Coral fish biodiversity

Biodiversity reservoirs, coral reefs and associated ecosystems are in grave danger from natural and human-made disturbances and no one with any common sense would deny this fact but it becomes increasingly complex when we attempt to understand the process better and aim to predict how strong any impact is and how soon certain changes will show. It is almost impossible to keep track of all the alarming news and filter out those that are based on sound science and come with reasonable predictions. The latest World Resources Institute assessment for example is one of those alarming ones with 75% of coral reefs reported as endangered worldwide, a figure that may reach 100% by 2050. Such numbers are concerning, particularly as coral reefs provide sustenance and economic benefits for many developing countries and fish biodiversity on coral reefs partly determines the biomass available for human consumption. What is lacking is a more detailed insight into the relationship of human impact and loss of biodiversity.

In a new study researchers sampled 1553 fish communities through underwater surveys in 17 Pacific countries. They assessed the taxonomic, functional and phylogenetic diversity levels of a group of fish species fished along a human density gradient ranging from 1.3 to 1705 persons per sq. km of reef.. While taxonomic density is essentially known as species richness - the effective number of different species in a given region - the other two measurements are more representative of the entire community structure. Functional diversity refers to the variety of biological processes, functions or characteristics of a particular ecosystem (with multiple ways to calculate it) and phylogenetic diversity is a measure of biodiversity which incorporates phylogenetic relationships between species (has been used for a while already and even in barcoding studies). The aim of the study was to quantify the effect of human activities by using and comparing all three measurements, while decoupling the influence of biogeography and habitat along a gradient of human pressure.

Over the whole range, species richness decreased by 11.7%, while phylogenetic and functional diversity dropped by 35.8% and 46.6%, respectively. Our results call for caution when using species richness as a benchmark for measuring the status of ecosystems since it appears to be less responsive to variation in human population densities than its phylogenetic and functional counterparts, potentially imperiling the functioning of coral reef ecosystems.

The simple species count alone seems to be a rather poor indicator of anthropogenic pressure, while the two other biodiversity components are far more affected by human density. A strongly affected region might still be able to maintain a rather large and stable number of species but the for example the diversity of ecosystem function (and perhaps resulting ecosystem services) drops dramatically. This stresses how important it is to conserve all the components of biodiversity but also how crucial it is to look at more than one measure of diversity. Seems trivial but this study was the first that actually comparing these particular three measurements and how they are affected by disturbances.