Populations Rise and Fall

Every winter, survey teams from Maryland and Virginia tow metal dredges along the bottom of the Chesapeake Bay. They catch blue crabs that lie buried in the mud until spring. From these catches, scientists estimate how many crabs live in the whole Bay. From the winter of 1991–1992 to the next winter, that estimate more than doubled. The Bay did not get bigger. So what changed?

Explore: run a crab cove for ten years

The simulation shows a cove of the Bay with a bar for the crab population in each of ten years. You choose how much underwater grass there is for young crabs to hide in. You choose mild or harsh winters, and you set the harvest to none, moderate or heavy.

• Bar: crabs in the cove at the end of the year
• Blue column: a harsh winter
• Square: a crab pot (1 is moderate harvest, 2 is heavy)

1. Underwater grass for young crabs to hide in

Low Medium High

2. Pick the years to change All 10 years

All years Years 1–5 Years 6–10

12345678910

3. Winter in those years

Mild Harsh Random

4. Harvest in those years

None Moderate Heavy

Resample random winters Reset

In this sketch, all the numbers are made up. The cove starts half full and holds 100 crabs at most. Each year new young crabs that survive are added: 25, 50 or 100 for every 100 crabs with low, medium or high grass. Crowding cuts this to nothing as the cove fills. The crowding rule is invented for this sketch. A mild winter then removes 15 of every 100 crabs and a harsh winter removes 50. Harvest then removes 0, 15 or 45 of every 100. A random winter is harsh 1 year in 3. There is one kind of crab, no ages, and no predators drawn.

Rule to keep: a population grows when more crabs are added than are lost, and shrinks when more are lost than are added.

Try this.

1. Set high grass, mild winters and no harvest. Predict the shape of the ten bars, then run it.
2. Keep everything the same, but make year 4 a harsh winter. Predict what happens in year 5 and in the years after, then test it.
3. With medium grass, set heavy harvest for five years, then cut it to moderate. Predict whether the population recovers, and how fast, then run it.

Explain: what makes a population grow or shrink

A population is all the members of one species that live in the same place. The blue crabs in the Bay are one population.

A population changes in a simple way. Births and crabs moving in add to it. Deaths and crabs moving out take away from it. When more are added than lost, the population grows.

A limiting factor is anything that keeps a population from growing larger. In an earlier lesson you met food, water, shelter, disease and predators as limiting factors. When one of them changes, the population can change too.

• A biotic factor is a living part of the environment, such as predators, food or grass.
• An abiotic factor is a nonliving part of the environment, such as temperature or salt in the water.

Here are some factors that can change the number of blue crabs.

Factor	Living or nonliving	How it can change crab numbers
Very cold winter	Nonliving	More crabs die before spring.
Low salt in cold water	Nonliving	In lab tests, fewer crabs lived through a winter in cold, low-salt water.
Big crabs	Living	Adult crabs eat many young crabs.
Shallow water and grass	Living and nonliving	Young crabs can hide there from big crabs.
Harvest by people	Human action	Crabbers remove crabs every year.

When there is less of a needed thing, more individuals need the same food or hiding place. They struggle to get enough. That struggle is competition. Think of musical chairs. Take away chairs, and more players are left standing. The game breaks down in one way: real crabs do not all lose at once. Some find shelter, and some are eaten or starve.

Changes can also move where a population lives. Lab tests suggest that cold limits how far north blue crabs can survive. So warmer winters could let crabs live farther north, though lab tests alone cannot show that the move has happened.

Explain: natural causes and human causes

Some changes are natural.

• A very cold winter can kill many crabs buried in the Bay.
• A flood can drop a population below its earlier size.
• A wildfire can kill grown trees. In some pine forests, the heat also opens cones and drops seeds, so new trees start.
• A climate shift over many years can change where a species can live.

People cause changes too.

• Pollution. In one study, all of the adult coho salmon placed in untreated rain runoff from a busy Seattle road died within 24 hours. All 20 coho in the same runoff after it flowed through soil survived.
• Habitat destruction. Stone walls and other hard shorelines can shrink the shallow water where young crabs hide.
• Overharvesting. Overharvesting means people catch or collect a species faster than it can replace itself. Managers set catch limits on blue crabs and on menhaden, a small fish, to guard against this.

When a change is large, an ecosystem can lose species. Biodiversity is the variety of living things in an area. When one population crashes, the animals that eat it and the things it eats can change too. Larger changes, such as dead zones and climate change, are in Big Changes (a later lesson).

Explain: limits that help a population recover

A population that drops does not always come back. In some places after very hot forest fires, few trees were left nearby to drop seeds, and the forest may not return.

In 2008, Virginia reported that too many blue crabs were being caught each year. Virginia and Maryland set a plan to catch about 34 percent fewer female crabs than in 2004 to 2007. Females carry the eggs, so leaving more of them can mean more young crabs later.

This is sustainable harvest: taking only as many as the population can replace, so it lasts for years to come. Managers also keep counting, because a limit is not a promise. The 2008 report warned that rebuilding could take several years.

Elaborate: read the winter crab survey

Survey winter	Total crabs (millions)	Adult females (millions)
1989–1990	791	117
1991–1992	367	167
1992–1993	852	177
2000–2001	254	61
2007–2008	293	91
2011–2012	765	95
2018–2019	594	191
2021–2022	227	97
2024–2025	238	108
2025–2026	349	81

These are Bay-wide estimates from the Winter Dredge Survey of the Maryland Department of Natural Resources and the Virginia Institute of Marine Science, published by Maryland DNR (opened October 2026). Each winter’s survey samples about 1,500 sites, not one station. The total also counts young crabs and adult males. Limits on female harvest began in spring 2008, after that winter’s survey. Maryland reported that about 12 percent of adult females died during the cold winter of 2025–2026.

1. Which winter had the highest total? Which had the lowest?
2. From 2024–2025 to 2025–2026, did the total and the adult females move in the same direction? By how much did each change?
3. Find the mean number of adult females for the last four winters in the table.

Design problem

Soapy water can run off driveways into streams. Plan a fair test of whether soapy water changes how a population of duckweed grows. Duckweed is a tiny floating plant that makes new leaves by budding.

One way to do it

• Question: How does the amount of dish soap in water affect the growth of a duckweed population?
• Independent variable: soap mixed into the pond water before filling the cups, for example 0, 0.5, 1 and 2 milliliters per liter.
• Dependent variable: the number of duckweed leaves, counted every two days for two weeks.
• Control: cups with no soap.
• Constants: the same pond water, cup size, water volume (200 milliliters), light, temperature and starting number of leaves (10).
• Repeated trials: four cups for each soap level.
• Safety: wear goggles when you measure soap, wash your hands after handling pond water, and do not dump the plants or soapy water outside.
• What results would mean: if cups with more soap end with fewer leaves than the control, soap acted as a limiting factor in this test.

Evaluate: check your thinking

The Bay has lots of crabs some winters and far fewer in others because the things that limit crabs keep changing. Cold winters, hiding places, big crabs and harvest all change how many crabs are born and survive. No single graph proves one cause, and careful limits on harvest can help the population recover over time.

Science practices you used: asking a question about a real pattern, using a model and naming what it leaves out, reading a data table and finding a mean, planning a fair test with a control, and making a claim backed by evidence and reasoning.