Coral are some of the most important aquatic organisms on planet earth. Not only are coral species primary producers but they create habitats and are the basis of some of the largest aquatic ecosystems. Yamano et al. (2011) studied coral species off the coast of Japan and found large-scale evidence of the poleward range expansion of modern corals. Using data collected from the temperate waters around Japan, they found northern range shifts and no southern expansion on ranges. Their findings suggest that tropical coral and their associated organisms will undergo rapid fundamental modifications with continued sea temperature increases. They warn that these modifications could damage native temperate coral species because of higher growth rates that tropical coral species exhibit. In addition to out-competing temperate corals tropical coral species could provoke the migration of tropical organisms that live around or on the tropical coral species. –Connor O’Boyle
Yamano, H., Sugihara, K., Nomura, K., 2011. Rapid poleward range expansion of tropical reef corals in response to rising sea surface temperatures. Geophysical Research Letters 38, L04601.
Long-term, large-scale observations were required for finding the effects of climate change on coral distribution ranges. Yamano et al. accomplished this by choosing the coastal areas around Japan where there are sub-tropical and temperate coral species with 80 years of national records. The records have detailed measurements of coral species and ranges including the regions sea-surface temperatures. The authors selected coral species from eight temperate region’s of Japan along a latitudinal gradient, distinguishing four distinct periods of coral exploration that were to be used for the study. The first period was in the 1930s, the second was in the 1960/70s, the third was 1980/90s, and the fourth period was in the 1990s to the present. Yamano et al. conducted surveys in the latest period, observing coral occurrence and identifying coral taxa. The eight study regions were located in areas that showed significant sea-surface temperature rises in winter and were in marine protected areas, to lessen anthropogenic impacts. The coral species that were compared were selected on three criteria: species had to occur in the area of interest, be distributed at depths of ten meters, and had to have distinct growth forms that could be identified without collecting them, because a lot of the old data were qualitative. Nine species were selected for data collection and their expansion rate was calculated by occurrence records based on the four periods. The authors used the year that new colonies of coral were found to keep calculations consistent.
The results showed that four of the nine species exhibited poleward expansion since the first time period. The other five species remained stable during the four time periods. Out of the species that exhibited northward range expansion, the authors singled out two tropical Indo-Pacific reef species, Acropora hyacinthus and Acropora muricata, to indicate the expansion of tropical species into temperate regions. All of the expanded species are labeled either “Near threatened” or “Vulnerable” by the International Union for Conservation of Nature (IUCN), showing the need for these species to migrate to more tolerant temperatures. The coral species that showed expansion also exhibited successful spawning, indicating that these species have the potential to repopulate. Temperate areas may serve as refuges for many tropical corals in a time when tropical oceans and seas are becoming dangerously warm. However these expansions could have large effects on temperate ecosystems, due to a heightened growth rate exhibited by tropical corals. Tropical coral species could fit into niches that were previously inhabited by a temperate coral species and out-compete them, disrupting an entire ecosystem. The tropical organisms that thrive on particular species of coral could start migrating northward and compete with local temperate organisms. An example of such a problem would be northern migration of certain toxic microalgae that live around tropical coral. The authors suggest that more interest should be paid to this aspect of our ocean’s corals and that the migration of tropical corals is a mostly unexplored domain in climate change models.