Engage: Collect macroinvertebrates either with or without the students. If doing so with students, please see the lesson in the “What is an ecosystem?” section of the Hudson River Ecosystem module titled “Aquatic Ecosystem Explorations”. This will give students some experience with the organisms. However, if you do not have time to complete the outdoor activity, simply collect some organisms ahead of time and display them at stations around the room. Ask students to rotate through the stations, writing down the major characteristics of the organisms they see:
-number of body sections
-number of legs, antennae, eyes
-other observable features: gills, tails, wings, etc
They should be able to identify the organisms using a simple dichotomous key, also available online under our Hudson River Ecosystem webpage. Once they have some familiarity with different aquatic organisms, show the students photos of the submerged aquatic vegetation (water celery) and the water chestnut; one photo is at the end of the lesson plan.
Ask: what differences would you expect to find in the different vegetation habitats? Why? What kinds of changes might you see between different habitats? Depending on students’ experience with the habitats in the river, you may need to help them by showing photographs of the water celery and the water chestnut and comparing it to different terrestrial systems (such as a meadow and a forest). Students may be more familiar with these types of habitats and will generate a list of different organisms more quickly than that of an aquatic system. Alternatively, you could collect macroinvertebrates in two different habitats (such as a stream and a pond) and compare these populations. Ask students to write down their predictions.
Explore: Advanced students should have access to computers with Excel. Ask them to open ‘Invasives and Macroinvertebrates’. There are two groups of data for two different study sites: Esopus Meadows and Cruger Island. Assign students to work on the different areas. Following the directions on the lab sheet, they should be able to create graphs showing the variation in species density in the two different habitats. If students don’t have access to Excel, or are not comfortable using it to create graphs, you can use the versions of the worksheets with the data embedded or the graphs embedded.
Explain: Macroinvertebrate survey results have varied greatly between water chestnut and water celery within the published literature, with some studies finding significantly fewer species in water chestnut beds, while other studies have shown little difference between habitats. A study by Feldman (2001) looked at the invertebrate fauna of water chestnut and water celery beds and found that the water celery supported two to six-fold more invertebrates (per m2 of the river bottom) than water chestnut. Encourage students to think about the differences between the upper, non-tidal Hudson and the area where the data they looked at comes from. The general hypothesis about the larger number of macroinvertebrates in the water chestnut bed is simply due to the larger amount of plant biomass. This is a potential benefit of the invasive plant, which has been shown to have very low levels of dissolved oxygen in the water chestnut beds during low tide. At times, the beds are hypoxic or even anoxic, which caused researchers to think that most aquatic organisms would flee the invasive plant beds. However, large numbers of macroinvertebrates call the beds home, regardless of the dissolved oxygen levels.
The water chestnut beds have created a number of changes in the river, including some positive changes such as an increase in population density in invertebrate communities, and the increased number of invertebrates when compared with water celery. Students may not feel comfortable with the variability expressed in this study, since many of them have been taught that “invasives are bad”. However, teachers should encourage a discussion about other types of invasives that have become residents in our country, including the apple tree and the earthworm. Some are ‘good’ and some are ‘bad’. Asking students to define these terms can lead to an interesting discussion on the changes that invasives have caused, realizing that some things may be ‘better’ rather than ‘worse’.
Extend: Students could use the Feldman paper, listed below in the References, to extend their knowledge about the different types of changes throughout the Hudson river. Feldman conducted a study in the freshwater, non-tidal areas of the Hudson, and found much different results. Once they have graphed the data, they can make a list of possible explanations for the differences.
Evaluate: Students should be able to explain why invasive species aren't always only "good" or "bad", depending on the types of changes they cause in the system.
Water celery seen floating below a single water chestnut plant.