Grasses, mowing, and ecological impacts

Introduction

In this three-part open-ended experiment, students will make field measurements and observations to understand how grasses and other plants commonly found in a school or personal yard are influenced by a rather extreme limiting factor – mowing.  If mowing were stopped for even one season, what differences would students find in the types of organisms that live there? Students will: 1) make basic observations of grass taxa and other associated species, 2) observe biodiversity differences between mowed and unmowed lawn, and 3) make observations of a transition zone between disturbed (mowed) and less disturbed environments. 

It is not expected that students will have prior taxonomic knowledge to carry out the activity. Students will do their best to distinguish between species in their study areas, but it won't be necessary to identify the species by name in order to discuss principles of biodiversity, abundance, traits, and other variables. The activity can also be used to foster discussion of topics such as why certain taxa preferentially survive in particular environments, natural and artificial selection, and adaptation.

Student worksheet: A downloadable pdf of a student worksheet for this activity includes some of the content on this web page, together with space for students to record observations and answers to questions. Please refer to the worksheet as you read the description of the activity below.

Grade level: 9-12, NGSS Standards HS-LS-1 through 5

Time required: Each part of this three-part activity can be in one 45-minute class period. 

Lesson format: Parts of this activity require sampling and observing grassy yards outdoors, such as in a school yard, park, or at home. Ideally students will participate in sampling, but samples could also be brought to the classroom by the teacher. The analysis and discussion can be done within the classroom. 

Activity Part 1: Making Observations of a Lawn

This activity is a good place for students to practice collecting observations about the area of lawn that they are exploring.  It is also a good place for students to practice writing good descriptions about the organisms they are observing.  Once students have made their samples (of a complete plant), have them record observations and take measurements of each sample, using as many senses as appropriate.  Students can then use these observations to write descriptions so that someone else could pick out the sample using the students' description only.  A fun side activity if time permits: after the students have written their descriptions, have them trade with another student and see if they can find the other student's sample among all the samples taken. 

This section is also a good place to practice writing good hypotheses to explain the patterns they observe. It is not necessary to focus on whether the students' hypotheses are correct; rather, focus on how students express and support their ideas. Use the resources provided to talk about the differences between a dependent and an independent variable and have the students write hypotheses in part 2 of the activity on biodiversity. It may be helpful to ask students to read over the instructions for part 2 before doing part 1.

Summary: Students will take several core samples in grassy settings to engage students in focusing closely on the plant species in parts of the yard. For each core they will describe: the most abundant plant species in each core, other less abundant plant species in the core, observations about the environment and soil, and speculate as to why one species might be more abundant than the others.

Standards: HS-LS4-1, HS-LS4-4

Materials:
Core sampler 4-in. (a bulb planting corer works great!)
Any small stone
Device for taking pictures
Materials to record observations
Weeds and grasses field guide
Magnifying glass

Activity Part 2: Comparing biodiversity between mowed and unmowed areas

This comparative study looks at possible ways that grasses have adapted to environmental stresses and is a good place for students to test ideas they expressed in the first activity about why particular species dominated their mowed plot. The template in the student worksheet guides students through this experiment and helps them organize their data.  An appropriate opening conversation could include asking students to try to explain the types of limiting factors that affect the success of plants occupying both plots. You may wish to have students watch this this short video about abiotic and biotic factors). 

Once the data has been collected from both sites, have the students check to see if the estimated percent cover adds up to 100%.  This becomes important when graphing the data collected.  Have the students graphically represent their data so that they may better examine their results (students may find this video “Pie Chart Hints” to be helpful). When examining the results, have students use the pie charts and list observations and inferences they notice when comparing the results from each site. You can use the questions under “Discussion and Conclusion” section of Part 2 to guide your class discussion. Don’t be afraid to contribute your own conclusions and reasoning, pointing out aspects you are unsure about, to affirm for students that part of doing science is making and supporting hypotheses when a "right" answer is not already known through previous research.

When students examining which plot has the greatest biodiversity, you may wish to have students read an introduction to biodiversity and its important on Earth@Home or watch a short video with David Attenborough on why biodiversity is important. 

Standards: HS-LS4-1, HS-LS4-2, HS-LS4-3, HS-LS4-4, HS-LS4-5

Materials:
Meter stick (or yard stick)
4 pencils and string to make a quadrat (for instructions, see the video "How to Make a Quadrat")
Data sheet
Device for taking pictures
Area of mowed lawn and unmoved lawn

Activity Part 3: Environment and biodiversity across a mowed-unmowed transition zone

This section of the activity is a good place for students to use the information and skills they gathered in the first two sections. In this section students will examine how these two sites (habitats) are different and similar and how the organisms living there have adapted to these habitats.  Since each species has a large number of possible adaptations, have the students concentrate on just a few  (sunlight, water, nutrients, and reproduction). Have students create a data table where they can record their observations.  Students can watch a short video on environmental transitions at a larger geographic scale in a bioregion of southern Ontario, Canada to deepen their understanding of the concept.

When students have finished their research, have them examine the transition zone and answer a few guiding questions:

1. What are some ways the plants in the areas on either site of the transition zone are different?
2. What are some characteristics of the grasses on the transition zone line?
3. What are some ways plants in this transitions zone are:
   • optimizing their sunlight time?
   • gathering water?
   • collecting nutrients from the soil?
4. How are plants optimizing their reproductive success?

Of course, answering these questions accurately requires extensive environmental data and knowledge of plant anatomy and physiology -- these are topics of ongoing scientific research. Students can, however, make meaningful hypotheses, and guided discussion of possible answers may help students understand some of the possibilities and which answers seem more likely.

You may find reviews of grass morphology and anatomy and of grass reproduction to be helpful.

Standards: HS-LS4-1, HS-LS4-4, HS-LS4-5

Materials:
Device for taking pictures
Area of at interface mowed lawn and unmoved lawn