Image showing fish diversity on a coral reef

The biodiversity of a coral reef is the variability within and between species on that reef; this is a multifactorial measure, which measures many features of the reef. Despite coral reefs occupying less than 1% of the sea floor, they are populated by 25% of marine species; coral reefs are believed to accommodate the largest biodiversity on Earth. 32 phyla are present in coral reef ecosystems, out of the 34 on Earth. This diversity can be measured and used to interpret the importance of a coral reef; reefs can house endemic species and support surroundings ecosystems. It can also create an understanding of the interrelationships that occur in these densely populated marine ecosystem.

Despite the development of many statistical measures of biodiversity, the multifactorial nature often means it is difficult to quantify. The lack of a reliable gage has led to much controversy on how to calculate biodiversity in a manner that is representative of each factor. For the past decade, the most quoted theory has been the “Neutral Theory of Biodiversity“. However, this measure has been shown to have limited application, particularly within the coral reef environment. A recent study, by Connell, highlighted these issues and suggested a new measure should be implicated.

Neutral Theory of Biodiversity

Since its publication in 2001, the neutral theory of biodiversity has been applied to most marine environments in order to explain and understand the interrelationships of species. While this theory take both abundance and species richness into account, it also assumes that the dominant species are interchangeable, or ‘neutral’, and the dominance of one species over another is purely down to chance.

Issues with the theory

Dominant species are those that have higher abundances and compose a greater proportion of the life present in an area, also known as biomass. These species generally have a disproportionate impact on the reef, such as corals providing habitats for other species to shelter, or sharks feeding on a large variation of organisms. Neutral theory does not take these individual species characteristics into account, attributing the dominant species to be down to chance, and predicts the removal of one such species will allow another to reach the same levels of abundance.

Field studies have found this is not the case. Multiple papers cite the neutral diversity theory as inaccurate, and diversity measured using weighted methods has found significant differences in actual and measured biodiversity. Connell’s study investigated 1185 sites, and is believed to be the most comprehensive of its kind. Neutrality performed substantially worse than other measures at the majority of locations. An example of the limitations of this theory, cited by Connell, took place within the Caribbean coral reef in the 1970s. Before this point, two dominant species of branching coral had composed most of the biomass. However, after damage to the reef occurred, and these species were reduced drastically, no other corals increased in size or abundance.

Different species present on a reef can have a different impact

Why is this important?

The understanding of biodiversity of the reef environment is important for both conservation and management reasons. A reduction in biodiversity, and therefore range of species, puts predation pressure on individuals and reduces food availability. The neutrality ignores species specific features and the complex interactions of environment and community that create an ecosystem. Management methods need to be updated to include these measures of species importance. The most common species are often those that have the largest impact on the ecosystem. Estimates predict that over 1 million species are linked to coral reefs. This diversity provides for tourism, medicine, coastal protection, and food for over 6 million people; the value of these services is believed to be over £280 billion. The loss of an important species could have a major impact on habitats left behind. An understanding of the biodiversity of these ecosystems is needed before they can be protected and managed.

Development of the neutral theory

Neutral biodiversity has been used as the accepted theory for over a decade, despite these gaps in application. Connell challenged the use of this theory and, through the use of already existing weighted measures, found it to be 99% inaccurate. In development of this criticism, some academics have suggested the use of multiple models; the complex and variable nature of coral reefs cannot be forced to fit a singular model. The application of many different models, for each factor impacting biodiversity, would provide a more representative measure of the biodiversity of the reef, and allow a full understanding of not only the species present, but every factor involved in diversity. This would allow scientists a clearer understanding of a reef, and how each species is important to the overall ecosystem, improving management and conservation of these important areas.

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