Ecology and Ecosystems: Energy, Populations, Cycles
Core ideas about energy flow, trophic efficiency, and population dynamics that govern ecosystems.
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Questions Covered in This Set
11 cards to master
What is the backbone sentence of ecosystem science?
Energy flows (one-way, sunlight → heat), matter cycles (carbon, nitrogen, phosphorus are reused indefinitely).
Gross primary production (GPP) vs net primary production (NPP)
GPP is the total chemical energy plants capture (~1–2% of incoming sunlight); NPP is what remains after plants respire roughly half of it away — the energy available to everything else.
What is the average trophic transfer efficiency, and why is it so low?
About 10% (range ~2–20%). Consumers don't eat everything, don't digest everything they eat, and burn most of what they digest just staying alive.
Why are food chains usually only 4–5 links long?
Because 10% losses compound — by the fifth level there is essentially no energy left to support a viable population.
Why are top predators necessarily rare and vulnerable?
They sit atop the energy pyramid and need enormous areas to intercept enough energy, so habitat loss hits them hardest (wolves, tuna, eagles).
Why does a hectare feed more people as grain than as beef?
Routing food through a herbivore costs a factor of ten in energy transfer.
Write the exponential growth model and its solution.
dN/dt = rN, giving N(t) = N₀e^(rt). Unchecked, one bacterium dividing every 20 minutes would outweigh Earth in ~2 days.
State the logistic growth equation and define K.
dN/dt = rN(1 − N/K); K is carrying capacity, the population the environment can sustain. Produces the S-curve.
What assumptions make the logistic model a simplification?
It assumes K is fixed, crowding acts instantly, and all individuals are identical — real populations overshoot, crash, or cycle.
What do Lotka–Volterra dynamics predict?
Coupled predator–prey cycles where the predator peak always lags the prey peak — as in the Canadian lynx–snowshoe hare record.
Density-dependent vs density-independent limits
Density-dependent (competition, disease, predation) strengthen with crowding and are stabilising; density-independent (drought, frost, eruption) hit regardless of population size.
