Notes from our class meeting:
– We spent some time talking about our ponds at home and how the Daphnias and worms are doing. Some of the students have had their Daphnias all disappearing, and I mentioned that Daphnias have a tendency to cycle in population. They’ll eat most of the food and sense that there isn’t an abundance anymore, lay their eggs and then die. Once the amount of food rises again, the eggs will hatch. One could spend an entire class learning how Daphnias “sense” less food, and how the eggs know how to hatch. What are the chemical signals? How do they induce hatching? Anyway, I also clarified that the worms won’t eat the Daphnias, they eat decaying organic matter (which is what? dead material, waste products from snails and Daphnias).
– Some more questions from our discussion: How are our ponds like an aquarium? Not like an aquarium?
The kids mentioned that our ponds are much smaller and aren’t open to the environment (closed system). Someone also mentioned we have very limited species in our ponds, which is very true. We are trying to have a representative from the major kinds of animals found in ponds, but a real pond has many, many species. Our ponds are like aquariums because they contain water and will have fish now! A final point I covered was that we aren’t planning on feeding the fish and other animals in our pond models, we are trying to make a self-sustaining pond where everyone gets their food from the pond.
– We have covered this before, but I returned to the idea of a cycle of life in the pond. We started with the fish: what do they eat? The Daphnia, worms, and even algae and protists. What are the Daphnia and worms eating? Algae for the Daphnia and decaying organic matter for the worms (although they will also eat bacteria, algae and protists). What are the algae eating? Ah, they aren’t eating other organisms, they are making their own food from the sun, just like the plants in the pond.
– You can draw a large circle showing what each animal is eating, and where the energy is coming from. Here’s an example below – this includes information about genetics, but it can be a food web as well. Try making your own cycle with your own pond and see how information you can add to it!
Returning to our discussion, I asked how many fish could a pond support? How could you figure that out if you were a scientist?
I mentioned the term “carrying capacity”, which is a fancy term for determining how many organisms of each species a habitat can support. Scientists actually try to figure this out by measuring how much food energy is available, how much carbon and oxygen, and another element that we haven’t talked about yet, nitrogen. Nitrogen is a component of many of our biological molecules, and is also a large part of the waste that animals produce. We’ll cover more about the nitrogen cycle for our next class, but in the meantime, here is a very interesting diagram to the left.
I also told the kids that I used air pumps when raising these guppies – why don’t they need them? (easy answer, the plants!).
And finally, everyone is taking fish home, and both the guppies and the mosquito fish love Daphnia and Tubifex worms. How are the Daphnia going to survive the fish?
What are some strategies? Making many babies was mentioned, and this is one of the strategies that Daphnia do use to try and survive. What are some other ones? More about this soon…