In the Convention of Biological Diversity the term biological diversity is defined as "the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems".Researchers have not been able to agree on a single explanation of biological diversity. It is generally accepted that biological diversity (or biodiversity, as it is sometimes called) is a hierarchical concept, where diversity is considered on several levels, most commonly the genetic, species, and ecosystem levels. These levels can be further split into smaller sections, if a finer resolution is desired. The concept of hierarchical levels of biological diversity is convenient in organizing information and in studying diversity, and has been adopted worldwide. Conserving biological diversity on all levels is more effective than directing all conservation efforts towards only one level, such as species diversity. Although the marine component of biological diversity is of immense importance to humankind, information about the status of marine living resources and ecosystems is less complete than that for terrestrial ecosystems. Oceans cover 71% of the world’s surface. The whole ocean is capable of supporting life. However, this life is unevenly distributed, with some areas having high species diversity and productivity while other areas are much poorer. Coral reefs, for example, are one of the most productive and diverse of all natural ecosystems.

Species diversity

Species diversity is the middle level in the hierarchy of biological diversity. Species diversity is often equated with biological diversity, although this is not the case. Species diversity refers to the variety of species in a certain region, and varies greatly among taxonomic groups and among geographic areas. In general, there is a greater number of small species than large ones, although the diversity of larger organisms is better known than that of smaller ones. For example, there are many more species of insects than species of sharks. Also, in the marine environment, the diversity of plants is lower than the diversity of animals. Marine species diversity is much higher in tropical regions than in temperate or arctic regions, with such exceptions as kelps and starfishes, which are most diverse in the cold waters of the Pacific Northwest U.S. and Canada. Within the tropics, the Indo-Pacific region has much higher species diversity than the Caribbean, because the Indo-Pacific, as an older ocean, has had more time for speciation to take place.Species richness, or the number of species within a certain area, is one of the most straightforward ways to measure biological diversity. Counting the exact number of species occurring in an area is a difficult task, however, because a majority of the species are likely to be very small and difficult to identify and count in the field. The uniqueness of an area may be assessed by the number of endemic species found there. A species is endemic to a particular area if it only occurs in that area and not elsewhere. The degree of endemism is an indication of an area’s importance in a wider context. Sites rich in endemic species can be seen as areas of active speciation or refuges for relict species. From the point of biodiversity conservation, it is important to identify areas with a high number of endemic species.

Ecosystem diversity

Ecosystem diversity is the highest level on the hierarchy of biological diversity. The composition, structure, and function of ecosystems are the three main ways in which ecosystem diversity is measured. Differing physical conditions favor different communities of species. The concept of ecosystem encompasses both the species composition of the communities and the physical structures that the communities exist in. The interactions within and between species, the ecology of the community, is also a part of ecosystem diversity, and one of the ways in which ecosystems differ from one another. The pathways of energy flow and proportions of organisms performing particular functions also distinguish one ecosystem from another. For example, coral reefs have high primary production by efficiently cycling the available nutrients, while deep sea hydrothermal vent communities rely on chemosynthesis instead of photosynthesis for primary production. Ecosystem diversity is harder to measure than species or genetic diversity because the boundaries of communities and ecosystems are often hard to define. Measuring ecosystem diversity necessitates the use of a consistent set of criteria to define communities and ecosystems.Ecosystem diversity not only occurs between different ecosystems but also among similar types of ecosystems. Wetlands in the Caribbean, for example are very different from wetlands in Northern Europe. Even within the Wider Caribbean region there are differences, where estuaries on a small, mountainous island might differ greatly from an estuary on the Caribbean coast of Central America. It is sometimes hard to classify marine ecosystems, and no universally accepted classification system exists. The population, food web, and community dynamics of marine ecosystems are not well understood, although ecosystem diversity in the sea is high.

Genetic diversity

Genetic diversity is the lowest level in the hierarchy of biological diversity, and it is the least studied. Genetic diversity refers to the variation of genes within a species. Each species consists of one or more populations of individuals. Individuals within a population are more likely to breed with one another than with the members of a different population. This is often due to proximity, and to the fact that different populations often breed in different locations. Green sea turtles, for example, return to the same nesting beach they left as hatchlings to breed. Although there is some interbreeding between members of populations using close-by nesting beaches, there is virtually no chance of green turtles from Australia and Nicaragua interbreeding. The time of breeding can also keep populations separate in some cases. Because of the limited genetic mixing between populations, they tend to diverge genetically over time. This happens because of mutation, natural selection and genetic drift. Because of this divergence, some populations have specific versions of genes (alleles) that are not present in other populations, or alleles very rare in one population may be abundant in another. This may be due to adaptations to local environmental conditions. Like ecosystem diversity, genetic diversity can be evaluated either between populations or within a population. Within a given population, individuals posses different variations of a particular gene. The process of evolution utilizes this genetic diversity, where changing environmental conditions may favor certain alleles. Populations with higher genetic diversity will likely have at least a few individuals who will be able to withstand a particular environmental change, reproduce successfully, and ensure the survival of the population. Because of the certainty of changing environmental conditions in the future, it is important that genetic diversity be conserved.

Other types of biological diversity

There are two complementary ways of looking at biological diversity that can be used along with the hierarchical approach. One is to consider the diversity of higher taxonomic groups, the phyla and the classes. The diversity of the higher taxonomic groups is much greater in the sea than it is on land or in fresh water. This is thought to be because the phyla originated in the sea and remain there, with only a small portion spreading to the land and to fresh water environments. Most of the species diversity on land comes from the phylum Arthropoda, and the members of its class Insecta outnumber the species in all other animal phyla. While Insecta are not numerous in the marine environment, many other phyla are either almost exclusively (sponges, bryozoans, coelenterates), or exclusively (comb jellies, echinoderms, lamp shells) marine. There is still much to be discovered about marine biological diversity, especially with very small planktonic organisms (picoplankton), small sized benthic organisms, and deep sea communities.Another way to look at biological diversity is to examine function. This can be done according to method of feeding, method of reproduction and the length of time to reach maturity. It is also possible to look at biochemical diversity. In ecosystems where there are a lot of sessile organisms and where the predation pressure is high (such as coral reefs) there is also likely to be a high diversity of defensive chemicals, which are of interest to the pharmaceutical industry.