Climate change is causing organisms to migrate into new geographic areas, with most organisms tracking the movement of suitable temperatures poleward. The movements appear to be more rapid for marine than for terrestrial fauna, perhaps because marine organisms have fewer barriers to dispersal, bu... Show moreClimate change is causing organisms to migrate into new geographic areas, with most organisms tracking the movement of suitable temperatures poleward. The movements appear to be more rapid for marine than for terrestrial fauna, perhaps because marine organisms have fewer barriers to dispersal, but many marine organisms do encounter barriers that inhibit their ability to colonize new habitats. On page 1135 of this issue, Muir et al. show that low wintertime irradiance limits the poleward expansion of staghorn corals. On page 1132, Deutsch et al. identify another barrier to marine species: the ratio of oxygen supply and metabolic demand, which determines in which parts of the ocean fish and invertebrates have enough oxygen to support their metabolic needs. These two studies remind us that climate change will reshape marine species habitats but not necessarily expand them. Some barriers to range expansions are essentially static with respect to climate change (see the figure, panel A). For organisms that depend on photosynthesis, light availability is one of those barriers. The amount of solar irradiance reaching the ocean surface and its attenuation with depth are not expected to change much with changes in climate. Reef-building corals, such as the staghorn corals studied by Muir et al., depend on photosynthetic symbionts, limiting them to depths with sufficient levels of photosynthetically available radiation (PAR) throughout the year. Near the equator, this depth is typically around 25 to 30 m. Outside the tropics, it shallows by about 0.6 m per degree latitude, and so do the depths at which these corals are found. Muir et al. show that as reef-building corals settle at higher and higher latitudes in response to warming temperatures, they will eventually be confined to waters too shallow to support growth. Low PAR also appears to similarly limit the poleward expansions of other benthic photosynthesizers. Show less
