In many ways, ecology is easier on land--it's easier to watch species go extinct, watch the weeds take over. But it's all happening in the oceans, too. The narrowly adapted endemics are going extinct, and the fast-reproducing generalists are taking over. Extinctions are caused by many of the same problems as on land: pollution, habitat destruction, and so forth.
One new problem, though, is unique to the oceans: acidification. Atmospheric carbon dioxide dissolves in the water and breaks down into ions, including hydrogen ions that slowly and inexorably pump up seawater acidity. As acidity increases, it gets harder and harder for animals to build skeletons and shells. And at a certain threshold, the skeletons and shells already in existence start dissolving. That's simple chemistry, and in parts of the deep ocean it's happening already.
One of my friends is writing a paper about this stuff, and it got me thinking. Squid already have all the weedy characters--fast generation time, high fecundity, generalist diet. Many of these traits they probably developed over evolutionary time as they struggled to compete with fish--the new kids on the block that began rapidly outcompeting the ancestors of squid in the Devonian. One of the first things squid did was lose their big, heavy shells. Now that loss turns out to make them one of the molluscs best adapted to acidic oceans. No shell, no need for calcification, and no problems!
I love squid, but I love biodiversity too. Everyone does, I think, even if they don't know the word. It's a big part of what makes this world an interesting and beautiful place to live, even if you never travel out of your hometown. You know your local flora and fauna, the stuff out-of-town visitors have never seen before. They've got their local flora and fauna that you've never seen before. You may never travel that far, but isn't it fun to know that there are tigers in India, lemurs in Madagascar, echidnas in Australia? What will happen if all this diversity becomes history instead of biology?
I love squid. But--dare I confess it?--I don't actually want them to take over the world.