In the 1990s, ecologists Deborah Letourneau and Lee Dyer studied a tropical forest shrub called the piper plant and the various species of insects that live on and near the shrub. A species of ant uses the piper plant as a home by hollowing out some of its branches and building colonies inside the hollow branch cores. The ants do not eat the plant’s leaves. Instead, the leaves are consumed mostly by caterpillars. When the ants encounter caterpillars or caterpillar eggs on the plant’s leaves, they either eat them or kick them off. Letourneau and Dyer added beetles that eat ants. The figure below compares the leaf area of piper plants in control plots to that of experimental plots to which they had added beetles that eat ants.
Identify: Write statements on the graph that show you can see directly, this is not what you are guessing or think is going on, you are just making notes about things that are on the graph.
Interpret: Use a different color to write statements on the graph that explains what it means, do not try to explain the whole graph at this point.
Caption: Write a caption for the graph. Start with a topic sentence and use your Identify and Interpret statements to build a coherent paragraph that summarizes what the graph is showing.
Which statement do Letourneau and Dyer’s results support?
a. Adding beetles reduced ant numbers and triggered a trophic cascade that increased the mean leaf area left on plants.
b. Adding beetles had little effect on this ecosystem, showing that it is primarily regulated from the bottom up.
c. Adding beetles reduced ant numbers and triggered a trophic cascade that decreased the mean leaf area left on plants.
d. Adding beetles reduced ant numbers and increased the caterpillar population size, proving that the caterpillars are a keystone species in this habitat
Draw two diagrams that show the food chains for both the experimental and control plots, and describe the effect of adding beetles on each species in the food chain.
Write a CER to answer the question of what happens when beetles are added to a test plot
Claim: Adding beetles to a test plot decreases leaf area on a piper plant.
Evidence: After 18 months, piper plants without beetles in their plot had 2000 cm2 of the plant covered with leaves, whereas in that same time period, piper plants in an area with beetles had a leaf area of 1000 cm2 per plant, or half the leaf area.
Reasoning: The size of a population at a trophic level in a food chain can be determined by bottom-up or top-down forces. Often, the top predator in a food chain serves as a keystone species that influences the size of populations at lower trophic levels. This is the case in the piper plant experiment. When only three organisms were in the food chain and ants were at the top, there was a positive effect on the amount of leaf matter on the plant. However, when beetles that prey on ants were added, the trophic cascade was negative. This is clearly illustrated in the diagrams I drew and in the graph above. These strong indirect forces are at play in many different ecosystems.