Use evidence to explain cause and effect relationships between resources and population size - Middle School Life Science

SECTION 1

Historical Context & Motivation

Have you ever wondered why some years you see tons of squirrels in your neighborhood, but other years there seem to be fewer? Scientists have asked similar questions for centuries. The connection between resources (things organisms need to survive, like food and water) and population size (the number of individuals of a species in an area) is one of the most important ideas in ecology.

Our anchoring phenomenon is this: On Isle Royale, a remote island in Lake Superior, the wolf population crashed from 50 to just 2 wolves between 1980 and 2016. Meanwhile, the moose population swung wildly — sometimes growing fast, sometimes dropping. What caused these dramatic changes? The answer involves resources, competition, and cause-and-effect relationships.

1798 • Malthus and Population Limits

Thomas Malthus argued that human populations grow faster than food supplies. He warned that limited resources would eventually stop population growth.

1859 • Darwin's Struggle for Existence

Charles Darwin explained that organisms compete for resources. Those best suited to their environment survive and reproduce more, a process called natural selection.

1958 • Isle Royale Wolf-Moose Study Begins

Scientists began tracking wolves and moose on Isle Royale. This became the longest-running predator-prey study in the world, showing how resources drive population changes.

2000s • Modern Ecology and Data Analysis

Ecologists now use satellite imagery, GPS tracking, and computer models to study how resources affect populations across entire ecosystems around the globe.

The big question that connects all these discoveries is: How do changes in available resources cause populations to grow or shrink? In this lesson, you will learn to use evidence — like data tables, graphs, and observations — to explain these cause and effect relationships.

SECTION 2

Core Principles & Definitions

Before we dig into evidence, let's define the key ideas. Every organism needs certain things to survive. When those things run out, the population is affected. Understanding these principles helps you think like an ecologist.

Limiting Factors

A limiting factor is any resource or condition that restricts how large a population can grow. Examples include food, water, shelter, sunlight, and space.

Carrying Capacity

The carrying capacity is the maximum number of organisms an ecosystem can support over time. It depends on the resources available in that environment.

Cause and Effect

A cause is something that makes an event happen. An effect is the result. For example, a drought (cause) can reduce a deer population (effect).

Competition

When resources are limited, organisms must compete (struggle against each other) to get what they need. More competition often means fewer organisms survive.

Evidence-Based Explanations

Scientists use evidence (data from observations, experiments, and measurements) to support claims about cause and effect. An explanation without evidence is just a guess.

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KEY TAKEAWAY

Think of an ecosystem like a cafeteria. If the cafeteria only has 100 lunches and 150 students show up, 50 students go hungry. The number of lunches is the limiting factor, and the 100-lunch limit is like the carrying capacity. If the school adds more food, more students can eat — the population the cafeteria can support grows!

SECTION 3

Visualizing Resources and Population Growth

The diagram below shows how a population changes over time when resources are limited. Notice how the population grows quickly at first, then levels off near the carrying capacity. This S-shaped curve is one of the most important patterns in ecology.

This S-shaped growth curve shows three phases. First, the population grows slowly because there are few individuals reproducing. Then growth becomes rapid as resources are plentiful. Finally, the population levels off near the carrying capacity (K) because resources become limited and competition increases.

Look at the pattern in the diagram. When resources are abundant, the population grows fast. When the population gets close to the carrying capacity, growth slows and eventually stops. This is a clear cause and effect relationship: limited resources cause the population to stop growing. If a drought suddenly removed half the water supply, you would expect the carrying capacity to drop — and the population would shrink to match.

SECTION 4

How Resources Control Populations

The Cause-and-Effect Chain

Population size does not change randomly. There is a logical chain of cause and effect. Let's trace through how a change in resources leads to a change in population.

Step 1 — Resource change: Something in the environment changes. Maybe there is more rain, which grows more grass.
Step 2 — Effect on organisms: More grass means herbivores (plant-eaters) have more food. They are healthier and can reproduce more.
Step 3 — Population response: More babies are born than organisms die, so the population increases.
Step 4 — Feedback: As the population grows, competition increases. Eventually resources run low again, and growth slows.

A Simple Population Equation

You do not need advanced math to understand this idea. Here is a simple way to think about population change.

POPULATION CHANGE

Population Change = Births − Deaths + Immigration − Emigration

Births = new individuals born. Deaths = individuals that die. Immigration = individuals moving in. Emigration = individuals leaving. When resources are plentiful, births go up and deaths go down, so the population grows.

When food is scarce, fewer babies survive and more adults die from starvation. Deaths increase and births decrease. The population shrinks. This is the cause and effect relationship we see again and again: resource levels are the cause, and population change is the effect.

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NGSS Connection: Crosscutting Concept

The crosscutting concept here is Cause and Effect. Scientists look for mechanisms — the step-by-step processes — that explain why a change in one thing leads to a change in another. When you explain how a drought causes a deer population to shrink, you are describing a mechanism.

SECTION 5

Types of Limiting Resources

Not all resources affect populations in the same way. Ecologists group limiting factors into categories. The diagram below shows common types of resources and how they connect to population outcomes.

This diagram shows three categories of limiting factors: food and water, space and shelter, and predators and disease. Each box shows how a change in a resource (cause) leads to a change in population (effect). Notice that scientists rely on evidence such as data tables, graphs, and observations to support these cause-and-effect claims.

Resource / Factor	When It Increases	When It Decreases
Food supply	Population grows — more energy for reproduction	Population shrinks — starvation and fewer births
Water availability	Organisms stay healthy and populations grow	Dehydration kills; organisms leave the area
Space / territory	More nesting sites; less fighting; population grows	Overcrowding leads to stress, disease, and decline
Predators	More prey are eaten; prey population shrinks	Fewer prey are eaten; prey population grows
Disease	More organisms die; population drops quickly	Fewer deaths from illness; population can recover

How different factors cause population increases or decreases

SECTION 6

Worked Example: Isle Royale Wolves and Moose

Let's return to our anchoring phenomenon — the wolves and moose of Isle Royale. Suppose a scientist collects the following data and wants to explain why the moose population changed.

Year	Wolves	Moose	Winter Snowfall
2005	30	540	Normal
2008	23	650	Low
2011	16	515	Very High
2014	9	1,050	Normal
2016	2	1,300	Low

Simplified Isle Royale population data

Explaining the Moose Population Boom

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Step 1 — Identify the Claim

We want to explain this claim: The moose population grew because the wolf population declined. Wolves are predators of moose. Fewer wolves means fewer moose are killed.
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Step 2 — Find the Evidence

Look at the data table. From 2005 to 2016, the wolf population dropped from 30 to just 2. During the same period, the moose population rose from 540 to 1,300.

Evidence: Wolf count decreased by 28; moose count increased by 760.
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Step 3 — Explain the Cause and Effect

The cause is the drop in wolves. With fewer predators, fewer moose were killed each year. The effect is that more moose survived and reproduced. Each year, births exceeded deaths, so the moose population grew.
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Step 4 — Consider Other Factors

Notice that in 2011, moose actually dropped even though wolves were declining. The data shows very high snowfall that year. Deep snow makes it hard for moose to find food. This tells us that multiple factors can affect a population at the same time.
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Step 5 — Write the Full Explanation

The moose population on Isle Royale increased from 540 to 1,300 between 2005 and 2016. The evidence shows that the wolf population dropped from 30 to 2 during this time. With fewer predators to kill moose, more moose survived each year, causing the population to grow. However, in 2011, very heavy snowfall reduced food access and the moose population temporarily declined. This shows that population change results from multiple cause-and-effect relationships acting together.

Complete explanation uses evidence + identifies cause and effect + considers multiple factors.

SECTION 7

Types of Evidence: Strengths and Limitations

Not all evidence is the same. Some evidence is stronger than others. As a developing scientist, you should be able to evaluate the strength of different types of evidence when explaining cause and effect relationships.

Type of Evidence	Strengths	Limitations
Controlled experiment	Can show direct cause and effect by changing one variable at a time	Hard to do with large wild populations; may not reflect natural conditions
Long-term field data	Shows real patterns in nature over many years	Many variables change at once; hard to isolate one cause
Computer model	Can simulate "what if" scenarios that are impossible in real life	Only as good as the data put into it; may oversimplify nature
Single observation	Quick and easy to collect; good starting point	One observation cannot prove cause and effect; could be coincidence

Comparing evidence types for studying populations

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KEY TAKEAWAY

Think about it like a detective solving a case. One fingerprint at the scene (a single observation) is interesting, but not enough to convict someone. DNA evidence, security footage, and witness statements together (multiple types of evidence) make a much stronger case. In ecology, the best explanations use multiple lines of evidence to support a cause-and-effect claim.

SECTION 8

Connecting to Broader Ecosystem Ideas

Understanding how resources affect populations is the foundation for many bigger ideas in ecology. As you continue studying science, you will build on these concepts. Here is how what you learned today connects to what comes next.

What You Learned Today	Where It Leads
Resources limit population size	Ecosystem stability — how ecosystems stay balanced over time
Competition for resources	Natural selection — organisms with better traits survive and pass genes on
Predator-prey relationships	Food webs and energy flow — tracing energy through entire ecosystems
Using data as evidence	Scientific modeling — building mathematical models to predict population changes
Carrying capacity	Human impact — how people affect carrying capacity through habitat destruction, pollution, and climate change

From today's lesson to future concepts

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NGSS Three-Dimensional Learning

In this lesson you practiced all three dimensions of NGSS. DCI: Organisms compete for resources, and populations change when resources change (MS-LS2-1). SEP: You constructed explanations from evidence. CCC: You identified cause and effect relationships. These three dimensions work together to help you think like a scientist.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A scientist notices that a rabbit population in a meadow has stopped growing even though there are no predators nearby. Which of the following is the most likely explanation? A) The rabbits have decided to stop reproducing. B) The meadow has reached its carrying capacity for rabbits. C) The rabbits are migrating to a new area every winter. D) A new predator species just arrived but has not been observed yet.

Show answer

The correct answer is B. When a population stops growing, it usually means the ecosystem has reached its carrying capacity — resources like food, water, and space are no longer enough to support more rabbits. Option A is wrong because animals do not consciously decide to stop reproducing; reproduction is driven by biology and resources. Option C is wrong because migration does not explain a population leveling off in a fixed area. Option D is wrong because the question states no predators are nearby, and speculation without evidence is not a valid scientific explanation.

Question 2: BASIC CALCULATION

In a pond ecosystem, 80 fish are born in one year, 50 fish die, 10 fish swim in from a connected stream, and 5 fish swim out. What is the population change for this year? A) 25 B) 35 C) 45 D) 135

Show answer

The correct answer is B. Use the formula: Population Change = Births − Deaths + Immigration − Emigration. Substituting: 80 − 50 + 10 − 5 = 35. The population grew by 35 fish. Option A (25) results from forgetting to add immigration. Option C (45) comes from adding emigration instead of subtracting it. Option D (135) incorrectly adds all numbers together without using subtraction.

Question 3: INTERMEDIATE

A graph shows that the deer population in a forest increased sharply for 5 years, then suddenly dropped. A second graph shows that rainfall in the area was very low during the year the deer population began to fall. Which statement best uses this evidence to explain the cause and effect? A) The deer left because they were bored with the forest. B) Low rainfall reduced plant growth, which decreased the deer's food supply, causing the population to drop. C) The deer population dropped because populations always go down after going up. D) Low rainfall made the deer drink less water, so they gained weight and had more babies.

Show answer

The correct answer is B. This answer correctly traces the cause-and-effect chain: low rainfall → less plant growth → less food for deer → population decline. It uses the graph evidence to build a logical mechanism. Option A is not scientific — animals do not make decisions based on boredom. Option C describes a pattern but does not explain the mechanism or use the rainfall data as evidence. Option D gets the cause right (low rainfall) but the effect wrong — less water does not cause weight gain and more reproduction.

Question 4: APPLIED

A city builds a new housing development on the edge of a wetland, destroying half of the wetland habitat. A student predicts that the frog population in the remaining wetland will decline. Which evidence would BEST support this prediction? A) A news article about frogs being interesting animals. B) Data showing the number of frog egg masses decreased by 60% in the two years after construction. C) A photo of one dead frog found near the construction site. D) A computer model showing that frogs can live in any type of habitat.

Show answer

The correct answer is B. Quantitative data (numbers) collected over time provide the strongest evidence for a population change. A 60% decrease in egg masses directly shows fewer frogs reproducing, which supports the claim that habitat loss caused the population to decline. Option A is an opinion piece, not scientific evidence. Option C is a single observation — one dead frog does not prove the whole population declined. Option D contradicts real ecology, since frogs depend on wetland habitats and cannot live just anywhere.

Question 5: CRITICAL THINKING

On a small island, scientists track a lizard population for 10 years. The population grows from 200 to 800, then fluctuates between 750 and 850. A new insect species is then introduced to the island, providing an additional food source for the lizards. Predict what will happen to the lizard population and explain the cause and effect using at least two pieces of reasoning. A) The lizard population will stay exactly the same because carrying capacity never changes. B) The lizard population will decrease because a new insect species will compete with lizards for space. C) The lizard population will increase and stabilize at a new, higher carrying capacity because more food is available. D) The lizard population will go extinct because too many insects will overwhelm the island.

Show answer

The correct answer is C. Here is the reasoning: First, the data shows the lizard population leveled off around 750–850, meaning it was near carrying capacity and food was a limiting factor. Second, the new insect species adds food resources. More food means more lizards can survive and reproduce (cause: more food → effect: more births and fewer deaths from starvation). The population should grow until it reaches a new, higher carrying capacity set by the increased food supply. Option A is wrong because carrying capacity absolutely can change when resources change. Option B is wrong because insects are food for lizards, not competitors for space. Option D is wrong because additional food supports a population rather than causing extinction.

SUMMARY

Lesson Summary

Summary

In this lesson, you learned that resources such as food, water, space, and shelter control how large a population can grow. Every ecosystem has a carrying capacity — the maximum number of organisms it can support. When resources increase, populations tend to grow. When resources decrease, populations tend to shrink. These are cause and effect relationships. Limiting factors like drought, predation, disease, and competition all play a role in determining population size.

Scientists use evidence — including data tables, graphs, experiments, and field observations — to explain these relationships. Strong explanations trace the mechanism (the step-by-step process) that connects a resource change to a population change. The Isle Royale wolf-moose study is a powerful example: as wolves declined, moose populations surged because fewer moose were killed by predators. Remember to use multiple lines of evidence and consider multiple factors when constructing your explanations.