Marine ecology studies the interrelationships between organisms and their aquatic environment. Living things do not exist in isolation. Individuals interact with their own species, other species and the physical and chemical environment surrounding them. Their activities can affect both their surroundings and other organism’s activities. Similarly, different factors within the environment can affect the activities of organisms.

Definitions

Species: a group of interbreeding individuals reproductively isolated from other such groups

Population: all individuals of a given species in an area

Ecological community: different species that tend to occur together in a particular geographical area

Ecosystem: the community, or group of communities, and their surrounding physical and chemical environment.

Ecosystems may be considered on different scales. At the largest scale, the entire planet may be considered as a single ecosystem, with its various terrestrial, freshwater and marine communities. At the other end of the scale, a rock pool may be considered an ecosystem. Both examples have the biological and abiotic (non-biological) components required for the ecosystem definition.

The major controlling factors for communities and ecosystems are energy, the physical environment and interactions between species. Populations and communities are controlled by tolerance levels to abiotic factors such as light, salinity, temperature and nutrient availability. Biological regulation of populations also occurs through the interactions of different species and individuals with one another, particularly competition and predation.

Competition: the interaction between individuals for a limited resource.

Competition can be intraspecific (between individuals of the same species) or interspecific (between different species). In a competitive interaction, either the competitors manage to share the resource or one excludes the other. With resources shared, the fitness of the competitors is reduced, i.e. their growth and reproduction are limited. The competitive exclusion principle states that no two species with exactly the same requirements can coexist in the same place at the same time. As populations increase, so competition for resources increases. Increased competition absorbs energy otherwise available for reproduction. Consequently, populations can be limited by competition.

Predation: the consumption of one species by another, including both traditional carnivores and grazers (herbivores) and seedeaters.

The ability of carnivores and herbivores to regulate prey populations varies considerably. Sometimes, the predator has little effect on the prey population. Removing it would have little discernible effect on prey population numbers. In contrast, the predator may be the most important factor regulating prey population numbers. If the predator is removed, the prey population can explode, occasionally having dramatic consequences for the rest of the community.

Keystone species: predators whose removal causes great changes in the presence and abundance of many species in the community, most of which are not the prey of the predator.

Biodiversity: the 'variety of life'.

This can be viewed at many different levels from the variety within a species (genetic), between species (taxonomic) and of ecosystems (ecological). Biodiversity is measured in a wide variety of ways. Most commonly for nature conservation, this is at the species level. The simplest measure is species richness, i.e. number of species present.

Eunice Pinn