration, birth or death between samples, and marked individuals mix randomly.

Carrying capacity and the sigmoid growth curve

When a few individuals colonise a new area with plentiful resources, the population grows. Plotted against time, growth follows a characteristic sigmoid (S-shaped) curve with three phases:

• Exponential (log) phase: resources are abundant, natality (birth rate) greatly exceeds mortality (death rate), and the population grows ever faster.
• Transitional phase: resources begin to run short, so natality slows and mortality rises; growth decelerates.
• Plateau (stationary) phase: natality and mortality are roughly equal, so the population levels off around the carrying capacity.

The carrying capacity is the maximum population size an environment can support sustainably over time. It is set by density-dependent limiting factors — competition for food, water, space, light, breeding sites, plus disease and predation — which act more strongly as the population becomes more crowded. Density-independent factors such as droughts, fires or storms can also reduce numbers regardless of density. Around the carrying capacity the population fluctuates by negative feedback: above it, mortality rises and natality falls, pushing numbers back down; below it, the reverse occurs.

Competition: intraspecific and interspecific

Whenever a resource is in short supply, organisms compete for it. The syllabus distinguishes two kinds:

• Intraspecific competition is between members of the same species. Because they have identical needs it is usually intense, and it is the main brake that holds a population near its carrying capacity.
• Interspecific competition is between members of different species whose requirements overlap. The greater the overlap in their needs, the stronger the competition.

Both natality and mortality, together with immigration (movement in) and emigration (movement out), determine whether a population grows or shrinks: a population increases when natality plus immigration exceeds mortality plus emigration.

Other interactions: herbivory, predation, symbioses and disease

Communities are held together by a web of named interactions, each of which you should be able to define and illustrate:

• Herbivory: an animal (the herbivore) feeds on a plant or alga, for example a caterpillar eating a leaf.
• Predation: one animal (the predator) kills and eats another (the prey), for example a lion hunting a zebra.
• Mutualism: a symbiotic relationship in which both species benefit, for example root nodule bacteria (Rhizobium) fixing nitrogen for a legume while receiving sugars, or mycorrhizal fungi exchanging minerals for plant sugars.
• Parasitism: one species (the parasite) benefits at the expense of the host, which is harmed, for example a tapeworm in an intestine or a tick on a mammal.
• Pathogenicity: a disease-causing organism (pathogen) harms its host, for example bacteria or viruses causing infection.

A clear way to remember the symbioses is by who gains and who loses: in mutualism both gain; in parasitism one gains and one is harmed. Always name a real example — examiners expect it.