AP Biology · Lesson 7 of 7 · Unit 8

Disruptions to ecosystems: adaptation, invasive species and human and geological change

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Topic 8.7 connects natural selection (Unit 7) directly to ecosystem dynamics (Unit 8). On the AP Biology exam, you will rarely be asked simply to define an invasive species or pollution; instead, you will analyze data from disrupted ecosystems in stimulus-based multiple-choice sets and in Free-Response Question 4 (Conceptual Analysis) or Question 6 (Analyze Data).

By the end you’ll be able to

Explain how an adaptation is a genetic variation favored by selection, including heterozygote advantage, and why mutations are not directed by specific environmental pressures.
Explain how an introduced invasive species can exploit a new niche free of predators or competitors or outcompete native species.
Describe how human activities, including those causing biomagnification and eutrophication, accelerate ecosystem change at local and global levels and cause extinctions.

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Core ideas (continued)
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What the exam asks

Evaluate hypotheses using ecological data: Given a table or graph with $\pm 2\,SE_{\bar{x}}$ error bars showing population sizes, species richness, or toxin levels before and after an environmental change, support or refute a claim.
Trace cascading effects: Predict how introducing a non-native species, adding excess nutrients, or altering a habitat affects populations across multiple trophic levels.
Link evolutionary mechanisms to ecological change: Explain how pre-existing genetic variation—not environment-induced mutation—allows a population to adapt when an ecosystem is disrupted.

Core ideas

Adaptation, random mutation, and heterozygote advantage

An adaptation is a heritable genetic variation that is favored by natural selection because it provides a reproductive advantage in a specific environment.

Mutations are random: Environmental disruptions (such as a new pesticide, warming waters, or an introduced pathogen) do not cause or direct organisms to mutate in order to survive. Mutations occur randomly prior to the selective pressure; the environment merely selects for phenotypes that already exist in the population.
Heterozygote advantage: In some environments, individuals heterozygous for a particular gene ( $Aa$ ) have higher fitness than either homozygote ( $AA$ or $aa$ ). This maintains both alleles in the population even if one homozygote is strongly selected against (for example, the sickle-cell allele in malaria-endemic regions).

Invasive species

An invasive species is an introduced, non-native species that spreads rapidly and disrupts the native community. Invasive species succeed because they:

Enter a new niche free of their natural predators, parasites, or pathogens (predator release).
Outcompete native species for limiting resources (food, light, nesting sites) due to faster reproduction or broader tolerance ranges.

Because native prey often lack evolutionary adaptations to evade the new predator or competitor, invasive species reduce local biodiversity and alter trophic cascades.