Are plants more toxic in the tropics?

Carina looking for jaboncillo plants in Costa Rica.

The activities are as follows:

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Long before chemists learned how to make medicines in the laboratory, and even long before there were chemists, people found their medicines in plants. To this day, people still extract some medicinal drugs from plants, while others that we used to get from plants are now manufactured in factories.

Why do plants make these chemicals that are often so useful to people? One reason is that plants can’t run away or hide from herbivores, the animals that eat them. So instead, many plants defend themselves using chemicals that are poisonous or toxic to herbivores. As pharmacists say, “the dose makes the poison,” meaning it all comes down to quantity. A tiny amount of caffeine helps you stay awake, but you wouldn’t feel so great if you ate a giant salad of coffee leaves. Similarly, an herbivore that tries to eat coffee leaves would get sick, so it will avoid eating coffee leaves. That’s why plants have evolved to make chemicals – because the chemicals discourage animals from eating the plants. This benefit helps plants survive and reproduce, and any benefit to humans is an unintentional side effect of evolution.

Carina is fascinated by the amazing ways that plants have evolved to avoid being eaten. She also loves researching tropical forests near the Equator. Tropical forests have many more kinds of plants and insects than temperate places, which are farther from the Equator. One important difference between the climates is that the tropics don’t have harsh winters that kill insects. Therefore, biologists think that tropical plants get eaten more by herbivores.

Some plants have high chemical diversity, and make many kinds of chemicals. Biologists have observed that some plants with high chemical diversity are especially difficult for herbivores to eat. Carina thought that maybe stronger insect attacks in the tropics would lead the tropical plants to evolve higher chemical diversity than temperate plants in order to better protect them from herbivory. She thought that over time, the individual plants that had more types of chemicals in their leaves would grow and reproduce more. This would allow them to pass on their traits to the next generation.

Jaboncillo plants with herbivore damage in Costa Rica.

To answer her question, Carina collected seeds from wild pokeweed plants in Michigan and Florida. She also collected seeds in Costa Rica from jaboncillo, a species closely related to pokeweed that lives in tropical countries in Latin America. She chose these locations because they vary in how close they are to the equator, and how severe their winters can be. Michigan has long and very cold winters (a temperate climate), Florida has mild winters with occasional freezing (a subtropical climate), and in Costa Rica temperatures never go below freezing (a tropical climate).

She started by growing 15-20 plants from each location in a greenhouse. Then, she extracted chemicals from their leaves and analyzed the chemical diversity of each plant. Chemical diversity is measured by an index that includes how many and how abundant different kinds of chemicals are. Carina predicted that the tropical plants would have the highest chemical diversity. She also predicted that the subtropical plants would have higher chemical diversity than the temperate plants.

Featured scientist: Carina Baskett, Michigan State University

Flesch–Kincaid Reading Grade Level = 10.8

The mystery of Plum Island Marsh

Scientist, Harriet Booth, counting and collecting mudsnails from a mudflat at low tide.

Scientist, Harriet Booth, counting and collecting mudsnails from a mudflat at low tide.

The activities are as follows:

Salt marshes are among the most productive coastal ecosystems. They support a diversity of plants and animals. Algae and marsh plants use the sun’s energy to make sugars and grow. They also feed many invertebrates, such as snails and crabs, which are then eaten by fish and birds. This flow of energy through the food web is important for the functioning of the marsh. Also important for the food web is the cycle of matter and nutrients. The waste from these animals, and eventually their decaying bodies, recycle matter and nutrients, which can be used by the next generation of plants and algae. Changes in any links in the food chain can have cascading effects throughout the ecosystem.

Today, we are adding large amounts of fertilizers to our lawns and agricultural areas. When it rains, these nutrients run off into our waterways, ponds, and lakes. If the added nutrients end up in marshes, marsh plants and algae can then use these extra nutrients to grow and reproduce faster. Scientists working at Plum Island Marsh wanted to understand how these added nutrients affect the marsh food web, so they experimentally fertilized several salt marsh creeks for many years. In 2009, they noticed that fish populations were declining in the fertilized creeks.

View of a Plum Island salt marsh.

View of a Plum Island salt marsh.

Fertilizer does not have any direct effect on fish, so the scientists wondered what the fertilizer could be changing in the system that could affect the fish. That same year they also noticed that the mudflats in the fertilized creeks were covered in mudsnails, far more so than in previous years. These mudsnails eat the same algae that the fish eat, and they compete for space on the mudflats with the small invertebrates that the fish also eat. The scientists thought that the large populations of mudsnails were causing the mysterious disappearance of fish in fertilized creeks by decreasing the number of algae and invertebrates in fertilized creeks.

A few years later, Harriet began working as one of the scientists at Plum Island Marsh. She was interested in the mudsnail hypothesis, but there was yet no evidence to show the mudsnails were causing the decline in fish populations. She decided to collect some data. If mudsnails were competing with the invertebrates that fish eat, she expected to find high densities of mudsnails and low densities of invertebrates in the fertilized creeks. In the summer of 2012, Harriet counted and collected mudsnails using a quadrat (shown in the photo) and took cores down into the mud to measure the other invertebrates in the mudflats of the creeks. She randomly sampled 20 locations along a 200-meter stretch of creek at low tide. The data she collected are found below and can help determine whether mudsnails are responsible for the disappearance of fish in fertilized creeks.

Mudsnails on a mudflat, and the quadrat used to study their population size.

Mudsnails on a mudflat, and the quadrat used to study their population size.

Featured scientist: Harriet Booth from Northeastern University

Flesch–Kincaid Reading Grade Level = 10.2

Click here for a great blog post by Harriet detailing her time spent in the salt marsh: Harriet Booth: Unraveling the mysteries of Plum Island’s marshes

If your students are looking for more information on trophic cascades in salt marsh ecosystems, check out the video below!

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Skills students will need

Before using Data Nuggets in your classroom, there are several concepts that students should be familiar with. Below is a collection of lesson plans and resources that will help prepare teachers and students before using Nuggets in the classroom.

What is a Hypothesis?

The Scientific Method

Graphing Skills

Basic Statistics