Two Urchins, Similar but Not

Many different species do similar things. The pencil sea urchin and green sea urchin, for example, are both grazers in the coastal waters of the Galapagos Islands.

When does biodiversity matter for the function of ecosystems, and when can we consider different species to be essentially the same? When, for example, is a grazer just a grazer, and when do we need to consider the distinctions between different species of grazers?

Green sea urchins and pencil sea urchins are quite similar to each other in many respects. They are both ball-shaped marine animals, and are covered in long spines. Both are found in the Galápagos Islands. They are grazers — they move along the rocky ocean bottom scraping away and eating algae, small animals, and whatever else is in their paths. Left to their own devices, they can rapidly strip the rock bare, removing habitat and resources that many other organisms depend on. It is tempting to think of green urchins and pencil urchins as ecologically interchangeable — an urchin is an urchin.

Credit...Jon Witman

But there are important differences. Green urchins tend to stick to shallow water, and are fast and active. Pencil urchins are more widespread, and slower. Work by Jon Witman’s laboratory at Brown University has now also revealed that predatory fish far prefer pencil urchins to green urchins. This means that green urchins and pencil urchins have very different impacts on the ecosystem when predators are absent than when predators are present. In the absence of predators, both urchins graze extensively. In the presence of predators, more pencil urchins are eaten and the impact of their grazing is greatly reduced. Green urchins go about their business as usual.

Although different species may have very similar ecological impacts under some conditions, their impacts can be very different from each other under other conditions. And these differences are often driven by complex interactions between species. Without understanding these species differences and interactions, it is very difficult to predict what will happen to an ecosystem that faces change like overfishing of predators or climate change.

This video was made by Sofia Castello y Tickell, a research assistant, and Robert Lamb, a graduate student. Both are in Jon Witman’s laboratory. The music is by Jahzzar.

More animations and images can be found at and, a project supported in part by the National Science Foundation.

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