Investigation 10: Energy Dynamics

An Energy Pyramid

Introduction
One cannot talk about science without also talking about energy. Energy is the power source for everything that happens in nature, from waterfalls to a growing plant to a bear catching a salmon in a river. For almost all organisms, life is focused solely on energy gathering.

Energy is input into the system through producer level plants performing photosynthesis to capture the energy of the Sun and make it available for organisms on Earth to use. The total energy output in an ecosystem is called its gross productivity, and is affected by the production of energy by the plants. Then first level consumers then consume the plants, but are only able to maintain 10 percent of the energy that the plant captured from the Sun. Then second level consumers eat the first level consumers, once again only recovering 10 percent of the energy that was gained by the organism lower in the food chain. Finally, the apex predators or third level consumers eat the second level consumers and conserve 10 percent of that energy.

But the question remains, how are energy and biomass related? Biomass is created when plants capture the energy of the Sun and use it to build up their own bodies, and then this biomass travels down the food chain to the consumers. For this experiment, the lab group of Vikram, Vinay, Shreyan and Mark decided to find out what sort of relationship energy and biomass have. If there is an energy change will the biomass change as well? Or will biomass stay the same because of the law of conservation of mass? There are questions we decided to explore in our experiment.

For our experiment, we had to model ecosystem energy dynamics to then determine flow of energy through the system. To do this we wanted to figure out net primary productivity of a system of our own design, which is the amount of energy produced minus the amount of energy used by respiration. Because the group could not magically track energy usage through the system, we looked at the change in biomass of our system over a few days, and then use this data to extrapolate the change in energy in the system.

For our system we decided to use mealworms and wheat bran because they were two easily obtainable materials and we have used mealworms in the past to great success. The purpose of this experiment was to track the movement and flow of energy through an environment and through organisms in that environment. We hypothesized that the system of worms and bran would have the same mass at the beginning of the experiment as it will at the end of the experiment because of the law of conservation of mass, regardless of the flow of energy. We think that energy will flow through the system, but energy and biomass are two separate components of life that are not related in a closed system. It seems as if the two are related in nature because the environment is open, but hopefully we can prove that in a laboratory experiment mass and energy are unrelated.

Mealworms in Bran

Procedure
First, the lab group had to measure out and weigh our materials to establish a baseline for our experiment. We weighed out a small sample of wheat bran that will be used to feed the worms. Then we pulled 12 worms out of their containers and weighed them as best as we could by shaking the residual bran off them so we got an accurate measurement. We then weighed our container that we will place them in and poked a few holes in the container to provide air to the worms. Combining the worms and the bran in the container, we put the lid on tight and set our worms aside.

We were not sure how many days was proper to leave the worms aside in order to get an accurate read on the change in mass of the worms, but we decided on 5 days for this trial. 5 days is a good amount of time because it will allow the worms to eat the bran but also not eat too much or die of thirst from lack of water because there is no water in our container.

After 5 days we came back into the lab and weighed out the worms, the bran and the container as a whole system again, and our results are listed below.

Data/Observations

	Weight of Worms	Weight of Bran	Total Weight of Container
Day 1	7.89 grams	8.42 grams	41.9 grams
Day 5	7.31 grams	8.33 grams	41.4 grams
Percent Change	-7.35%	-1.07%	-1.19%

Our results were, at first glance, quite surprising and interesting. On the one hand, the total weight of the bran decreased as expected. It makes sense that the weight of the bran decreased because, obviously, the worms ate bran over the five day period, although we did expect the bran mass to decrease by more than just .09 grams, or 1%. However, on the other hand, we had expected the decrease in the weight of the bran to be countered by an increase in the weight of the worms. To our complete surprise, the weight of the worms actually decreased by .58 grams – or 7.35% – over the five day period. Thus, although the worms obviously consumed around .1 grams of bran, the total biomass of the worms decreased. 

Then we determined the amount of energy in the worms and the wheat bran so we could see the flow of energy. Our initial hypothesis was that the energy of the mealworms would increase as they consumed the energy from the bran and used it. To find the energy of each component, we used the conversion factor given to us by the lab manual, which was 6.5 kcal/gram for the mealworms and 4 kcal/gram for the dry mass of the wheat bran. We also had to calculate the dry mass of the mealworms, which was 36% of their total mass as stated by the lab manual. Here are our calculations:

	Weight of worms (grams)	Dry Weight of Worms (grams)	Weight of Bran (grams)	Energy of Worms (kcal)	Energy of Bran (kcal)
Day 1	7.89	2.8404	8.42	18.4626	33.68
Day 5	7.31	2.6316	8.33	17.1054	33.32
Net Change	-0.58g	-0.2088g	-0.09g	-1.3572 kcal	-0.36 kcal

We were quite confused at these results because, though we knew there would be a net energy loss due to energy being expended by the mealworms through simple respiration and the energy would be given off as heat, the data shows that energy lost by the mealworms was greater than the energy lost by the bran. So even though the worms consumer food, they still lost biomass and had a net decrease in total energy in the system.

Conclusion

Overall, our group set out to find the connection between biomass and energy. Since energy is such an important part of life, we looked at the energy conservation through the use of mealworms and wheat. Our results showed a decrease in the energy of the worms especially because of the decrease in the weight of the worms after leaving the worms with the bran over the weekend. When we found out the mass of the worms actually decreased, we realized our experiment could have been subject to some errors. First, we hypothesized that maybe temperature had a factor with the loss of the weight. We also thought that maybe the worms could have endured a lot of stress, and this could have also contributed to the loss of weight.

But, looking back at the data, perhaps our experiment wasn't as fraught with error as we previously thought. Perhaps the reason why the worms lost mass was because of their energy consumption. The worms used more energy than they took in, so perhaps they had to dip into their storage of fats in their bodies to survive, and therefore lost some weight. 

This hypothesis, however, brings up another question. Why did the worms have to dip into their fat storage for energy with such an abundant food source? I think the lack of water in the container contributed to this process. Perhaps the worms used the water already in their bodies to perform cellular respiration, and in doing so used up their water reserves in their bodies. So therefore, their weight loss was actually just a loss of water weight. This was one hypothesis that we pursued as a possibility for why the worms lost weight and energy.

In conclusion, we did not receive the results we expected, but this means we made some sort of error somewhere, and repeating the experiment would help us pinpoint this error for more accurate data and results.