Disruptions in Ecosystems: Study Guide (Biology 8.7)

Ecosystems are always changing

No ecosystem stays the same for long. A new predator arrives. A river fills with fertiliser. The sea warms for a year. Over millions of years, continents move. Each change is a disruption: it alters which organisms survive, how many there are and how energy and matter flow between them.

This last topic of the course pulls together two big ideas. From evolution: a disruption changes which variations are favoured. From systems: a disruption to one part spreads through the whole ecosystem. For every disruption, ask three questions:

1. What changed? (a new species, a new chemical, a new climate)
2. Who gains and who loses? (which populations grow or shrink, and why)
3. What is the evidence? (data, and whether it supports the claim)

Variation meets a changing environment

Adaptations come from variation that already exists

An adaptation is a heritable trait that gives an organism an advantage in a particular environment. It begins as a genetic variation. Selection favours it only where it helps. The same trait can be neutral, or even harmful, somewhere else.

Mutations are random with respect to need. The environment does not cause the mutations that an organism needs. Here is what happens instead:

• Mutations arise by chance in every generation, in all kinds of genes.
• Most are neutral or harmful. A few, by chance, suit conditions that may appear later.
• When the environment changes, individuals that already carry a helpful variant survive and reproduce more. The variant becomes more common.

So the environment is a filter, not a designer. A pesticide does not create resistance genes in insects. It kills the insects without them. The rare resistant insects, already present, leave more offspring.

Heterozygote advantage

Sometimes neither homozygote does best. Heterozygote advantage means the heterozygote (for example, Aa) has a higher relative fitness than either homozygote (AA or aa). Relative fitness compares how well each genotype survives and reproduces, with the best one set to 1.

The best-known human example involves the gene for haemoglobin. People with two copies of the sickle-cell allele have sickle-cell disease. People with one copy (sickle-cell trait) are much less likely to die of malaria caused by Plasmodium falciparum. Where this malaria is common, heterozygotes have the highest fitness. As a result, the sickle-cell allele stays in the population, even though it is harmful in the homozygous form. Where malaria is absent, the heterozygote has no advantage. Again, the environment decides which genotype wins.

Invasive species

An invasive species is a non-native species that spreads quickly and harms its new ecosystem. People introduce species on purpose (as crops, garden plants or pets) or by accident (in cargo, soil or ship ballast water). Most introduced species fail. The ones that become invasive usually succeed in one of these ways:

How the invader succeeds	What it means
Escape from enemies	Its predators, parasites and diseases were left behind in its native range, so fewer individuals die.
An open niche	It uses a resource that no native species uses well, so it meets little competition.
Outcompeting natives	It grows or reproduces faster, or uses shared resources (light, food, space) more efficiently.

Its population can then grow almost exponentially. Native species decline, and the whole food web shifts.

• Kudzu is a fast-growing vine native to East Asia. In the 1930s, the United States promoted it to control soil erosion. It climbs over trees and shrubs and blocks their light, so the native plants beneath it die.
• Zebra mussels are native to the Black Sea and Caspian Sea region. They were found in the North American Great Lakes in 1988, probably carried in ships’ ballast water. They filter huge volumes of water, removing the plankton that native animals feed on. They also attach to native mussels in large numbers and smother them.

Human activities speed up change

Natural change is often slow. Human activity accelerates change, both locally and globally. When change is faster than populations can adapt or move, extinctions follow.

• Habitat loss. Logging and urbanisation remove or break up habitats. Small, isolated populations have less genetic diversity and are more easily lost.
• Monocropping. Growing one genetically uniform crop over a large area means every plant has the same weaknesses. In the Irish potato famine (1845–1852), potato blight destroyed a crop that relied heavily on one variety. Blight is caused by Phytophthora infestans, a water mould, not a true fungus.
• Introduced diseases. Dutch elm disease is a fungal disease spread by bark beetles. It was carried to North America on imported logs in the late 1920s. By 1989, more than three-quarters of the region’s elms had been lost.
• Climate change. Warming shifts the range of species, changes the timing of seasons and can leave species unable to find suitable habitat.

Two disruptions need special attention: biomagnification and eutrophication.

Biomagnification

Some pollutants, such as the insecticide DDT and mercury compounds, are persistent (they break down very slowly) and fat-soluble. An organism stores them in its fat instead of excreting them. Recall from Topic 8.2 that only about 10% of the energy at one trophic level passes to the next. A consumer therefore eats a large mass of food to build a small mass of its own body, and it keeps nearly all the pollutant from that food. The concentration rises at each step up the food chain. This is biomagnification. Top predators carry the highest concentrations.

DDT is the classic real case. A breakdown product of DDT made the eggshells of birds such as ospreys, peregrine falcons, bald eagles and brown pelicans thinner. The eggs broke during incubation, and these bird populations fell sharply. The United States banned most uses of DDT in 1972.

Eutrophication

Eutrophication is the over-enrichment of water with nutrients, mainly nitrogen and phosphorus, often from fertiliser runoff or sewage. It follows a chain of cause and effect:

The key step is easy to miss. The oxygen crash usually comes after the bloom peaks, when the algae die and decomposers break them down. Aerobic decomposers respire, and respiration uses oxygen. Species that cannot leave, or that need high oxygen, die. Repeated dead zones can remove species from a lake or coastal area entirely.

Geological and meteorological events

Disruptions also come from Earth itself. They change habitats and alter where species live. Biogeography, the study of where species are found, records these changes.

• El Niño. Every few years, surface water in the eastern tropical Pacific becomes unusually warm. Upwelling of cold, nutrient-rich water off South America weakens. With fewer nutrients, plankton decline, and so do the fish and seabirds that depend on them. Anchovy fisheries off Peru and Chile have collapsed during strong El Niño years.
• Continental drift. Over tens of millions of years, moving plates split land masses apart and change climates. Populations on separate continents then evolve independently. This helps explain why Australia, isolated for a long time, has so many marsupials found nowhere else.
• Meteor impact. About 66 million years ago, a large asteroid struck Earth. Dust and debris blocked sunlight and stopped much of the photosynthesis on land and in the sea. About three-quarters of plant and animal species died out, including all non-avian dinosaurs. Mammals then diversified into the niches left empty.

The same pattern runs through every case: a sudden or large change removes some species and opens niches for others.

Worked example 1: biomagnification in a lake food chain

Question. Use the fictional data in Figure 1 for pesticide P.

(a) Calculate the concentration factor (how many times greater) from each trophic level to the next, starting from phytoplankton. (b) Calculate the factor from the water to the fish-eating bird. (c) Explain why the concentration rises at each level. (d) A simple model says that if only 10% of the biomass at one level is passed on and all of P is kept, the concentration should rise about tenfold per step. Compare this with the data.

(a) Factor per step. Divide the higher concentration by the lower one.

1. Zooplankton ÷ phytoplankton: 0.04 ÷ 0.004 = 10
2. Small fish ÷ zooplankton: 0.5 ÷ 0.04 = 12.5
3. Large fish ÷ small fish: 3.0 ÷ 0.5 = 6
4. Bird ÷ large fish: 24 ÷ 3.0 = 8

(b) Water to bird. 24 ÷ 0.00002 = 1 200 000, so the bird’s concentration is 1.2 million times that of the water. (Phytoplankton take up P directly from the water and concentrate it 200 times: 0.004 ÷ 0.00002 = 200.)

(c) Explanation. P is fat-soluble and persistent, so organisms store it rather than excrete it. Each consumer eats many times its own body mass in food over its life, because most of the energy in that food is used in respiration or lost. The P in all that food stays behind in a smaller body mass. So its concentration rises at every step.

(d) Comparing with the model. The model predicts 0.004 × 10 × 10 × 10 × 10 = 40 ppm in the bird. The measured value is 24 ppm, and the steps range from 6 to 12.5 rather than exactly 10. The model gives the right order of size. Real factors differ because transfer efficiency is not exactly 10%, some P is excreted or broken down, and animals have different fat contents and diets.

Interpretation. The top predator is at greatest risk, even though the water contains almost no P. That is why monitoring water alone can miss a dangerous pollutant.

Worked example 2: testing for heterozygote advantage with chi-square

Question. In a fictional laboratory study, researchers genotyped 400 beetle larvae, all offspring of Aa × Aa crosses: 100 AA, 200 Aa and 100 aa. The larvae were then exposed to a parasitic fungus. 240 adults survived: 46 AA, 150 Aa and 44 aa. Does survival depend on genotype? Use p = 0.05.

Step 1: state the null hypothesis. “Survival does not depend on genotype.” If that is true, the survivors keep the 1 : 2 : 1 ratio of the larvae.

Step 2: expected counts. 240 × ¼ = 60 AA, 240 × ½ = 120 Aa, 240 × ¼ = 60 aa.

Step 3: calculate χ² = Σ (o − e)² ÷ e.

Genotype	o	e	(o − e)² ÷ e
AA	46	60	(−14)² ÷ 60 = 3.27
Aa	150	120	30² ÷ 120 = 7.50
aa	44	60	(−16)² ÷ 60 = 4.27
		Total	χ² = 15.03

Step 4: degrees of freedom = number of categories − 1 = 3 − 1 = 2. Critical value at p = 0.05: 5.99.

Step 5: conclude. 15.03 > 5.99, so reject the null hypothesis. A difference this large would happen by chance less than 5% of the time if survival did not depend on genotype.

Step 6: interpret. Survival rates were 46 ÷ 100 = 46% (AA), 150 ÷ 200 = 75% (Aa) and 44 ÷ 100 = 44% (aa). Setting the best genotype to 1, relative fitness is about 0.61, 1 and 0.59. The heterozygote does best, so the data support heterozygote advantage in the presence of the fungus.

Check the claim’s limits. The chi-square test shows that survival is linked to genotype. The direction comes from comparing o with e: there were more heterozygotes than expected and fewer of both homozygotes. A control group without the fungus is needed to show that the fungus causes the difference. Suppose the no-fungus group gave 57 AA, 126 Aa and 57 aa survivors (240 in total). Then χ² = 0.60, which is below 5.99, so you would fail to reject the null. That is what you would expect if the fungus is what changes fitness.

Common misconceptions

• “Organisms mutate because they need to.” Mutations arise at random. The environment selects among variants that already exist.
• “Individuals adapt by natural selection.” Individuals can respond during their lives, but populations evolve. Selection changes allele frequencies across generations.
• “Heterozygote advantage means the dominant allele is better.” It means the combination of two different alleles has the highest fitness. Both homozygotes do worse.
• “Any introduced species is invasive.” Most introduced species never spread. A species is invasive only when it spreads and harms the new ecosystem.
• “Algae use up the oxygen in eutrophication.” Living algae release oxygen by day. The crash comes mainly from aerobic decomposers breaking down dead algae.
• “Biomagnification happens because predators are more sensitive.” It happens because a persistent, fat-soluble toxin builds up in each larger mass of food eaten.
• “Rejecting the null hypothesis proves my hypothesis.” It shows that the difference is unlikely to be chance alone. It supports a claim; it does not prove it, and it does not identify the cause without a controlled comparison.
• “Failing to reject the null means the null is true.” It means the data do not give enough evidence against it.

Where this leads

This is the last topic of the course, so use it to connect earlier ideas. Disruptions act through natural selection (Natural Selection), change food webs and energy flow (Energy Flow Through Ecosystems) and test an ecosystem’s resilience (Biodiversity). The chi-square method is the same one you used in Mendelian Genetics. Now test yourself with the practice questions, then use the revision notes and the checklist. When you have finished, return to the course roadmap to plan your review of the whole course.