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Human Impacts on
Ecosystems

Explore how human activities disrupt biodiversity, alter biogeochemical cycles, and reshape ecological systems at local and global scales.

AP Environmental Science

What Is an Ecosystem?

An ecosystem is a community of living organisms (biotic factors) interacting with their physical environment (abiotic factors). These interactions drive energy flow and nutrient cycling that sustain all life.

Biotic Factors
Producers, consumers, decomposers, and their relationships
Abiotic Factors
Temperature, water, sunlight, soil composition, pH

Ecosystem Services

Ecosystems provide critical services that support human survival and well-being. These services have enormous economic and ecological value.

Provisioning
Food, clean water, timber, fiber, genetic resources
Regulating
Climate regulation, flood control, disease regulation, water purification
Supporting
Nutrient cycling, soil formation, primary production, oxygen generation
Cultural
Recreation, aesthetic value, spiritual enrichment, ecotourism

How Humans Disrupt Ecosystems

Human Activities Habitat Loss Pollution Overexploitation Climate Change Invasive Species Nutrient Loading

Transport pathways determine where pollutants reach receptors

A field,road and river above a soil and groundwater cutaway showing several transport contexts.Dissolved pollutants can move with groundwater, while adsorbed pollutants may travel with sediment. Chemical form affects transport and exposure; intake, absorbed dose and tissue concentration are different quantities.

Dissolved and sediment-bound differ

Dissolved and sediment-bound differ. Dissolved pollutants can move with groundwater, while adsorbed pollutants may travel with sediment. Chemical form affects transport and exposure; intake, absorbed dose and tissue concentration are different quantities.Dissolved pollutants can move with groundwater, while adsorbed pollutants may travel with sediment. Chemical form affects transport and exposure; intake, absorbed dose and tissue concentration are different quantities.

Biodiversity Loss: The Core Crisis

Biodiversity refers to the variety of life at all levels: genetic, species, and ecosystem diversity. Current extinction rates are estimated at 100 to 1,000 times the natural background rate.

Species Extinction Rate Time Background Current Rate 1800 1900 1970 2024

The HIPPO Framework

Ecologist E.O. Wilson developed the HIPPO acronym to categorize the five major threats to biodiversity, ranked by severity.

H
Habitat loss: Deforestation, urbanization, and land conversion destroy the physical spaces species need to survive.
I
Invasive species: Non-native organisms outcompete, prey on, or introduce diseases to native species.
P
Pollution: Chemical, thermal, and nutrient pollution degrades habitats and directly harms organisms.
P
Population growth: Expanding human populations increase demand for resources and land.
O
Overexploitation: Overhunting, overfishing, and overharvesting deplete species faster than they can reproduce.

Fragmentation vs. Destruction

Intact Habitat Fragmented Destroyed Full connectivity Isolated patches Total habitat loss
Fragmentation Destruction
Habitat area Reduced, split into patches Eliminated entirely
Edge effects Increased significantly Not applicable
Gene flow Restricted between patches Completely severed
Recovery potential Possible with corridors Requires full restoration

Bioaccumulation & Biomagnification

Top Predators 10 ppm Secondary Consumers 2 ppm Primary Consumers 0.5 ppm Producers 0.04 ppm Toxin concentration

The Eutrophication Cycle

1
Nutrient input: Excess nitrogen and phosphorus enter waterways from agricultural runoff, sewage, and fertilizers.
2
Algal bloom: Abundant nutrients fuel rapid growth of algae on the water surface, blocking sunlight.
3
Oxygen depletion: When algae die, decomposing bacteria consume dissolved oxygen, creating hypoxic conditions.
4
Dead zone: Aquatic organisms suffocate. Fish, invertebrates, and plants die, creating a lifeless area.

Greenhouse Effect & Climate Change

Sun Solar radiation Atmosphere (GHGs) Earth's Surface IR radiation Trapped heat Some escapes

Positive Feedback Loops

Positive feedback loops amplify change: the output reinforces the original process, causing runaway effects. These loops make climate change especially dangerous.

Temperature Rises Ice Melts Albedo Decreases More Heat Absorbed Ice-Albedo Feedback Loop

How to Analyze Human Impact

Use this step-by-step framework on the AP exam to evaluate any human impact scenario.

1
Identify the activity: What human action is occurring? (agriculture, mining, urbanization, etc.)
2
Trace the pathway: How does the activity interact with the ecosystem? (direct removal, pollution, habitat change)
3
Identify affected components: Which species, cycles, or services are disrupted?
4
Evaluate feedback loops: Does the impact trigger positive or negative feedback?
5
Propose mitigation: What strategies could reduce, reverse, or prevent the damage?

Example: Deforestation in the Amazon

Scenario: A region of tropical rainforest is cleared for cattle ranching. Analyze the ecosystem impacts using our framework.

Activity
Clear-cutting for cattle pasture
Pathway
Direct habitat removal, soil exposure, carbon release
Affected
Biodiversity loss, carbon cycle, water cycle, soil fertility

Example: Invasive Cane Toads in Australia

Scenario: Cane toads, introduced in 1935 to control beetles, became one of Australia's most damaging invasive species.

Cane Toads Introduced 1935 Poison native predators (quolls, snakes, goannas) Outcompete natives for food and habitat Ecosystem Disruption

Example: Ocean Acidification

Scenario: As atmospheric \( \text{CO}_2 \) levels rise, oceans absorb approximately 30% of emitted \( \text{CO}_2 \). This lowers ocean pH through the following reaction:

Coral Reefs
Reduced calcification rates; coral bleaching and structural weakening
Shell Organisms
Mollusks and echinoderms struggle to build calcium carbonate shells
Food Webs
Phytoplankton decline ripples through entire marine food chains

Common Mistakes on the AP Exam

✕ Confusing Bioaccumulation with Biomagnification
Bioaccumulation occurs in a single organism over time. Biomagnification occurs across trophic levels in a food chain. They are related but distinct processes.
✕ Positive Feedback = Good
Students assume "positive" means beneficial. In ecology, positive feedback means the effect amplifies itself: it is often destabilizing and harmful (e.g., ice-albedo feedback).
✕ Ignoring Indirect Effects
FRQs require tracing impacts through multiple trophic levels. Stating only the direct effect (e.g., "fish die") without explaining the cascade earns partial credit.

Summary of Human Impacts

Impact Type Primary Mechanism Key Example Mitigation
Habitat Loss Land conversion Amazon deforestation Protected areas, corridors
Pollution Chemical contamination DDT biomagnification Regulation, remediation
Invasive Species Competitive exclusion Cane toads in Australia Biosecurity, biocontrol
Climate Change Enhanced greenhouse effect Arctic ice loss Emissions reduction
Eutrophication Nutrient overloading Gulf of Mexico dead zone Buffer zones, low-input farming
Overexploitation Harvest exceeds reproduction Atlantic cod collapse Quotas, marine reserves

Question 1 of 5

Which of the following best describes biomagnification?

Question 2 of 5

A farmer applies excessive nitrogen fertilizer. Rain washes the fertilizer into a nearby lake. Which sequence of events most likely follows?

Question 3 of 5

The ice-albedo feedback loop is an example of a positive feedback loop because:

Question 4 of 5

In the HIPPO framework, which factor is considered the greatest threat to global biodiversity?

Question 5 of 5

Which of the following, if true, would most strengthen Researcher A's argument?