Lesson 3 of 10
Objective: Students will be able to differentiate exponential growth from logistic growth and explain how various limiting factors ultimately determine the carrying capacity of an ecosystem.
This lesson is a direct follow up to the first part of the Population Ecology lesson sequence, describing populations. This lesson covers how patterns of population growth can follow predictable patterns, depending on a number of factors such as the species' maximum reproductive potential (biotic potential), the limiting factors such as disease and competition the restrain unrestricted, exponential growth, and the ultimate carrying capacity that an ecosystem can support.
The lesson essentially consists of two parts:
1. A pre-class textbook reading and homework assignment focused on close reading techniques, critical-thinking questions, and content vocabulary development.
2. An in class presentation that provides supplementary examples to review the concepts and vocabulary from the chapter along with a class discussion seeking to draw students into more critical examination of the topic at hand and assist in their ability to connect the concepts to their personal experiences.
The textbook reading comes from Environmental Science: Your World, Your Turn by Jay Withgott.
If you do not have that particular textbook, I would recommend finding a similar chapter or chapters and modifying the lesson accordingly.
Alternately, the powerpoint attached to the Direct Instruction section covers most of the same concepts and vocabulary as the chapter. If you have a shorter class period, you may want to skip the reading assignment and assign the discussion questions as homework. You could then hold the class discussion on the following day.
In my case, I assign the textbook reading on the meeting previous to this lesson. In that way, students will have already covered the concepts on their own and the powerpoint presentation will be less of a lecture and more of an opportunity for students to ask questions and clarify their understanding.
Connection to Standard:
In this lesson, students will prepare for class by reading and determining the central idea of a text, establish familiarity with relevant scientific vocabulary, and then draw evidence from the text to support arguments and opinions presented as part of their participation in a group discussion.
Like I mentioned above, I assign the textbook reading as a homework assignment to be completed upon arrival to this class period. The powerpoint presentation is then more of a review and an opportunity for students to ask questions.
Wondering WHY I use lectures as a pedagogical strategy? Watch this video.
Wondering HOW I use the Powerpoint to differentiate instruction? Watch this video.
Wondering why I choose to have a reading assignment AND a lecture on the same content? Read this rationale.
Wondering how you might use this lesson's resources if you don't plan on presenting a lecture? Read this reflection.
When class begins, I ask students to get their homework out and first give them about 10 minutes to discuss the critical thinking questions with their group members. During this time, I walk around and put a stamp on completed homework and answer any questions that students bring up. If students bring up a good question or insightful comment, I ask them to please remember to bring that up in the larger class discussion to follow the presentation.
Affording this time before the presentation allows students to "field test" their answers with a smaller group, increasing their confidence to participate in the larger discussion. Also, because the discussion is graded by groups, it allows the ideas of individual members to influence the thinking of their peers which may lead to greater insights or even new questions. Finally, while I walk around, I listen to the nature of student discussions and get a better sense of what kinds of questions may be floating around the room, allowing me to emphasize certain aspects of the lesson or offer more detailed examples to scaffold the instruction.
After I have stamped all the homework assignments, I distribute the note sheet that accompanies the presentation. As I've mentioned in previous lessons, offering students a note sheet provides a readymade study guide for later and allows students to focus on their thoughts and the concepts being discussed as opposed to focusing all of their attention on copying down copies amounts of notes.
Please Note: I find it important to really do thorough checks for understanding on a few points:
- The difference between a K and r selected species is important to understand differences in biotic potential and population growth patterns between different kinds of species. On the one hand, it's important for students to understand that K and r species are really on a spectrum, and that some K selected species can exhibit characteristics of r selected species, and vice versa (e.g., most trees produce many seeds, and therefore have high biotic potential, but actually have slow population growth because so few of those seeds actually become trees) . Like many concepts in science, the notion of K and r selection is a useful tool for describing patterns in nature, they are not absolute categories into which all species fit neatly. Conceptually, the two types of selection aren't too difficult for students to comprehend, but it can be challenging to remember which is which. Any kind of mnemonic will do, but I tell my students that r selected species have lots of offspring and therefore grow rapidly, whereas K selected species don't grow as rapidly because they spend a lot of time and energy taking care of their Kids. I'm sure a better mnemonic exists somewhere, but this seems to work for my students.
- It can sometimes be misleading to see the "S" curve of logistic growth and believe that populations always just "level out" at their carrying capacity. Even though the presentation goes into some detail to dispel this notion with the concept of "dieback", I try and make sure to continually repeat the point that populations almost always fluctuate around the carrying capacity. On the one hand, so many graphs both in the presentation and in textbooks show a "flat" end to the logistic curve that it's important to remind students it's not that smooth. On the other hand, I would hate for students to be misled into believing it "all works out in the end" and that populations stabilize more or less perfectly. To illustrate this better and really bring home this point, I ask students to describe what "dieback" would look like for human populations. Responses include such feelgood topics as "starvation", "water wars", and "genocide". I hate to go too dark on this point, but I really feel like it's important for students to understand what these abstract graphs would mean on a much more personal level.
Following the presentation, I let students know that we will wrap up by having a class discussion to review the concepts of the lesson. Again, depending on your class length, it may be preferable to have this follow-up discussion on the following day.
The discussion protocol for this lesson:
all groups are required to participate in the discussion and will receive a “participation” grade for the day
groups with more than one member that participate will receive a higher participation grade
groups that participate more frequently will receive a higher grade
These criteria make the group collectively responsible for their grade and accountable to each other. If no one in the group participates, the group as a whole will receive a failing grade. If only one member of the group participates, regardless of how often, the group can’t receive any grade higher than a C.
To keep track of participation, I begin by making a map of the class with the group tables labeled by group name. Since there are four students at each table, as a student from a particular group participates, I make a tally mark in the position of that student in their group. In this way, I can tally how often the group participates, which members are participating, and how often. To determine "average" participation, I add up all tally marks and divide by the number of groups, rounding down. I then use this rubric to determine their participation grades.
See this discussion guide for a more detailed explanation of how to lead this particular discussion, but I would bring the following to your attention as key points in the discussion:
1.On the clarification question regarding the difference between immigration and emigration, students often get stuck on "immigration" in the political sense of the word. This is definitely understandable considering that many of my students are either children of immigrants of immigrants themselves. Outside of this particular community, however, this may still be an issue as the dialogue on immigration seems to make the term synonymous with the political question of Latin American immigration. Besides just clarifying what the two terms mean, it might be helpful to discuss the political topic in ecological terms. By this I mean to ask, "What causes animals to migrate, whether that be immigration or emigration?" Students generally will answer something to do with resource availability or competition. This could lead to a deeper discussion about the economic and social "resources" that might drive human migrations, hopefully helping students to understand the terms better in their ecological senses as they apply to other species.
2. The question of whether human population growth will stabilize as logistic growth, or whether we will follow a "rise and crash" pattern is a great opportunity to have some early discussions about the global human population and how we are affecting the natural resources we need to survive. This question is open to student interpretation and I would likely allow students a bit of time with their groups to discuss what they think and if they can come to some consensus or not. I’m hoping that students that take too pessimistic a view consider that humans are conscious of the ways populations behave in nature and, in ideal conditions, may be able to alter our behavior to avoid a population crash. As students offer limiting factors that affect humans, I write them on the board. A few that I would bring up and add if students don’t offer them would be water, food, arable land, energy (so much of our technology and infrastructure that have pushed back historical limiting factors of famine and disease rely precariously on ready access to fossil fuels), living space, disease, war, climactic weather, etc. It might be useful to graph human population growth on an overhead or on the whiteboard to help students visualize the dramatic exponential growth we have been experiencing. This image is a great example of a fairly dramatic representation of our very recent exponential growth.