Learning target: I can use the terms predation, competition, mutualism, commensalism and parasitism to classify new interactions by their effect on each partner, and explain how the terms relate to one another.
Start with the word bank and the symbol table. The rule for every interaction in this lesson is the same: ask what the interaction does to each partner, helped (+), harmed (−) or no effect (0), and the pair of symbols names the interaction. Then apply the rule to new situations in Part B. None of the examples here are from your textbook, so you cannot answer by memory; you have to use the rule.
You will need: This worksheet, Pencil, Colored pencils for the concept map (optional)
Read each situation. Decide the effect on the first-named species and on the second-named species, then name the interaction. Situation 1. A mistletoe plant sinks root-like structures into the branch of an oak and draws off water and minerals; the branch beyond the mistletoe often weakens, but the oak lives for years. Situation 2. A great blue heron stands in the shallows and swallows a frog. Situation 3. Two kinds of hummingbird visit the same patch of flowers; when both are present, each bird gets less nectar than it would alone. Situation 4. Bees collecting nectar carry pollen between squash flowers; the squash plants set fruit and the bees get food. Situation 5. A songbird builds its nest in the fork of a maple's branches; the maple is neither helped nor harmed as far as scientists can tell. Situation 6. A deer browses the new shoots of a shrub, leaving the shrub alive but smaller. Situation 7. A tapeworm lives in a dog's intestine and absorbs food the dog has digested; the dog loses weight. Situation 8. Microbes in a cow's stomach break down grass fibers the cow cannot digest on its own; the microbes get food and a warm home, and the cow gets nutrients from the grass. Situation 9. Two shrub species on a dry hillside need the same scarce soil water; both grow more slowly when planted together than apart. Situation 10. A hermit crab moves into the empty shell of a snail that died months ago.
| Effect on first species (+, −, 0) | Effect on second species (+, −, 0) | Name of the interaction |
|---|---|---|
| Situation 1 | ||
| Situation 2 | ||
| Situation 3 | ||
| Situation 4 | ||
| Situation 5 | ||
| Situation 6 | ||
| Situation 7 | ||
| Situation 8 | ||
| Situation 9 |
For any situation, ask only two questions. Question 1: Does the first species end up better off, worse off, or the same? Question 2: Does the second species end up better off, worse off, or the same? Write the two symbols first, then look them up in the grid. If both are minus, it is competition. If one is plus and the other is zero, it is commensalism. If one is plus and the other is minus, ask whether the gainer eats the other outright (predation) or lives on or in it and takes a little at a time (parasitism).
Some interactions do not sit still in one box. Oxpecker birds on African buffalo eat ticks from the buffalo's skin, which helps the buffalo, but they also peck at open wounds and drink blood, which harms it. Write a paragraph explaining which two of the five interaction names could describe oxpeckers and buffalo, what evidence a scientist would need to collect to decide which fits better, and why the answer might be different in the dry season than in the wet season.