Ask any sixth-grade student in Kodiak this year about marine ecosystems and they should have some of these answers for you. During the past three weeks, sea urchins, kelp, crabs, sea stars, snails and hermit crabs have visited Kodiak Middle School classrooms as parts of model ecosystems in desktop tubs. Students learned about three different marine ecosystems and looked for connections between key organisms.
There was the sea otter (represented by a stuffed animal, not the real thing), that preys on sea urchins, which in turn graze on the stipes of kelp. Students worked out that a lot of sea urchins could decimate a kelp forest by biting off the kelp stipes at the bottom, so that the kelp floats away. The sea otter keeps the urchin populations at bay to prevent this clear-cutting of a kelp bed and the little decorator crab still has a place to hide in.
Another tub contains hermit crabs and snails. How are these connected? When the snails die, their shells turn into homes for the hermit crabs. Small periwinkle snails are so common in the summer that one cannot walk in the intertidal without stepping on them. If they get crushed, they provide food for hermit crabs, but their sheer numbers mean that there is ample real estate for little hermits. What happens when the small hermits grow up? They need a bigger snail. Dogwinkles are a good choice for a crab that has outgrown its starter home. However, those are not as numerous and the competition for their shells becomes daunting. For the winners, growing up doesn’t get any easier; when they need even bigger shells, there is no snail big enough in the intertidal to provide homes for fully grown hermit crabs. If your community gets too small, the youngsters have to move out into the world. For hermit crabs this means going into deeper water where they stand a chance to score the shell of a hairy triton, a larger snail that inhabits muddy and sandy nearshore ocean bottoms. A hermit crab in a hairy triton shell often doubles as a ride for a sea star or sea anemone stowaway.
So can an ecosystem also exist in the open ocean, where there is no structure like kelp forests or tide pools to live in? Plankton are the drifiting organisms that are carried on the ocean currents. The most common among the plankton animals are the copepods. Copepods are in the plankton the equivalent for fish in the nekton, the larger strong swimmers living in the open ocean: Just like there are many different fish adapted to inhabit all kinds of places and suit all kinds of lifestyles, there are also many different copepods of various sizes and lifestyles. They range in size from less than a needle’s head to about half an inch and are the most numerous organisms in the open ocean. No surprise that they have a lot of predators. There are the amphipods, distant relatives that are ferocious hunters with large eyes and well developed claws for grabbing. Also the arrow worms, built to torpedo through the water and chomp copepods with specialized jaws, and there even are large copepods that feed on their smaller relatives. You may have heard that it’s an “eat or get eaten” world out there, but most copepods are primarily grazers on tiny algae with an occasional egg or smaller larva. Their predators include numerous filter feeders that grow on rocky structures underwater, and they are essential for fish. Most fish that grow to be strong swimmers utilize copepods for food during part of their life. Herring never kick the habit of eating copepods and rely on their ample fat reserves for all their life.
Perhaps most fascinating and impressive, however, is the ecosystem in a single drop of water put under a microscope. We usually collect a tiny bit of the slimy growth found on the side of a dock and put it in a jar. From this sample, a single strand of algae in a drop of water makes a great miniature habitat to see cells of algae, tiny animals, both copepods and others, and sometimes even bacteria.
Marine ecosystems come in all sizes from the entire world ocean to a single drop of water. It really is up to us to define the ecosystem of our focus. No matter how large or small, their study can be fascinating and rewarding. However, the most important lesson in this all, is that all parts of an ecosystem are connected, that animals and plants rely on each other and that any change in factors like temperature, light levels, or current strength impacts life for all involved. And all are involved, including us.