Sunday, May 31, 2009

Managing ecosystems is hard!

When I was an undergrad, I kept aquaria for the fish. Now I'm keeping aquaria again, but I'm doing it for the ecosystem. I had a 10 gallon tank left over from a failed effort to develop a multitrophic level system for my students to manage - plus a guppy. The guppy was supposed to be the predator in the system - but alas, it turns out guppies are largely herbivorous, or at least they don't like to eat Daphnia. (Fish purists at this point are aghast that I choose guppies, but I was trying to come up with a LOW COST educational exercise.) I also failed miserably to grow free floating algae, and hence all my Daphnia died. But the guppy lived. Thus, my first effort at an engineered ecosystem ended with a tank full of hard, alkaline water, a coat of green slime, and a guppy. That's when I started thinking - ecosystem management isn't easy, is it?

I messed around with adding a few plants to compete with the algae for nutrients, snails and shrimp to eat the algae, but it was a lost cause. Nothing I did put a dent in the algae - although I was still trying "ecosystem management" - no chemical herbicides. Eventually I allowed myself to try physical removal, but even that was only ever a temporary fix. The algae always grew back. I did discover quite a diverse microfauna of flatworms, Hydra (these I had added in culture), and some copepods. No idea where the copepods and flatworms came from.

So - physician, heal thyself! What were my objectives? I decided that I wanted to create a stable ecosystem with at least two trophic levels and no green slime. Allochonthous productivity (i.e. fish flakes) would be allowed. I started with a completely new substrate, reduced lighting, reverse osmosis filtered water (no phosphorous!), and a larger pool of aquatic plants (primary production). Same snails (Herbivores), shrimps (detritivores) and guppies (ammonia factories? Eye candy?). So far, things are going well. My female guppy has produced lots of baby guppies, water quality is excellent, and the snails keep the glass spotless. The hornwort, duckweed, and Amazon sword plants are doing great, the other plants are clinging to life or have succumbed to ... probably snail consumption. I provide a trace element fertilizer and feed the guppies once a day with a variety of dried foods or crushed peas. The snails get a piece of lettuce to chew on once a week.

So - what have I learned about ecosystem management?
  1. The abiotic environment matters duh, that should have been a no brainer - after all the definition of an ecosystem is a community of species plus the abiotic environment. But I think terrestrial ecology doesn't give one the appreciation of power of the abiotic environment that an aquatic environment does.
  2. Biological control isn't perfect Even though my snails keep the glass and substrate clean, the green slime persists in odd corners, especially up in the fine leaves of the hornwort.
  3. Top down controls need support Although I haven't tried this yet, I think if I were to stop feeding the snails "on the side" they would starve to death pretty fast. There is NO algae in the tank. Keeping them fed means that I have more herbivores than the primary production in the tank can support, and thus makes the top down control of algae more effective.
  4. Models are critical especially when your sample size is one. Now, I don't have a simulation model of my aquarium, but I do have a pretty good conceptual model of what's going on in there. My model isn't perfect - for example my understanding of water chemistry is pretty rudimentary. But it provides a framework for thinking about what's going on, and categorizing suprises - like the fact that the water is suddenly all hazy this morning. What's going on there?
So the question remains - how to teach undergrads about ecosystem management? We expect that they will do this as professionals. Is the first time they do it going to be when they're responsible for a National Wildlife Refuge?

And if anyone wants alot of baby guppies ...

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