Topic 8.7 Notes – Disruptions in Ecosystems

1. How Environmental Change Drives Evolution

Evolution happens when allele frequencies change in a population over time. The environment doesn’t “create” useful traits. It changes which traits are favored.

Populations already contain genetic variation, produced by:

• Mutations (random DNA changes)
• Recombination during meiosis
• Sexual reproduction

When the environment shifts, some phenotypes survive and reproduce more successfully. Over generations, those alleles become more common.

Random mutations vs nonrandom selection

This distinction shows up constantly on tests.

Mutations

• Random changes in DNA sequence
• Occur before selection acts
• Not caused by environmental need
• Can be beneficial, neutral, or harmful

Natural selection

• Nonrandom process
• Favors traits that increase fitness (reproductive success) in a specific environment
• Changes allele frequencies over time

If a population of bacteria becomes antibiotic resistant, the antibiotic didn’t cause resistance mutations to appear because the bacteria “needed” them. The mutation already existed. The antibiotic simply killed non-resistant bacteria, allowing resistant ones to reproduce.

That sentence structure is exactly how you’d explain it on a short-answer response.

What counts as an adaptation

An adaptation is:

• A heritable genetic variation
• That increases fitness
• In a particular environment
• And becomes more common over generations

Environment matters. Thick fur is adaptive in cold climates. It reduces fitness in hot ones. Adaptations are always context-dependent.

Heterozygote advantage

Sometimes the heterozygous genotype has the highest fitness.

Classic pattern:

• AA → lower fitness
• aa → lower fitness
• Aa → highest fitness

This is called heterozygote advantage and it maintains genetic diversity in a population. Both alleles stay in the gene pool because heterozygotes have a survival benefit. On the AP exam, they may give you relative fitness values and expect you to identify this pattern.

2. Invasive Species and Ecosystem Disruption

An invasive species is a non-native organism that spreads and causes harm.

They may be introduced:

• Intentionally (erosion control, agriculture)
• Unintentionally (ballast water, trade, travel)

Once introduced, they often exploit a new niche.

Why they spread so successfully

In their new environment, invasive species often lack:

• Predators
• Competitors
• Parasites

This removal of limiting factors allows rapid population growth, sometimes close to exponential.

Ecological effects

• Outcompete native species for food, space, light, nutrients
• Disrupt food webs
• Reduce biodiversity
• Alter nutrient cycling

Two classic examples you should recognize:

• Kudzu → grows rapidly and smothers native vegetation
• Zebra mussels → clog infrastructure and remove plankton from aquatic systems

If you see a graph showing explosive population growth after introduction into a new region, think “release from limiting factors.”

3. Human Activities That Disrupt Ecosystems

Human impact accelerates ecosystem change beyond natural rates.

Biomagnification

This involves increasing toxin concentration at higher trophic levels.

Here’s the pattern:

1. Toxins (like mercury or DDT) enter water.
2. Producers absorb small amounts.
3. Primary consumers eat many producers.
4. Concentration increases at each trophic level.
5. Top predators accumulate the highest levels.

Biomagnification of DDT across trophic levels

Notice how the concentration increases from water at the bottom to fish-eating birds at the top. Each step up the food chain results in a higher toxin concentration.

Key points:

• Toxins are often fat-soluble
• Stored in tissues
• Can cause reproductive failure (ex: eggshell thinning)

Eutrophication

This is nutrient pollution in aquatic systems.

1. Excess nitrogen and phosphorus enter water (runoff, sewage).
2. Algal blooms form.
3. Algae die.
4. Decomposers break them down.
5. Oxygen levels drop.
6. Fish and other organisms die (hypoxic “dead zones”).

Eutrophication leading to hypoxia in aquatic systems

Focus on the lower layer in the diagram where decomposers consume oxygen as they break down sinking organic matter. This increased cellular respiration by bacteria is why dissolved oxygen drops.

You should be able to explain why oxygen drops. Decomposition increases cellular respiration by bacteria, which consumes dissolved oxygen.

New diseases

When pathogens enter populations with no resistance, mortality can be extreme.

Examples:

• Dutch elm disease → major loss of elm trees
• Potato blight → crop collapse and famine

Loss of dominant species reshapes community structure.

Habitat change

• Global climate change → shifts temperature and precipitation, coral bleaching, migration pressure
• Logging and urbanization → habitat loss and fragmentation
• Monocropping → low biodiversity, high vulnerability to pests

Reduced biodiversity often means reduced ecosystem resilience.

4. Geological and Meteorological Disruptions

Natural events also reshape ecosystems.

El Niño

A periodic warming of Pacific surface waters that shifts rainfall patterns globally. It can cause drought in some regions and flooding in others, altering marine and terrestrial ecosystems.

Continental drift

Movement of tectonic plates:

• Separates populations → speciation
• Connects landmasses → migration and competition

Biogeography is built on this.

Meteor impact

The asteroid impact 66 million years ago triggered rapid climate change and mass extinction, including non-avian dinosaurs. After mass extinction, open niches allow adaptive radiation.

5. Ecosystem Response and Recovery

Ecosystems respond to disturbance in different ways:

• Resistance → little change despite disturbance
• Resilience → recover after disturbance
• Shift to new stable state → permanent structural change

Recovery depends on:

• Biodiversity
• Severity of disturbance
• Connectivity to other ecosystems
• Rate of environmental change

Disruptions create new selective pressures. Populations must adapt, migrate, or go extinct. That connection between ecology and evolution is the heart of this topic.