Do insects prefer local or foreign foods?
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How does insect herbivore damage compare for native, exotic, and invasive plant species?
Featured scientist: Elizabeth Schultheis from Michigan State University
Background: Insects that feed on plants, called herbivores, can have big effects on how plants grow. Herbivory can change the size and shape of plants, the number of flowers and seeds, and even which plant species can survive in a habitat. A plant with leaves eaten by insect herbivores will likely do worse than a plant that is not eaten. Plants that naturally grow in an area without human interference are called native plants. When a plant is moved by humans to a new area and lives and grows outside of its natural range, it is called an exotic plant. Sometimes exotic plants become invasive, meaning they grow large and fast, take over habitats, and push out native species. What determines if an exotic species will become invasive? Scientists are very interested in this question. Understanding what makes a species become invasive could help control invasions already underway and prevent new ones in the future.
Because herbivory affects how big and fast a plant can grow, local herbivores may determine if an exotic plant thrives in its new habitat and becomes invasive. Elizabeth, a plant biologist, is fascinated by invasive species and wanted to know why they are able to grow bigger and faster than native and other exotic species. One possibility, she thought, is that invasive species are not recognized by the local insect herbivores as good food sources and thus get less damage from the insects. Escaping herbivory could allow invasive species to grow more and may explain how they become invasive.
To test this hypothesis, Elizabeth planted 25 native, 25 exotic, and 11 invasive species in a field in Michigan. This field was already full of many plants and had many insect herbivores. The experimental plants grew from 2011 to 2013. Each year, Elizabeth measured herbivory on 10 individuals of each of the 61 species, for a total of 610 plants. To measure herbivory, she looked at the leaves on each plant and determined how much of each leaf was eaten by herbivores. She then compared the area that was eaten to the total area of the leaf and calculated the proportion leaf area eaten by herbivores. Elizabeth predicted that invasive species would have a lower proportion of leaf area eaten compared to native and noninvasive exotic plants.
Activities:
Let’s look at one leaf to see how Elizabeth collected herbivory data. Here is an example of a leaf from an invasive plant:
Observe the shape of the leaf. Does it appear some of the leaf area is missing and has been eaten by herbivores?
Each grid cell represents 1 square centimeter (cm).
Estimate how many grid cells the leaf covers and how many grid cells are missing from the leaf.
Area of leaf eaten by herbivores =
Total area of leaf =
Proportion leaf area eaten by herbivores = area of leaf eaten / total area of leaf =
Scientific Question:
How does insect herbivore damage compare for native, exotic, and invasive plant species?
What is the hypothesis?
Find the hypothesis in the Research Background and underline it. A hypothesis is a proposed explanation for an observation, which can then be tested with experimentation or other types of studies.
Draw Your Predictions:
On the three leaves below, draw your predictions for the amount of herbivory on the three plant types. Use one color to fill in areas that the herbivores have eaten. Use another color to fill in the rest of the leaf. Remember to fill in the legend to show which colors you used.
More materials for this dataset including videos, photos, student guide, teacher guide, and grading rubric are available at http://datanuggets.org/2014/01/do-insects-prefer-local-or-foreign-foods/
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