Mowing for monarchs, Part II

In Part I you explored data that showed monarchs prefer to lay their eggs on young milkweeds that have been mowed, compared to older milkweed plants. But, is milkweed age the only factor that was changed when Britney and Gabe mowed patches of milkweeds? You will now examine whether mowing also affected the presence of monarch predators.

A scientist measuring a milkweed plant.
A scientist, Lizz D’Auria, counting the number of monarch predators on milkweed plants in the experiment.

The activities are as follows:

The bright orange color of monarch butterflies signals to their enemies that they are poisonous. This is a warning that they do not make a tasty meal. Predators, like birds and spiders, that try to eat monarch butterflies usually become sick. Many people think that monarch butterflies have no enemies because they are poisonous. But, in fact they do have a lot of predators, especially when they are young.

Monarchs become poisonous from the food they eat. Adult monarchs lay their eggs on milkweed plants, which have poisonous sap. When the eggs hatch, the caterpillars chomp on the leaves. Young caterpillars are less poisonous because they haven’t eaten much milkweed yet. And monarch eggs are not poisonous at all to predators.

Britney and Gabe met with their friends, Doug and Nate, who are scientists. Doug and Nate thought that Britney and Gabe’s experiment might have changed more than just the age of the milkweed plants in the patches they mowed. By mowing their field sites they were also cutting down the plants in the rest of the community. These plants provide habitat for predators, so mowing all of the plants would affect the predators as well. These ideas led to another potential explanation for the results Britney and Gabe saw in their data. Because all plants were cut in the mowed patches, there was nowhere for monarch predators to hang out. Britney and Gabe came up with an alternative hypothesis that perhaps monarch butterflies were choosing to lay their eggs on young milkweed plants because there were fewer predators nearby. To test this new idea, Britney and Gabe went back to their experimental site and started collecting data on the presence of predators in addition to egg number. Remember that in each location, they had a control patch, which was left alone, and a treatment patch that they mowed. The control patches had older milkweed plants and a full set of plants in the community. The mowed patches had young milkweed plants with short, chopped plants nearby. For the whole summer, they went out weekly to all of the patches. They counted the number of predators found on the milkweed plants so they could compare the mowed and unmowed patches.

Predators of monarch butterflies.
There are many different species that eat monarch butterfly eggs and young caterpillars. These are just a few of the species that Gabe and Britney observed during their experiment.

Featured scientists: Doug Landis and Nate Haan from Michigan State University and Britney Christensen and Gabe Knowles from Kellogg Biological Station LTER.

Flesch–Kincaid Reading Grade Level = 8.2

Additional resources related to this Data Nugget:

Picky eaters: Dissecting poo to examine moose diets

Moose chomping on a forest plant

When you eat at a restaurant, do you always order your favorite meal? Or do you like to look at the menu and try something new? Humans have so many meal options that it can be hard to decide what to eat, but we also have preferences for certain food over others. Animals have fewer decisions to make. They have to choose from food options available in their environment. Do animals search for specific food types or eat any food they find?

Scientists who study the ecology of the remote Isle Royale National Park are interested in knowing more about how moose decide which plants to eat. Isle Royale is a large (44 miles long and 8 miles wide) island found within Lake Superior. On the island, wolves are the main predators of moose. The wolf and moose populations have been studied there for over 60 years, making it the longest continuous study of predator-prey dynamics.

In recent years, the wolf population struggled to rebound because there were very few adults reproducing. Without their natural predators, the moose population has increased dramatically, in 2000 there were approximately 500 moose, but since that time the population has grown to over 2,000 moose! Moose are browsers, meaning they eat leaves and needles, fruits, or twigs that are found on woody plants. Having too many moose on the island would take a toll on the island’s plant community. Bite by bite, moose may be chomping away at the forest and changing the Isle Royale ecosystem as we know it.

To try to fix this problem, the National Park Service is working to restore the wolf population by relocating adults from other Lake Superior packs to the island. However, this will take several years and in the meantime moose will continue to have an effect on the plant community. Scientists Sarah, John, and their colleagues realize how important it is to monitor which plants the moose are eating. The scientist team wanted to know whether moose simply eat the plants that they come across, or if they show preference for certain plants. 

Surveying woody plants in Isle Royale National Park

One thing that could affect moose food preference is the nutrition level of the different plants. In the winter, deciduous plants lose their leaves, unlike conifers that are green all year round. In the winter, moose end up eating the edges of twigs from deciduous plants, but can still eat needles of conifers. Needles are easier for moose to digest and have more nutrients than twigs so the scientists thought moose would seek out coniferous plants, like balsam fir and cedar, even if they were less common in the environment.

Starting in 2004, the scientist team selected 14 sites across the island and started collecting moose poop, also called fecal pellets, at the end of winter. Back in the lab, the fecal pellets were examined closely under a microscope to determine what the moose were eating. Many plants have identifiable differences in cellular structures, so the scientists were able to look at the magnified fragments and record how much balsam fir, cedar, and deciduous plants the moose had been eating. 

To understand preference, the scientists also needed to know which plants were in the area that the moose were living. They did plant surveys at the beginning and end of the study to estimate the percent of different woody plants that are in the forest. Because woody plants are long-living, the forest didn’t change too much from year to year. 

Once they had the forest plant surveys and the moose diets analyzed from the fecal pellets, they were able to analyze whether moose selectively eat. If a moose was randomly eating the plant types that it came across, it would have similar amounts of plants in its diet than what is found in the forest. If a moose shows preferencefor a plant type, it would have a higher percent of that food in their diet than what is found in the forest. Moose could also be avoiding certain food types, which would be when they have a lower percent of a plant type in its diet than in the environment.

Featured scientist: Sarah Hoy, John Vucetich and John Henderson from Michigan Technological University.Support for this lesson was provided by the National Park Service with funding from the Great Lakes Restoration Initiative.

Flesch–Kincaid Reading Grade Level = 10.1

Additional teacher resources related to this Data Nugget:

The study and results described in this Data Nugget have been published. If students are curious to know more about the study design and how sites were selected, there is an approachable methods section available in the article:

  • Hoy, S.R., Vucetich, J.A., Liu, R., DeAngelis, D.L., Peterson, R.O., Vucetich, L.M., & Henderson, J.J. 2019. Negative frequency-dependent foraging behavior in a generalist herbivore (Alces alces) and its stabilizing influence on food-web dynamics. Journal of Animal Ecology.

There have been several news stories about this research:

Website with more information on the Isle Royale Wolf-Moose Study, including additional datasets to examine with students.