Resource Availability in Ecosystems
Remove one piece of a food web, and the whole web can change.
Topic 4 of 5Grade 6 · Life Science
- Teacher:
- Mr. Mohamed Fawzy
- School:
- Oceans Academy
- Unit:
- From Cells to Ecosystems
How this topic is built
- Covers workbook pages 89 and 90 (Week 9: Disrupting Food Webs, the trophic cascade chart) and both workbook questions.
- Important: the workbook's "Pacific Ocean Trophic Cascade" story is not real. It is replaced by the real, well-documented orca–sea otter–urchin–kelp cascade from Alaska (see the note on slide 7).
When a piece goes missing
A food web shows how organisms are connected. So what happens when one piece is taken away?
A species' numbers can drop because of illness, human actions (like overhunting, pollution, or clearing land), or natural disasters. The species may become endangered (at risk of disappearing) or even extinct (gone forever, with none left anywhere).
Because everything in a food web is connected, losing one species affects the others.
From the workbook (page 89)
All three causes (illness, human interaction, natural disaster) are kept. "Human interaction" is given examples so students can answer workbook question 1.
Quick Check 1
From the workbook, page 90, question 1 (written as multiple choice).
Workbook question
The workbook asks students to write three ways an organism can become endangered or extinct. Expected answer: illness (disease), human interaction, natural disaster. In class you can ask it as an open question first, then use this slide.
Invasive species
A native species belongs naturally in an area. An invasive species is a plant or animal that is not native. It was brought in, usually by people, and it causes harm in its new home.
There, it may have few or no natural predators, so its numbers can grow out of control. It can outcompete native species: win the contest for food and space, leaving less for them.
Not every species from somewhere else is invasive. Tomatoes first came from South America, but Virginia's garden tomatoes don't take over forests.
A species is invasive only if it spreads and harms the ecosystem, the economy, or people's health.
Corrections from the workbook (page 89)
- The workbook defines invasive species as any species living where it "doesn't belong," brought "by human interference or a natural disaster." Scientists call a species invasive only when it is non-native and causes harm, and most are spread by people (storms can occasionally carry them).
- "Since the invasive species has no predators... it will grow out of control and continue to destroy the native ecosystem." Softened to "may have few or no natural predators" and "can": not every non-native species behaves this way.
Sort it: native or invasive in Virginia?
All of these live in Virginia. Where did each one come from, and does it cause harm?
The planted mix-ups
- Mute swan: beautiful, so students assume it belongs. It is from Europe and eats large amounts of bay grasses.
- White-tailed deer: they can be "too many" in Northern Virginia, but they are native.
- Honeybee: came from Europe, but it is managed by beekeepers and is generally not listed as invasive.
Sources
Virginia Department of Wildlife Resources; Virginia Department of Agriculture (spotted lanternfly); Chesapeake Bay Program (blue catfish, mute swan).
Quick Check 2
From the workbook, page 90, question 2.
Change to the workbook question
Workbook choice C was "They bring diseases." That is actually true for some invasive species, so it can't be a wrong answer. It was replaced with a real misconception ("They always die out quickly"). The question and answer B are unchanged.
Trophic cascades
Some predators keep a whole food web in balance. If an apex predator (or another important predator) disappears, the animals it ate can multiply. Then they eat more of the level below them.
This chain reaction through a food web is called a trophic cascade. Trophic means "feeding level," and a cascade is like water tumbling down steps. The ecosystem changes, sometimes a lot.
Corrections from the workbook (page 89)
- "Different tropic levels" is a typo for trophic.
- "Eventually leading to the demise of the ecosystem": cascades change ecosystems, sometimes a lot, but they don't always destroy them. The slide says "changes, sometimes a lot."
- Cascades can start from other important predators too, not only apex predators.
A real trophic cascade: Alaska's kelp forests
In the 1990s, scientists watched this happen around the Aleutian Islands of Alaska. Tap each animal and the kelp.
Replaces the workbook's story (page 90)
The workbook's "Pacific Ocean Trophic Cascade" says Navy sonar made orcas decline, so sea lions increased, and sea lions ate sea otters. This chain is not real: sea lions do not eat sea otters, and no study shows this cascade.
The real, documented cascade (Estes and others, published in Science, 1998): around Alaska's Aleutian Islands, orcas began eating many more sea otters, likely because the seals and sea lions they usually hunted had become scarce. Sea otters dropped sharply, sea urchins multiplied, and kelp forests turned into bare "urchin barrens." Same animals, same idea, real evidence.
Picture fix
The workbook draws a river otter standing on land. Sea otters float on their backs in the ocean; the slide draws a sea otter.
Quick Check 3
Follow the cascade step by step.
What this checks
Students must follow two steps. The most common error is stopping after one step ("otters go down, so kelp goes down because otters help kelp").
Kelp forest simulator
Simplified model · Not to scale
How to use it
- Start with many otters: a healthy kelp forest.
- Press "Fur hunters remove otters." Watch the urchins rise first, then the kelp fall. The delay shows why it's called a cascade.
- Press "Otters protected" to show the recovery that really happened after 1911.
Accuracy note
The model shows the direction of change, not real population numbers.
Home experiment 1: oil and feathers
Human actions can harm species. What does an oil spill do to a bird's feathers?
Expected result
The feather from plain water shakes dry and fluffs up. The oiled feather clumps, stays heavy and flat, and won't fluff even after drying. Dish soap removes much of the oil, which is why wildlife rescuers use it.
Note
Use craft-store feathers. Collecting feathers of most wild birds is illegal in the U.S. under the Migratory Bird Treaty Act.
Why kelp forests matter
Kelp is a giant seaweed that grows in tall underwater forests along the Pacific coast. Kelp forests are a habitat, a home, for fish, crabs, and many other sea animals. They give them food, shelter, and safe places to raise their young.
When urchins wipe out the kelp, many of these animals lose their homes.
It happened before. From the 1700s to the early 1900s, hunters killed sea otters for their thick fur until almost none were left. In many places, urchins took over and the kelp forests disappeared.
In 1911, a treaty (an agreement between countries) protected sea otters. As otters came back, they ate the urchins, and in many areas the kelp forests grew back too.
From the workbook (page 90)
Kept: kelp forests provide habitat for fish, crustaceans, and other sea life, and without them many animals lose homes and breeding grounds ("places to raise their young").
Source
The 1911 North Pacific Fur Seal Treaty protected sea otters; their recovery and the return of kelp is one of the best-known examples in ecology.
Quick Check 4
Use the history of sea otters.
What this checks
The same cascade, started by a human action instead of orcas. Connects the cascade idea to "human interaction" from slide 1.
Home experiment 2: salty water and seeds
In winter, road salt washes off Northern Virginia roads into soil and streams. Can salt harm producers?
Expected result
Most seeds in cup A sprout within 3 to 5 days. Cup B shows few or no sprouts, and they are weak.
Local link
Several Northern Virginia streams have high salt levels in winter and spring because of road salt, and local agencies work on reducing it.
A comeback story: Chesapeake Bay bald eagles
In the 1970s, fewer than 100 pairs of bald eagles nested around the Chesapeake Bay. DDT, a chemical sprayed to kill insects, washed into the water and moved up the food web, into fish and then into eagles. It made their eggshells so thin that they broke.
In 1972, the United States banned DDT, and bald eagles were protected by law. Today, thousands of pairs nest around the Bay. Human actions can harm a species, and human actions can help it recover.
Sources
Center for Conservation Biology (William & Mary) Chesapeake Bay eagle surveys; U.S. Fish and Wildlife Service. Surveys counted fewer than 100 pairs in the 1970s and more than 3,000 pairs in recent years.
Why this slide
Links the DDT story to food webs (poison builds up as it moves up the levels) and gives students a hopeful, local example.
Quick Check 5
Think about the eagle comeback.
What this checks
That humans can reverse harm. Choice C targets the common idea that animals "adapt" quickly on purpose.
Invasive species in Virginia
Northern snakehead
This fish from Asia was found in the Potomac River near Fort Belvoir in 2004. It eats native fish, frogs, and crayfish. Anglers in Virginia are asked to keep any they catch.
Blue catfish
Brought to Virginia's rivers for fishing in the 1970s and 80s, blue catfish now spread through the Chesapeake Bay's rivers and eat blue crabs and native fish.
Spotted lanternfly
First found in Winchester, Virginia, in 2019. It sucks sap from grapevines and trees, hurting Virginia's vineyards. If you see one, report it.
Mute swan
Brought from Europe as a decoration for ponds and estates. In the Chesapeake Bay, mute swans eat large amounts of bay grasses that native animals depend on.
Sources for the local examples
- Northern snakehead in the Potomac (Dogue Creek, 2004): Virginia DWR, U.S. Fish and Wildlife Service.
- Blue catfish in Virginia's tidal rivers: Virginia DWR, Chesapeake Bay Program.
- Spotted lanternfly first detected in Winchester in January 2019: Virginia Department of Agriculture and Consumer Services.
- Mute swans and bay grasses: Chesapeake Bay Program.
Vocabulary flip cards
Say the meaning out loud, then tap the card to check.
Game idea
"Invasive or not?" Read a species' story aloud; students hold up the matching flip card term.
Final quiz: Resource Availability in Ecosystems
Answer all 8, then check your answers. Every answer comes with an explanation.
Answer key
1 A · 2 A · 3 B · 4 C · 5 C · 6 D · 7 D · 8 B
Both workbook questions are Quick Checks 1 and 2.