Fall2011_Lab12

=Lab 12 -- Community structure=

Quiz: We'll have a quiz on Wed the 16th over Population and Community structure stuff (community structure stuff from the first 2 days, Nov. 7th and 9th).

Worksheet:
- Use the data from below to fill out your worksheet. You should have 9 groups total (the total of the 8am and the 10am class). Use this data to calculate the mean, variance, etc. and answer questions 1-3 on the worksheet.

Mini-lecture:
- Each ecosystem is made up of many populations. Populations are groups of individuals that are of the same species. Each population eats specific things, such as detreitevores, herbivores, carnivores, saprophages, etc. In the soil/litter ecosystem, we can assume that saprophages, detrietivores, and herbivores are in the same trophic class. Trophic class refers to the lifestyle of the species, and how it gets its energy. Carnivores eat living things, and so they get their energy from the individual herbivores, saprophages, herbivores, detrietivores, etc. So when we think about how many carnivores are possible in an ecosystem, we find that the carnivores cannot live beyond their means. They cannot grow in number, more than there is food to eat. This leads to a pyramid-like structure if you count up all the animals in each trophic level. At the bottom are the organisms that get energy from what is most abundant. In terrestrial ecosystems this is usually plants, but in our soil there are no plants, because sunlight does not penetrate the soil very well. But there are loads of bacteria and other unseen microbes like fungi and tiny worms. These form the base of the trophic pyramid. Most of the energy in the soil ecosystem is located in these microbes. Herbivores eat the plant material that is in the soil or has fallen onto the soil, but there is not much energy in this plant mass, compared to the energy in the trophic level of the microbes. The microbes eat biomolecules of amino acids, lipids, carbohydrates, waste products, and other mostly carbon-sources. These carbon sources are like the sunlight in terrestrial ecosystems. It's everywhere. These carbon sources come from dead plant and animal material that has been broken down by detrietivores, as well as saprophages which eat recently dead organisms (think of a vulture). These as well as the herbivores make up the middle-tier of our trophic pyramid in this soil lab.

Data:

Click on this link to get the class data for your worksheet:

@https://docs.google.com/spreadsheet/ccc?key=0AvVyF7Sp08nJdHBRSkx4M05oQjRBZUlxZlNCNVIxM3c&usp=sharing

Notes for 2nd week:
We looked at agar plates after we poured the compost tea onto the plates. I also took one plate and poured sterile water onto a plate of agar and we were able to compare these two different experiments. How did they compare? How do these results explain what you see between the two kinds of agar plates (water with or without being soaked in leaves). What is the dependent variable? What is the independent variable? What kind of animals make up the stuff you saw in the agar plate? Where do they fall in the trophic pyramid? What do these organisms eat when they are in the agar plate? What do they eat when they are in the "real-world" ecosystem? In your lab book, they are called "microfauna", and in class and in my notes, I call them "microbes". Your book makes a distinction between microfauna and microflora. What is the difference?

We also worked on the trophic pyramid. We compared our results to the idealized results I showed in class, or that you saw in your lecture book. I asked a question on the last quiz about why carnivores are on the top of the idealized trophic pyramid (meaning, they make up the smallest proportion in the trophic level). Some folks answered that this was because, if there were more carnivores around, they would all starve, because there weren't enough prey to keep the carnivores alive.

But if the carnivores die off, then the prey are allowed to increase in numbers, because they aren't getting eaten by the carnivores. Imagine rabbits and foxes. As the number of foxes go down, the rabbits are not being killed as often (because there are fewer foxes), and so the rabbit population goes up. So, if there is more prey around, the foxes have more food, and so the population of foxes begins to increase.

Predator and prey populations do affect how many individuals there are at any given time. But if you look at the proportion of predators to prey, this doesn't explain why there are always fewer predators compared to the amount of prey. This has more to do with how much energy and what kind of nutrients an organism needs to survive. Rabbits contain a lot of energy and nutrients, but not all of these rabbit-nutrients and rabbit-energy are bio-available. For example, if you eat a piece of meat, this includes energy and nutrients, but the major source of energy is in protein, and it takes a lot of digestion and catabolism to turn the protein into energy for you to use. The nutrients in protein include essential amino acids, which are building blocks for you to repair and maintain cell structures such as microtubules, and are molecules that your body can't create on its own. But among the many types of essential amino acids, some amino acids are hard to find in meat, and so you have to eat lots and lots of meat in order to get enough to stay alive. Animals with specialized diets have evolved to make best use of the bioavailable molecules in their prey, but organisms that don't have these adaptations must eat a variety of different prey. That way, the fox doesn't have to eat 15 rabbits per day in order to get enough of a particular amino acid (or whatever nutrient rabbits might not provide).

All of this is to say, that trophic levels do depend on the organisms below them for their survival, but the explanation for why there is a pyramidal shape to the different trophic levels has to do with possible bottleneck-ing of energy and nutrient needs, as well as predator-prey or competition interactions. If there is a limit to the amount of energy and nutrients to go around (as opposed to just the numbers of individual prey), then this energy/nutrient limitation keeps predators (or herbivores, or detrietivores, or whatever) from getting bigger and bigger in population size.

An important aspect of this explanation is the focus on the energy and nutrients in food, as opposed to the number of individuals food items (whether it's rabbits or plants or berries). The best way to think about trophic levels is in biomass, as opposed to how many predators or how many prey are in an ecosystem. The biomass of an individual is basically how much something weighs. You can also look at biomass in terms of just energy or in terms of a certain nutrient (for example, the grams of nitrogen in a piece of food).

If you are still asking whether it's okay to explain the trophic pyramid with the predator-prey interactions, the answer is: sort of. But this is not the whole story. Consider an ecosystem with carnivorous ants (soldier ants video from MacGyver tv show) -- these predators will populate an ecosystem differently than an ecosystem with lion predators. One lion might eat 100 caribou over the course of its lifetime. But one carnivorous ant might eat only 1 caribou over the course of its lifetime. But the biomass and the energy and nutrient needs will always have the same proportion. Predators (or herbivores, or detrietivores) face a bottleneck of energy and nutrients when it comes to eating things from the trophic level below them. This bottleneck has to do with the needs of the organism, and how much energy and nutrients the organism actually gets when it consumes its food. There is always some inefficiency in converting food into energy and nutrients (just like there are no perpetual motion machines), and so this means there is a higher proportion of eaters compared to the food itself.

Here's a good pdf with an explanation of how energy and nutrients move in ecosystems: Ecosystem chapter -- search for "energy flow" (in your browser menu, goto "edit", and then "find")