The carrying capacity (denoted by K) of an environment is the maximum population size of a species that the resources of that particular environment can support and sustain indefinitely, without being degraded. When a population grows logistically, it increases until it reaches the carrying capacity, and then it levels off, because the limited resources cannot support any further increase. If the population exceeds K, it declines back due to shortage of resources.

An age pyramid is a diagram that shows the distribution of the individuals of a population in different age groups (pre-reproductive, reproductive and post-reproductive) at a given time. Its shape indicates the growth status of the population: a broad-based (triangular) pyramid indicates a rapidly growing (expanding) population with many young individuals; a bell-shaped pyramid indicates a stable population; and an urn-shaped (narrow-based) pyramid indicates a declining population.

Mutualism is a close, positive interaction between two different species in which both the species are benefited. Examples: (1) lichens, which are a mutualistic association between a fungus and an alga (the fungus provides shelter and absorbs water and minerals, while the alga provides food by photosynthesis); and (2) mycorrhiza, an association between a fungus and the roots of higher plants; and (3) the pollination of flowers by insects, where the plant is pollinated and the insect gets nectar as food.