STUDENT INSTRUCTIONS

Questions and Methods

Biological control is achieved when an invasive plant's natural enemies (herbivores) are introduced to reduce the fitness of the targeted invasive plant. Schutzenhofer and Knight (2007) conducted an experiment to artificially examine how increased levels of herbivory on an invasive plant species, Lespedeza cuneata, impact the fitness (growth and seed set) of this species, i.e., would biological control have the potential to curb the growth rate of this invader. Their experiment was set up by first finding plants that were of different size classes: small (1 branched individuals), medium (2-5 branched individuals), large (6-10 branched individuals), and extra-large (>10 branched individuals). For each size class they had five different clipping treatments: control (no augmented herbivory), 20%, 40%, 60%, and 80% augmented herbivory (see image 1a &1b). The purpose of the clipping treatments was to augment herbivory. Clipping consisted of manually removing leaf tissue by hand to mimic a leaf-chewing herbivore (herbivorous biological control agent). As previous work has found that ambient levels of herbivory on this species are minimal, the researchers assumed that all leaves were equivalent with regards to original amounts of damage. For example, in the 20% augmented herbivory treatment, 20% of all tissue was manually removed and plants in this treatment were estimated to have a total amount of 20% "herbivory".

Results

At the end of the growing season they measured the amount of seed produced by all plants (seed set). They found that the treatments did not affect seed set of plants in any size class. Then, in the next year, they determined if the plants survived and whether or not the plants decreased or increased in size (to measure growth). The only size class in which clipping treatments affected growth and survivorship was the small size class, which is shown in Figure 1a and 1b.

1. Interpret the results found by the researchers using Figure 1a and 1b.

1. How does increasing levels of herbivory affect the growth of small plants (Figure 1a)?

2. How does increasing levels of herbivory affect the survival of small plants (Figure 1b)?

2. Pair up with your neighbor and share your conclusions.

3. Form a generalized statement about the effect of herbivory on small plants.           

4. How might the artificial herbivory imposed by the researchers be different from actual insect herbivory?

5. Why do you think there was not an immediate effect of clipping on seed production, but there was an effect on survival the following year?