27 May: Abundance as important as rarity for biodiversity

The Australian Institute of Marine Science and a team of international researchers have published a study today in the US Proceedings of the National Academy of Sciences (PNAS) that dismisses the ‘Neutral Theory of Biodiversity'. The study is important as it shows that the really abundant species of plants and animals often offer the most ecosystem services, such as providing habitats for fishes, or keeping reefs clear of seaweed.

"We found that it is often the common species, not the rare ones that are most important to healthy ecosystems. Those species had particular traits that made them so abundant, and therefore critical to a functioning healthy reef system," said AIMS research scientist, Dr Julian Caley, a co-author of the study.

This study overturns the long-used theory by using mathematical methods. It is the largest study of its kind. The aim of neutral theory is to explain diversity and the relative abundances of species within ecosystems. However, the theory has an important flaw: it fails to capture the importance of highly abundant species that dominate marine communities.

Using neutral theory, species become common or rare as a consequence of random processes: chance variation in who a predator happens to eat, or whose dispersing offspring happen to land on a vacant bit of real estate on the seafloor. This study shows that these random processes are not strong enough to explain the large differences between common and rare species.

The researchers give examples of how neutral theory can be incorrect, such as when they studied reefs in the Caribbean. Until the 1970s, Caribbean reefs were dominated by two species that were close relatives of the branching corals that dominate the reefs of the Great Barrier Reef. However, when these species were nearly lost as a consequence of overfishing and other forms of reef degradation, no other coral species increased to fill the gap.

The researchers looked at 14 different marine ecosystems sampled at 1185 locations across the globe. The datasets range from the polar to tropical regions, from deep-sea to shallow coral reef environments and intertidal zones. It includes vertebrates as well as invertebrates.

For interview or more information, please contact Dr Julian Caley, j.caley@aims.gov.au 0439472148.

Title: Commonness and Rarity in the Marine Biosphere

Journal: Proceedings of the National Academy of Sciences.