Great Barrier Reef Corals Show Flexible Assembly Rules across a Steep Climatic Gradient

Coral Reefs are some of the most fascinating, important, and vulnerable ecosystems in the world. They hold a high environmental value due to species abundance and biodiversity, containing more species per unit area than any other marine environment. Healthy reefs also contribute to local economies through tourism and fishing. Over the past few decades, coral reefs have been degrading due to climate change and other anthropogenic causes such as overfishing and pollution. Hughes et al. (2012) evaluated the composition of assemblages in Australia’s Great Barrier Reef using multiscale sampling and analyses determined that the assembly rules, a set of ecological rules determining patterns of assemblage composition, of corals in the Great Barrier Reef are flexible, and do not change in response to latitudinal climatic drivers. Because the diverse pool of species they sampled are able to assemble in different configurations across a large range of environments, Hughes and colleagues support the hypothesis that coral reef assemblages will change extensively in the future, but not necessarily collapse due to climate change as long as greenhouse gas emissions are reduced sufficiently.—Kelsey Waite
Hughes, T.P., Baird, A.H., Dinsdale, E.A., Moltschaniwskyj, N.A., Pratchett, M.S., Tanner, J.E., Willis, B.L., 2012. Assembly rules of reef corals are flexible along a steep climatic gradient. Current Biology 22, 736-741.

Hughes and colleagues studied regional scale patterns in the composition of coral reef formations in Australia’s Great Barrier Reef. They used a multiscale sampling approach in which 33 reefs, within five different regions, were sampled over a 12-month period. The coral composition, as well as the number of coral colonies and percent of coral coverage was taken at each reef. Over 35,000 colonies were categorized into 12 ecologically relevant groups (taxa) depending on their physiology, morphology, and life history. The variation in abundance of each of these 12 groups was then analyzed across the five regions.
Coral assemblages have two main habitats: reef crests (1–2m depth) and reef slopes (6–7m depth). The characteristic faunas of reef crests and reef slopes differ greatly. Hughes et al. suggest that assemblages do not show an increasing or decreasing trend with latitude or latitude-related temperature gradients since different taxa flourish while others decrease or remain constant in abundance as the environment changes. On crests, 9 of the 12 taxa varied among the five regions, while 7 out of 12 varied on the reef slopes. Only one of the 12 taxa was uniformly abundant on both the crests and slopes, the other 11 taxa showed significant spatial variation. Hughes and colleagues suggest that this spatial variation may be due to disturbance events such as cyclones, crown-of-thorn starfish predation, episodes of bleaching, or to pulses of recruitment by more dominant species. 

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