Ecosystems are disrupted by invasive species, by human activity and by geology, on timescales that differ by a factor of a million. The mechanism is the same; the speed is not.
Learning objective 8.7.A Explain the interaction between the environment and random or preexisting variations in populations, 8.7.B explain how invasive species affect ecosystem dynamics, 8.7.C describe human activities that lead to changes in ecosystem structure and dynamics, and 8.7.D explain how geological and meteorological activity leads to changes in ecosystem structure and dynamics.
Four populations, four starting points, one destination
Three genotypes with three fitnesses: the heterozygote is the fittest, and both homozygotes pay a price. Four populations start at very different allele frequencies. Watch where they go.
Cost to the dominant homozygote, s—
Cost to the recessive homozygote, t—
Predicted equilibrium, t / (s + t)—
Where all four runs finish—
The same recursion, without the heterozygote's advantage
Give one allele a plain advantage instead and nothing balances. The interesting part is not where the two runs end up — it is that only one of them ever stops moving.
Balanced run, generations 150–200—
Directional run, generations 150–200—
Directional run reaches 0.99 at—
Born homozygous recessive, at balance—
What an adaptation is, why the costly allele stays, and who owns the balance
Two hypotheses that make the same prediction
Grow many identical cultures from single cells, then expose each to something that kills all but the resistant. Under one hypothesis resistance appears because of the exposure; under the other it was already there. Both are set to produce the same expected number of survivors, so no average can separate them.
Hypothesis
Expected
Observed mean
Variance
Variance ÷ mean
Largest culture
Cultures with none
What to measure, and what it settles
An invader that is worse at everything
The introduced species grows more slowly than the native and tolerates less crowding. By every measure that does not involve a predator it is the inferior organism. Run it with and without the predator that eats the native, and nothing else changes.
Native ends at—
Invader ends at—
Invader's growth rate—
Invader's crowding ceiling—
What makes a species invasive
Biomagnification: one number, used twice
A consumer must eat about ten units of prey to build one unit of itself. That is why nine tenths of the energy is lost going up — and why a substance the body cannot excrete becomes ten times more concentrated at the same step. Same number, opposite directions.
Level
Energy
Share of the original
Toxin, ppm
× the water
Share × factor
Why only some substances do this
Eutrophication: what actually kills the fish
Nutrients arrive over one season and then stop. Follow the four curves and note carefully when the oxygen bottoms out relative to when the algae peak.
Algae peak at step—
Oxygen bottoms out at step—
Lowest oxygen reached—
Fish remaining—
What kills them, and why a little is not a problem
The same mechanism, at wildly different speeds
Every disruption the CED names, on a log scale because no linear axis can hold them. The mechanism is identical in every case; only the speed differs, and the speed is what decides whether a population can track the change.
Disruption
CED objective
Kind
Order of magnitude, years
Why speed matters, and one oddity in the CED's own list
Check yourself
Answer these in your own words before you open the answers.
Define an adaptation, and explain why the phrase “in a particular environment” cannot be dropped from that definition.
Define heterozygote advantage.
Explain why heterozygote advantage keeps a costly allele in a population instead of removing it.
Four populations started at different allele frequencies and finished at the same one. Explain what that tells you about where the equilibrium comes from.
Explain what happens to the balance if the environment stops penalizing one of the homozygotes, and what that tells you about who the polymorphism belongs to.
State what it means to say that mutations are not directed by specific environmental pressures.
Two hypotheses about resistance predict the same number of survivors on average. Describe a measurement that distinguishes them, and say which result the real experiment gives.
Explain how the introduction of an invasive species can allow it to take over, and why an invasive species need not be a better organism than the native one.
Explain biomagnification, and why it is the energy pyramid read upside down.
Explain what actually kills the fish in a eutrophicated lake, and why the deaths come after the algal bloom rather than during it.
The disruptions in this topic span eight orders of magnitude in speed. Explain why the speed of a change matters as much as its size.