“Using traits to assess nontransitivity of interactions among coral species”

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Kristin Precoda, Andrew P. Allen, Liesl Grant, and Joshua S. Madin

In a competition between two coral species, who wins is only partially predictable based on species traits, allowing coexistence

Competition for light and space: Acropora hyacinthus (center bottom to upper left) with damaged edge partially surrounding Platygyra daedalea (center), at Sesoko-jima, Okinawa, Japan.
(Credit: Andrew H. Baird)

A longstanding puzzle is how a large number of species can coexist indefinitely when they’re all competing for a single, limited resource. Why doesn’t a single species crowd out all the others? A possible explanation is that as in the rock-paper-scissors game, no one species outcompetes all the rest. This paper presents evidence that, in the highly diverse world of coral reefs, competitions between coral species are generally not hierarchical, although few follow the rock-paper-scissors model exactly.

Montipora sp. (lower left quadrant) with damaged edge surrounding Dipsastraea sp. (mounding, center) and partially surrounding Favites stylifera (mounding, center right), which also borders damaged edge of Acropora gemmifera (center right edge). Sesoko-jima, Okinawa, Japan.
(Credit: Andrew H. Baird)

More than a hundred coral species can coexist on a reef, using multiple strategies to compete with each other for sunlight and living space. Kristin Precoda, Andrew Allen, Liesl Grant, and Joshua Madin gathered a list of wins, losses, and ties between pairs of 111 coral species from 16 prior studies in regions including the Caribbean, the Great Barrier Reef, Taiwan, Hawai‘i, and the Red Sea. They linked each species to traits that could influence its competitive abilities. For example, the tentacles of coral species with larger polyps may have a longer “reach,” and corals that grow upward and outward may shade flatter species. They found that several traits helped predict which species was likely to win, especially for less closely related competitors. However, the outcome for any particular species pair was strongly influenced by as yet unexplained factors, and the same species in a given pair may not always win. The results suggest that the outcome of competition in these species-rich communities is quite nuanced. Read the Article