So, a little background info. The marshes are a very human influenced region. We've done a lot of things to them - built dikes in them, built over them, used them for agriculture, etc. We've also done some things indirectly. Blue crabs, while tasty, have taken a hit due to our fishing of them. This poses a problem - if there aren't many blue crabs, what's eating what they eat? And if nothing's eating what they eat, how does that effect the next level down?
In short, the problem (one of them) is this: snails, a prey item of blue crabs, are on the rise. They're bad news for the marsh grasses, particularly Spartina, which constitutes most of the marsh, because when they feed on the grass, they essentially make a big open wound on the plant. They then proceed to poop in the open wound, which causes fungus to grow. Fungus is one of their very favorite foods, so the cycle goes on. This alone doesn't kill the plants very quickly, but when you have a dry season, this stresses the plants out and you end up with a big, bare patch of marsh devoid of plants. These are called die-offs.
So much of our research in the Silliman lab focuses on different aspects of this problem. Mine specifically is looking at how another pair of predators of the snails, the white and black clawed mud crabs, helps control snail numbers.
Remember the cages I made early on? These puppies?
These have been the focus of my trial run. Essentially, I have 12 of these (4 with black claws, 4 with white claws, and 4 with nothing in them as control treatments, essentially just there for baseline info to compare the crabs to).
Now you may be wondering, why put the crab in the cage? Won't the snails not get eaten then? Which is actually precisely the point.
You see, when a mud crab is around a snail, the snail can detect it via chemical cues in the water. Snails are especially sensitive to these chemicals and have been shown to react to a lot of different predatory species. Detection of a crab makes them climb the nearest plant shoot to get away, a fear effect commonly called a "non-consumptive effect," or NCE, i.e the snail reacts to the predator but not in a way in which it gets eaten. This is not to say the snails don't get eaten -- they do, but this is another aspect of the puzzle and kind of a cool one in my book.
This, ironically enough, is actually a big problem. Spooked snails stay up on the grass and graze the ever-loving life out of it. This is called a "trait mediated indirect interaction", or TMII. By suppressing the snails, the crab indirectly causes the grass to feel an effect, in this case a negative one.
The specific interaction my research aims to address is spatial scale. So, for instance, do the snails react way far away from the crab, or is it relatively isolated? This trial run has been precisely about that, and any differences in the reaction you see when you expose the snails to either species.
Initial data is looking kind of neat. We're looking specifically at how high the snails climb in response and how many do it versus staying on the ground, where they tend to eat lots of bacteria and stuff from the mud. The interesting thing is it only seems to spread out over essentially a third of a meter (close to a foot or so). So while the snails' climbing has a negative effect, it may only have it on a fairly small scale.
My future and main project will likely look at how the size of the crabs or how many you have and where you have them effects the process. I.e, if you have a lot in the same spot, or a lot of really big crabs, do you see a proportional change in the stuff outlined above?
It'll be fun, but that's for another day. Ecology is delightfully complicated, no?
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