People Change Habitats

In the 1970s and 1980s, people put blue catfish into Virginia’s James, Rappahannock and York river systems. Today these big fish are common in parts of those rivers.

Blue catfish are native to the Mississippi River basin and other waters outside Virginia’s Bay rivers. People stocked them here to give anglers big fish to catch. Their numbers grew fast in the 1990s. What does this new fish mean for the crabs, small fish and native catfish already living there?

Explore: release a new fish into a model river

The simulation shows a Virginia river as four population bars: blue catfish, native white catfish, blue crabs and small fish. A food bar shows how much prey is available. You choose how many blue catfish to release, how much shoreline habitat is lost to building, and how hard people fish for blue catfish.

Blue catfish released

None A few Many

Fishing for blue catfish

None Some Heavy

Shoreline habitat lost to building: 0 percent

Play 20 years Reset Year 20 of 20

In this sketch, nothing has changed, so every bar stays on its dashed line.

Made up for this sketch, in relative units, not measurements: a few and many releases, the food supply, the share of blue catfish fished each year, and how much each kind loses to habitat loss or to being eaten. In this sketch, blue catfish are made to use the shared food slightly better; the lesson does not say why they spread. Catfish are not changed by lost habitat, and the food bar does not change with habitat loss. Small fish losing habitat is an assumption of this sketch. In this sketch, blue catfish are made to take more small fish than crabs. Four species, the same food every year, no seasons, no random scatter. Not to scale.

Rule to keep: when two species use the same limited food, more of one leaves less for the other, and people can raise or lower a population by what they do to its habitat and its competitors.

Try this.

1. Predict what happens to the white catfish bar when you release many blue catfish. Then test it.
2. Keep the blue catfish released. Predict how losing half the shoreline habitat changes the crabs and small fish. Test it.
3. Predict whether heavy fishing for blue catfish brings the other bars back all the way, part of the way, or not at all. Then play 20 years and check.

Explain: people are part of ecosystems

Your lunch, your tap water and the wood in your house all came from ecosystems. People are living parts of ecosystems. We depend on them for food, water, energy and other resources.

A habitat is the place where an organism lives and finds the food, water and shelter it needs (more on habitats). A population is all the members of one species living in one area at the same time.

People change habitats in many ways:

• Land use is how people use an area of land, such as for farms, homes, roads or forest.
• Farming changes fields. Rain can carry fertilizer and manure from fields into streams.
• Taking resources from nature, such as wood, coal, fish or gravel, changes habitats too. Digging gravel out of a stream changes the shape of the streambed.

Some of these actions disrupt ecosystems:

• Habitat destruction is damage that removes or ruins a habitat, such as filling in a marsh to build homes. Animals that lived there must move or die, so their populations there often drop.
• Pollution is harmful material added to air, water or soil. Extra fertilizer in a river can feed fast algae growth, and later the water can run low on oxygen (the steps are here).
• Bringing in an invasive species is a third kind, explained next.

A food web links species by who eats whom (food webs). So a change in one population can spread through the whole web. Some populations go up, and others go down.

People and ecosystems also affect each other through climate, water supply, air quality, energy production, ocean acidification and waste management. Ocean acidification means ocean water becoming more acidic as it takes in carbon dioxide from the air.

Not all human impacts are negative. In Virginia’s coastal bays, people spread about 74.5 million eelgrass seeds. A 2020 study reported that underwater grass grew back over an area of 3,612 hectares, and the total mass of fish there rose (Helping Ecosystems Recover).

Explain: a new fish in the river

A native species is a species that lives in a place naturally, without people bringing it. An invasive species is a species brought to a new place that spreads and harms native species. Being new is not enough; it must spread and cause harm.

	Blue catfish	White catfish
Native to Virginia’s Bay rivers (James, Rappahannock, York)?	No. Native to the Mississippi River basin and other waters	Yes
How it got here	People stocked it: James and Rappahannock in the 1970s, York River system in 1985	Already here
What it eats	Plants and animals, including fish and sometimes blue crabs	Prey similar to what blue catfish eat

Blue catfish can get very big. The U.S. Geological Survey lists the longest one at 165 cm. That is about as long as many adults are tall.

Competition is when two or more species need the same resource, such as food or space, and there is not enough for all of them (more on competition). Competition does not need a fight. If blue catfish eat many of the small fish and crabs that white catfish also eat, less food is left for the white catfish. Scientists report that the two catfish eat similar prey, so they may compete.

A river’s food supply is a bit like a pizza at a party. If a new guest eats a lot, less is left for everyone else. The comparison breaks because a river’s food grows back, and its amount changes with the seasons.

Effects can spread. If blue catfish eat many blue crabs, fewer crabs may be left for other crab eaters, including people.

Elaborate: read the James River surveys

Scientists from the Virginia Institute of Marine Science (VIMS) pull a net along the river bottom at many places and count the catfish caught. The catfish index uses the tows, or net pulls, from September to November. From those counts they calculate a survey index. It tracks how common a fish is from year to year. It is not a count of every fish.

Year	Blue catfish survey index	White catfish survey index
1988	0.00	0.20
1990	0.00	1.83
1995	8.97	1.31
2000	17.44	0.05
2005	47.07	0.15
2010	16.45	0.41
2015	58.84	0.20
2019	317.77	0.23
2020	47.74	0.32
2022	54.42	0.16

Source: VIMS trawl survey of the James River, five-minute daytime bottom tows, September to November each year, older fish (age 1 and up, judged by length); Tuckey and Fabrizio, 2023 annual report, Tables 9 and 19. The two indexes use different math and different parts of the river, so compare each fish with itself over time. Values from 2015 on were adjusted for a new boat and net. The years are not evenly spaced. A 0.00 means no fish of that kind were caught in that survey, not that none lived in the river.

1. In which year does the blue catfish index first rise above zero in this table? Describe its trend after that.
2. Compare 1995 with 2022. About how many times larger was the blue index? By about what percent did the white index fall?
3. Find the mean of the ten white catfish values. Why should a scientist not trust the 2019 blue catfish value by itself?

Design problem

A town plans new homes beside a creek. Some people want lawns down to the water. Others want to keep a strip of plants along the bank as habitat. Plan a test, using trays of soil, of whether a strip of plants keeps soil out of the creek.

One way to do it

• Question: Does a strip of grass at the low end of a soil tray reduce the soil washed off by rain?
• Independent variable: a strip of grass sod 10 cm wide at the low end, or bare soil.
• Dependent variable: the mass of soil in the runoff, in grams, after the water dries in a pan.
• Control: trays of bare soil with no grass strip.
• Constants: the same soil, tray size, tilt, and 1 liter of water poured from the same watering can for 1 minute.
• Repeated trials: three trays of each kind; find each mean.
• Safety: wear gloves, wipe up spills so no one slips, and wash your hands.
• Meaning: less soil from the grass trays supports keeping plants along the bank. A tray is a model, not a real creek.

Evaluate: check your thinking

When people brought blue catfish into Virginia rivers, the new fish spread. In the James River surveys, blue catfish became far more common while native white catfish became rarer. The two eat similar prey, so they may compete for food without fighting. Land use, habitat destruction and pollution change populations too.

Science practices you used: reading a data table and finding a mean, comparing changes over time, testing a model and naming its limits, planning a fair test, and making a claim from evidence.