AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/8/8-7)
Unit 8 · Topic 8.7
8.7 Disruptions in Ecosystems
Ecosystems change when new pressures arrive. Which individuals survive depends on variation already present, since mutations aren't directed by need, and sometimes heterozygotes survive best. Invasive species, human activities that cause biomagnification and eutrophication, and geological and weather events all reshape ecosystems and can drive extinctions.
Key terms
- invasive species
- biomagnification
- eutrophication
- heterozygote advantage
- adaptation
- habitat change
Variation meets a changing environment
An adaptation is an inherited trait that selection has favored because it helps its owner survive or reproduce in a particular environment. When an ecosystem is disrupted, whether a population survives depends on whether some individuals already carry useful variation. Mutations are random: they aren't produced because an organism needs them. The environment then determines which existing variants succeed.
Sometimes the heterozygous genotype has higher fitness than either homozygote. This is heterozygote advantage. People with one sickle-cell allele and one normal hemoglobin allele are more resistant to malaria than people with two normal alleles, and they don't have sickle-cell disease, which people with two sickle-cell alleles do. So where malaria is common, selection keeps the sickle-cell allele in the population even though it's harmful in homozygotes.
Invasive species
An invasive species is one introduced, on purpose or by accident, to an area outside its native range, where it spreads and causes harm. Invaders often succeed because they've left their natural predators, parasites and competitors behind, so they can exploit an open niche or outcompete native species for resources.
Kudzu, a vine from Asia, was planted widely in the southeastern United States in the 1930s and 1940s to control erosion. It grows so fast that it smothers trees and shrubs. Zebra mussels, native to the Black and Caspian Sea region, arrived in the Great Lakes in ships' ballast water in the 1980s. They filter huge amounts of plankton from the water, crowd out native mussels and clog water intake pipes.
Human impacts
Human activity speeds up ecosystem change at local and global scales. Logging and urbanization destroy and fragment habitats. Monocropping (planting one crop over huge areas) lowers diversity and invites disease. Global climate change shifts temperature and rainfall patterns faster than many species can adapt. Introduced diseases, like Dutch elm disease (a fungus spread by bark beetles that killed tens of millions of American elm trees) and potato blight, can devastate species with little resistance. Two processes are especially important:
- Biomagnification: some toxins, such as the pesticide DDT and mercury, aren't broken down or excreted and build up in body fat. Each predator eats many prey and keeps their toxins, so concentrations rise at each trophic level. Top predators end up with the highest doses. DDT caused thin eggshells in birds like bald eagles and peregrine falcons, and populations crashed until DDT was banned in the U.S. in 1972.
- Eutrophication: fertilizer or sewage runoff adds excess nitrogen and phosphorus to lakes and coastal waters. Algae grow explosively (an algal bloom). When the algae die, decomposers break them down and use up dissolved oxygen through cellular respiration. Fish and other animals suffocate, creating low-oxygen dead zones, like the one that forms in the Gulf of Mexico most summers.
Geological and weather events
Natural events also change habitats and where ecosystems are found. El Niño, a periodic warming of surface waters in the eastern Pacific, weakens the upwelling of cold, nutrient-rich water off South America, so plankton and the fish that eat them decline. Over millions of years, continental drift has split and joined landmasses, isolating populations and changing climates. About 66 million years ago, a large asteroid impact helped cause the mass extinction that ended the non-bird dinosaurs, opening niches for mammals. Biogeography, the study of where species live, records these changes.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Tracing eutrophication
After heavy spring rains wash fertilizer from farms into a lake, dissolved oxygen near the lake bottom drops from 8 mg/L to 2 mg/L over the summer and many fish die. Explain the chain of events.
Show the solutionHide the solution
- Step 1: Rain carries nitrogen and phosphorus from fertilizer into the lake. These nutrients were limiting algae growth.
- Step 2: With extra nutrients, algae grow rapidly and form a bloom.
- Step 3: As the algae die, they sink, and decomposers (mostly bacteria) break them down.
- Step 4: Decomposers use oxygen in cellular respiration, so dissolved oxygen falls, especially near the bottom where the dead algae collect.
- Step 5: Fish need dissolved oxygen for cellular respiration; at 2 mg/L, many can't get enough and die.
Answer: Fertilizer nutrients cause an algal bloom; decomposition of the dead algae uses up dissolved oxygen, and the low oxygen kills fish.
- Example 2Calculator allowed
Calculating biomagnification
Mercury concentrations in a lake food chain are: water 0.00002 ppm, phytoplankton 0.004 ppm, zooplankton 0.03 ppm, small fish 0.3 ppm, large predatory fish 2.0 ppm. By what factor is mercury concentrated in large fish compared with the water? Explain why.
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- Step 1: Factor = 2.0 ÷ 0.00002 = 100,000.
- Step 2: So large fish have 100,000 times the mercury concentration of the water around them.
- Step 3: Mercury builds up in tissues instead of being excreted, and each consumer eats many organisms from the level below, keeping their mercury.
- Step 4: Since energy (and biomass) shrinks at each level but the mercury stays, its concentration rises at each step.
Answer: About 100,000 times more concentrated, because mercury is retained in tissues and accumulates as it passes up each trophic level.
- Example 3
Heterozygote advantage
In a region with frequent malaria, the sickle-cell allele (S) has a frequency of about 0.1, while in a region with no malaria, it's much lower. People with SS have sickle-cell disease. Explain why the S allele is more common where malaria is present.
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- Step 1: Three genotypes: AA (normal hemoglobin, no protection against malaria), AS (no sickle-cell disease and resistant to malaria) and SS (sickle-cell disease).
- Step 2: Where malaria is common, AS individuals survive and reproduce better than AA individuals, who are more likely to die of malaria, and better than SS individuals, who have the disease.
- Step 3: Because AS individuals have the highest fitness, selection keeps both alleles in the population; S isn't removed even though SS is harmful.
- Step 4: Where malaria is absent, AS has no advantage, but SS is still harmful, so selection lowers the S allele's frequency.
Answer: Where malaria is common, heterozygotes (AS) have the highest fitness, so selection maintains the S allele; without malaria, there's no heterozygote advantage and the allele becomes rarer.
Common mistakes
- Saying organisms mutate to survive a new pressure. Mutations are random and not directed by the environment; selection acts on variation already present.
- Saying eutrophication kills fish because algae use up the oxygen. Living algae add oxygen in daylight; the big drop comes from decomposers respiring as they break down dead algae.
- Confusing biomagnification with bioaccumulation alone. Bioaccumulation is buildup in one organism over time; biomagnification is increasing concentration up the food chain.
- Calling any non-native species invasive. A species is invasive when it spreads and harms the ecosystem it was introduced into.
On the exam
- Expect cause-and-effect questions about a disruption: give each link of the chain (nutrients → algae → decomposers → low oxygen → fish die).
- Data questions may ask you to evaluate a hypothesis about a disruption, including whether to reject the null hypothesis; tie your decision to the numbers.
Connected topics
Videos
Check yourself
4 questions on 8.7 Disruptions in Ecosystems. Pick an answer to see if you got it, and why.
A plant species brought from another continent spreads rapidly through a wetland and crowds out native plants. Which of the following is the most likely reason it spreads so successfully?
| Sample | Pesticide concentration (parts per million) |
|---|---|
| Lake water | 0.0001 |
| Zooplankton | 0.05 |
| Small fish | 0.6 |
| Large fish | 2.5 |
| Fish-eating birds | 20 |
Experimental data: concentration of a long-lasting, fat-soluble pesticide measured in a lake ecosystem
Which of the following best explains the pattern in the data?
Which organisms in this ecosystem are most at risk of harm from the pesticide?
Fertilizer runs off farm fields into a lake, and a few weeks later many fish in the lake die. Which sequence of events best explains this?
0 of 4 answered