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Optional: Dispersal Reading
Engage:
In small groups, show students two aerial maps: one historic and one current laminated map of the schoolyard you are studying.
Note: If students are unfamiliar with aerial maps, it might be a good idea to show students some aerial photos of different places on a smartboard and examine them together first. This will help them get oriented to the type of maps they will be working on. Ask students questions such as:
When you feel your students can identify features on an aerial map, pass out the two maps.
Give students 5-10 minutes for the following task:
Provide wet or dry erase markers for students to mark where they see differences. After 5-10 minutes, display the two maps side by side on a Smartboard and go around asking students first to identify the areas where there are changes. Next ask what is different. In some cases, the changes are in development (new houses, a new school, new roads). Other changes might be the loss of farm land, forest growth, or loss of forest. Have students identify as many as possible.
Next, focus student’s attention to areas of new trees/forest and ask them how they think they got there. If your schoolyard does not show any changes from past to present, you can use the maps attached and/or find some images from nearby sites that students are familiar with that do show changes. Generate a list of possible reasons. Students may indicate some of the following:
Help lead students to the idea of seeds with questions. “How can the forest expand?” “What would a tree need to start growing?” Some students might not realize that trees grow from seeds. If that is the case, show them a maple seed and ask them what it is. If students have a hard time coming up with ideas or if they are struggling with background knowledge, show them some videos about seeds:
Explore:
With the idea of seeds in their minds, have students investigate possible dispersal methods of seeds. Students might not be familiar with the term “dispersal.” If this is the case, simplify this idea for them by explaining what dispersal is: the process of distributing seeds over a specific area. Split students up into groups and give each group the bag of various different types of seeds and fruit. Tell students: Sort the seeds based on how you think the seeds are dispersed, or moved, from place to place. Ask students to make a list of the types of dispersal methods that they come up with. After the allotted time, have groups share with each other their criteria for sorting the seeds and the dispersal methods they came up with. Have groups share their ideas for the dispersal of certain seeds. This will be a good formative assessment which will help you determine what students already know or intuit about how seeds are dispersed.
Next, students will conduct an experiment on wind dispersal. Students will create wind dispersal mechanisms for a seed and collect data on time aloft and distance traveled. The student worksheet outlines the procedure for the activity of designing a seed and testing its viability for wind dispersal. Note: This investigation is only a study of wind and will not show water, animal or another method of seed dispersal.
Explain:
Have students compare their models of seeds to determine which seed model went the farthest. Ask students if they had to make a trade-off when creating the model - for example, did heavy seeds travel as far as light seeds? Was it possible to make a heavy (large) seed that also traveled far? Show students a coconut and a milkweed seed-pod. If you do not have these seeds, show them pictures on a smartboard or projector. Discuss what the possible trade-offs are with students. Have them generate some ideas. Some ideas might include: large seeds won’t go very far, will have less offspring and more food stored whereas small seeds like the milkweed will go farther but have less food, so they may not have enough resources to germinate. Explain that size and shape play a key role in plant life because plants tend to either produce a large number of small seeds or a small number of large seeds. Large seeds typically have enough energy stored in the seed to grow in low lighting (such as underneath a canopy) where small seeds need an abundance of light early on.
In understanding seed dispersal, it is important to recognize these multiple factors:
You may choose to have students take notes on the accompanying graphic organizer.
Wind and Water
Ask students to compare the seeds that have been provided as well as the seeds they made in the activity to discuss what about the seed made it go far. Some plants produce seeds with aspects that make them easily able to be carried by the wind to a new location. These can include wings as in a maple tree or parachutes like a fluffy dandelion seed. Wind dispersed seeds cannot be very large or heavy; plants that produce wind-dispersed seeds tend to make lots of seeds. Seeds that are carried by water vary in size and shape, but must be able to withstand water. Water-dispersed seeds can travel very long distances, but this may mean that they land in a place where germination is difficult or impossible.
Self-Propulsion
Some seeds disperse by generating a high amount of turgor pressure until they literally explode; this usually happens because as the pod dries out, the cells contract and "shrivel", thus causing the pop to release the seeds. These seeds, however, typically stay close to the parent plant.
Animal Dispersal
There are many different ways that seeds are dispersed by animals. Some plants have juicy fruit that contain seeds such as apples, blackberries, and strawberries. Animals (and humans alike) like to eat them. The animal digests the fruit but the seed passes through the digestive system and often ends up far from the parent plant. It takes a lot of resources for a plant to create a fruit surrounding a seed. Seeds within fruits tend to be smaller than seeds that do not have a fruit covering. In addition to fruit, plants may create seeds that are sticky such as mistletoe and birds end up with seeds stuck to their beaks. Or a plant may have a burr or a hook that gets attached to an animal and gets carried far away. Animals such as squirrels gather and move acorns, burying them in the ground. However, these seeds do not always end up in a place that is favorable for germination, or they may be partially eaten and thus destroyed.
Human Dispersal
Humans have been both intentionally and unintentionally altering the landscape for millennia, and this includes altering the distribution of seed-bearing plants. One reason for this is related to the increased mobility of people and goods associated with globalization. We build roads through many different landscapes, and frequently plant specific vegetation along these pathways. Also, some species are better adapted to habitat edges, so creating more edge by building roads may help those species thrive. In cities and towns, trees that are resistant to air pollution or are easy to care for are often planted along streets. Some cities such as Portland, OR are specifically using plants to absorb storm water runoff. All of this intentional plant dispersal can alter native ecosystems and promote introduced species in habitats where they may become invasive.
For example, in the Adirondacks, it is clear that the invasive plants have been moved mostly by people due to the way the plants cluster around roads:
Humans also unintentionally move plants around when seeds get stuck to our shoes or on our clothing, and we can disperse these seeds much farther than other animals are able to. In addition to the changes due to travel, we use land for agriculture and buildings. Many homeowners alter their landscapes to fit their wants and needs, with ornamental plants or restricting native plant growth with chemicals. Show students the two maps again and engage them in a discussion as to why people might plant trees in certain areas. Students should consider a variety of purposes including shade, aesthetics, size, storm water management and erosion.
Extend:
Evaluate:
Use the diagram of Cary Institute Plots provided as a handout to answer the two questions:
Choose two tree species and:
1. Predict their dispersal mechanism based on where they are located.
2. Describe characteristics of the seed you would expect to see.
