Ecological niches

A niche includes the biotic and abiotic interactions that influence growth, survival and reproduction, including how a species obtains food.

Plants, eukaryotic algae and several groups of prokaryotes, including cyanobacteria, photosynthesize.

All animals are heterotrophs, and most are holozoic. Food is ingested, digested internally, absorbed and assimilated.

Mixotrophs both photosynthesize and feed. Euglena is a freshwater example, and many others live in ocean plankton. Facultative mixotrophs can use either mode, whereas obligate ones need both.

Many fungi and bacteria secrete enzymes onto dead matter and absorb the products. They are also called decomposers.

Archaea are one of the three domains of life. They include photosynthetic, chemosynthetic and heterotrophic organisms, and many live in extreme environments.

Teeth show diet. Paranthropus robustus had very large, flat molars and strong jaws, suited to tough plants. Homo sapiens has small, flat teeth that suit a mixed diet. Homo floresiensis had large premolars and a strong jaw, and tooth wear suggests a mainly tough plant diet with some meat.

Leaf-eating insects have chewing or piercing mouthparts. Plants resist with spines, stings and toxic secondary compounds in seeds and leaves. Some herbivores have metabolic adaptations that detoxify these substances: the proboscis monkey has gut bacteria that neutralize toxins in the leaves it eats.

Predators and prey both have chemical, physical and behavioural adaptations for finding, catching and killing prey, or for resisting predation.

Trees grow up to the canopy. Lianas climb on them, epiphytes grow on their branches, and strangler figs begin high up and send roots down. Shade-tolerant plants survive in low light on the forest floor.

A fundamental niche is the potential of a species, based on adaptations and tolerance limits. A realized niche is its actual extent when it competes with other species.

Each species has a unique niche. If two species compete, one may be eliminated, or both may be restricted to part of their fundamental niche. Gause showed this in 1934: grown together, Paramecium aurelia excluded P. caudatum.