by Amanda Kahn
In case you don’t know about it, xkcd.com has a fabulous blog section called What-If. In it, the author responds to readers’ questions, and one from a few weeks ago caught my eye because the last paragraph talked about sponges. The question was, “How much would the sea level fall if every ship were removed all at once from the Earth’s waters?” The answer to that was six micrometers, but at the end, the author brought up another sea-level question (often used as a joke): how much deeper would the ocean be if it didn’t have sponges in it?
Imagining that a single group of animals can affect sea level that much seems preposterous at first. After all, I imagine that even though there are very few blue whales left in the ocean, in terms of biomass there is surely more displacement caused by whales, sharks, and fish than by sponges, which appear lower on the food pyramid. In the ocean, food pyramids are inverted, meaning there is more biomass at higher food levels (called “trophic levels”) than at lower ones. This is opposite than what we often see in ecosystems on land. By some quick back-of-the-envelope calculations, blue whales don’t even occupy 0.000000001% of the volume of the ocean (it’s somewhere around 10e11%, if you really wanted to know).
Sponges do, however, cover large portions of the ocean and can dramatically affect the water they live in. Sponges and other filter feeders can process vast volumes of water during the filtering process, resulting in major changes in water clarity and plankton concentrations. So while these animals do not take up a lot of space in the ocean, they are still very important and have a strong impact in some regions. In case you don’t have time to watch the whole video below (it’s super interesting! I highly recommend watching it from the beginning, if you’ve got time), skip to 4:22 to see the filtering ability of oysters.
I didn’t think a single species could have a large impact on sea level until I remembered the elephant in the room: Homo sapiens. With mounting evidence that global temperature is rising and local climates are changing around the world, humans are already causing a much greater change in sea level than removing any one particular species, all the ships in the ocean, or even removing some islands.