AP Biology: Ecosystem Disruptions – Invasive Species, Eutrophication, Biomagnification

Ecosystem Disruptions – Invasive Species, Eutrophication, Biomagnification

Concept Summary

• Invasive Species: Non-native organisms introduced to an ecosystem, often outcompeting native species due to lack of predators or rapid reproduction, disrupting biodiversity and ecosystem stability.
• Eutrophication: Excessive nutrient enrichment (e.g., nitrogen/phosphorus) in aquatic systems, leading to algal blooms, oxygen depletion, and dead zones that harm aquatic life.
• Biomagnification: Increasing concentration of toxic substances (e.g., mercury, DDT) in organisms at higher trophic levels due to accumulation through the food chain, posing risks to apex predators.
• Keystone Species Impact: Invasive species can displace keystone species, causing cascading effects on ecosystem structure and function (e.g., loss of predators-overgrazing by prey).
• Human Role: Anthropogenic activities (e.g., agriculture, shipping, pollution) accelerate all three disruptions, requiring mitigation strategies like regulation or restoration.

Core Questions

WHAT (definitional)

Q: What is an invasive species? A: A non-native organism that spreads rapidly in a new ecosystem, often causing ecological or economic harm. Trap/Clarification: Not all non-native species are invasive—only those that disrupt ecosystems qualify.

Q: What is cultural eutrophication? A: Human-induced nutrient pollution (e.g., fertilizer runoff) that accelerates eutrophication beyond natural rates. Trap/Clarification: Natural eutrophication occurs slowly; cultural eutrophication is rapid and anthropogenic.

WHY (causal/explanatory)

Q: Why do invasive species often outcompete natives? A: They lack natural predators, have high reproductive rates, or exploit unfilled niches in the new ecosystem. Trap/Clarification: Invasives don’t always "win"—some fail if the environment is unsuitable.

Q: Why is biomagnification dangerous for apex predators? A: Toxins accumulate at each trophic level, reaching lethal concentrations in top predators (e.g., eagles, humans). Trap/Clarification: Biomagnification-bioaccumulation (bioaccumulation occurs within one organism over time).

HOW (process/application)

Q: How does eutrophication lead to dead zones? A: Nutrient overload-algal bloom-algae die/decompose-bacterial respiration depletes oxygen-hypoxia kills aquatic life. Trap/Clarification: Dead zones are not caused by toxins but by oxygen depletion from decomposition.

Q: How is biomagnification quantified? A: Measure toxin concentration (e.g., ppm) at each trophic level; expect exponential increase (e.g., phytoplankton: 0.04 ppm-large fish: 1.5 ppm). Trap/Clarification: Fat-soluble toxins (e.g., DDT) biomagnify more than water-soluble ones.

CAN (conditions/possibilities)

Q: Can eutrophication occur in terrestrial ecosystems? A: Yes, but it’s rare—typically affects aquatic systems due to nutrient runoff into water bodies. Trap/Clarification: Terrestrial "eutrophication" (e.g., nitrogen deposition) alters soil chemistry but lacks the dead-zone effect.

Q: Under what conditions does biomagnification not occur? A: If toxins are metabolized/excreted (e.g., water-soluble compounds) or if the food chain is short (e.g., producer-herbivore). Trap/Clarification: Biomagnification requires persistent toxins (e.g., heavy metals, POPs).

Quick Facts & Traps

• Fact: Zebra mussels (invasive) filter plankton-reduce food for native fish but clarify water, increasing benthic plant growth.
• Trap: "All algae are harmful"-Reality: Only harmful algal blooms (HABs) produce toxins; most algae are benign or beneficial.
• Fact: DDT biomagnification caused eggshell thinning in bald eagles, nearly driving them extinct in the 1960s.
• Trap: "Eutrophication is irreversible"-Reality: Reducing nutrient inputs (e.g., buffer zones) can restore ecosystems over time.
• Fact: Invasive species cost the U.S. ~$120 billion/year in damages and control efforts.
• Trap: "Biomagnification = bioaccumulation"-Reality: Bioaccumulation is within an organism; biomagnification is across trophic levels.

Rapid-Fire True/False

• Statement: Invasive species always reduce biodiversity. Answer: FALSE Why the common mistake happens: Some invasives fill empty niches (e.g., honeybees in North America) or increase local diversity temporarily.

• Statement: Eutrophication only occurs in freshwater systems. Answer: FALSE Why the common mistake happens: Marine dead zones (e.g., Gulf of Mexico) are also caused by eutrophication from river runoff.

• Statement: Biomagnification affects primary producers the most. Answer: FALSE Why the common mistake happens: Producers have the lowest toxin concentrations; apex predators have the highest.