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As you know, Dr. Lucardi is interested in invasive plants and weeds. Below is a Natural Inquirer article about a topic related to her interests. It focuses on an invasive tree species in Hawaii. Use the links to see an example of developing a testable question (introduction), planning to test your question (method), analyzing your data (results), and explaining it all (discussion).
Visit Don't Litter the Stream to see the whole article.
Introduction | Method | Results | Discussion
Don't Litter the Stream
Figure 1: Albizia growing on the island of Hawai‘i
Figure 2: Albizia was brought to Hawai‘i from islands located north and northeast of Australia
Figure 3: Algae in St. Marks National Wildlife Refuge, Florida. Photo courtesy of Steve Hillebrand and U.S. Fish and Wildlife Service
Figure 5: This downstream area was invaded by albizia trees.
 Figure 6: Caddisfly larvae live in the water. Caddisflies are not native to Hawai‘i.
 Figure 7: Amphipods, native to Hawai‘i, look like shrimp
 Figure 8: Crayfish, not native to Hawai‘i, also live in the water. You might have found crayfish in streams near your home or school. Photo courtesy of the U.S. Fish and Wildlife Service
 Figure 9: Swordtails, a popular aquarium fish, are found in Hawaiian streams but are not native to Hawai‘i
.png) Figure 10: Guppies are a popular aquarium fish found in Hawaiian streams. Guppies are not native to Hawai'i.
Figure 11: A ration-mass spectrometer
Figure 12: The scientists found a greater proportion of one type of nitrogen in the plant and animal species living in the stream area with albizia. Although the pattern was similar for every plant and animal species studied, not all the differences can be considered certain. When scientists are fairly certain that an observed or measured difference is not caused by chance, they say it is a significant difference. In this figure, significant differences are shown by the addition of an asterisk (*) above the bar. Introduction
From 1920 to 1950, a tree species called albizia (Falcataria moluccana) was brought to Hawai‘i from islands located north and northeast of Australia (figures 1 and 2). Albizia trees were planted to grow new forests and for landscaping. A fast-growing tree, albizia now grows all over Hawai‘i. Albizia has been so successful, it is taking over the places where native trees have grown.
The scientists in this study had observed an increase in nitrogen in some Hawaiian streams. The places where nitrogen was increasing were areas with albizia trees growing along the streams. When albizia leaves fall to the ground, they eventually decay. The nitrogen in the leaves goes into the soil.
Over time, this nitrogen can get into streams. Some of the albizia leaves fall directly into the stream. The scientists wondered if aquatic organisms were eating these albizia leaves. If the organisms were eating the albizia leaves, then albizia trees may be changing the nature of Hawaiian stream food webs.
This is because, naturally, Hawai‘i has little nitrogen in its plants, soil, and streams. Usually, aquatic organisms in Hawaiian streams eat algae (figure 3). Algae are aquatic primary producers (See “Thinking About the Environment”). The scientists thought that albizia leaf litter might be replacing algae as the base of the food web. They wanted to know whether organisms living in Hawaiian streams with albizia trees growing nearby were eating albizia leaf litter instead of native algae.
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Method
The scientists had to find streams to study that were alike in almost every way. To answer their question, the presence of albizia trees growing nearby should have been the only difference between the streams.
The scientists located two different areas of one stream. This stream had an upstream area that had no albizia trees and a downstream area that was invaded by albizia trees (figures 1 and 2).
The scientists measured three times the amount of nitrogen in the stream’s water where the albizia trees were growing. Otherwise, the stream areas were similar. The scientists collected samples of the following from each area:
- Algae
- Caddisfly larvae (moth-like organisms whose larvae are aquatic (figure 6)
- A species of amphipod (small, shrimplike organisms) (figure 7)
- American swamp crayfish (figure 8)
- Swordtails (figure 9)
- Guppies (figure 10)
In the downstream area, the scientists also collected albizia leaves from the shore. All of the samples were collected within a few weeks of each other in August 2006.
Using technology called an isotope ratio mass spectrometer (spek trä mə tər, image at right), the scientists identified which kind of nitrogen atoms and what proportion of each was present in everything but the albizia leaves.
They did this identification by testing small pieces of the consumers’ tissues. Then the scientists used a special computer program to identify the source of food in the consumers’ diets.
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Results
The scientists found that the plants and animal species living in the stream area with no albizia contained a smaller proportion of one type of nitrogen than the species living in the area with albizia (figure 12). Figure 12. The scientists found a greater proportion of one type of nitrogen in the plant and animal species living in the stream area with albizia. Although the pattern was similar for every plant and animal species studied, not all the differences can be considered certain. When scientists are fairly certain that an observed or measured difference is not caused by chance, they say it is a significant difference. In this figure, significant differences are shown by the addition of an asterisk (*) above the bar.
In a food web, some animals eat plants and some eat plants and other animals. These animals may themselves become food for other animals.
Tables 1 and 2 show the estimated percentage of each food source for each consumer. Note that in Table 2, albizia is included as a food source.
Table 1. The estimated percentage of food source for consumers in the stream area with no albizia trees. (Note: < means “less than,” > means “greater than,” and ≈ means “approximately”) 
(Click table for larger image)
Table 2. The estimated percentage of food source for consumers in the stream area with albizia trees.
(Click table for larger image)
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Discussion
Because amphipods and caddisfly larvae are a food source for other animals, the higher levels of nitrogen will be passed up the food web. If albizia continues to spread across Hawai‘i and especially along streams, the natural stream food web could change from one of native origin to one of invasive origin. If albizia trees invade across Hawai‘i, many other changes could occur that would change the stream food webs across Hawai‘i.
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