Several centuries ago, a simple mathematical calculation proved the world was round and changed how people thought about their environment. In a similar fashion, two University of Rhode Island scientists and their Brown University colleague have made a simple discovery that may force biologists, ecologists, and evolutionists to question some long-standing beliefs about diversity in the ocean.
In an article to be published in the August 19 issue of Nature, URI Graduate School of Oceanography (GSO) paleoceanographers Scott Rutherford and Steven D'Hondt with Brown University paleoceanographer Warren Prell show that the diversity of planktonic foraminifera, one-celled animals that float in the ocean, is greatest in the middle latitudes of the world's oceans. Comparison of their results to other studies suggests that this is true of zooplankton in general; all of the microscopic animals that float in the ocean appear to be most diverse at middle latitudes. This finding runs counter to the traditional model that biodiversity peaks at the equator and declines towards the polar regions of the Earth.
Their analysis of Atlantic Ocean data shows that scientists can use sea-surface temperature to estimate the number of planktonic foraminiferal species at each spot in the ocean with 90 percent accuracy. The temperatures are derived from satellite observations that measure the ocean's "skin" temperature -- the top layer of water to a depth of about one centimeter. Their discovery goes one step farther by showing that oceanic temperature at a depth of 50, 100, or 200 meters predicts diversity nearly as well, suggesting that temperatures at the sea surface are highly correlated to the structure of the water column beneath the surface.
The scientists explain their results by suggesting that the number of zooplankton species at each site is primarily controlled by the physical structure of their environment. As D'Hondt explains, "All zooplankton need food and many zooplankton reproduce sexually. For these reasons, they need to live in places where they can count on encountering both prey and other members of their own species. If two different populations of a single species consistently reproduce in separate places or at different times, they may eventually evolve into two different species. Conversely, if a species is composed of a single large population that generally meets and reproduces at the same time and place, its members have little opportunity to separate into different species."
At the poles, the water temperature is approximately constant with depth and zooplankton may be forced to stay near the surface in order to consistently meet each other and find abundant prey. At the equator, there is a shallow surface layer of nearly uniform temperature and a deeper region of cooler uniform temperature with a sharp transition between; this structure may support more species than the structure at the poles because it provides a greater diversity of zones where prey may aggregate and reproductive partners can form separate populations. In middle latitudes, the temperature of the water changes more gradually with water depth, providing an even greater diversity of vertically segregated zones where different species can exist. x-x-x
Journal
Nature