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Biology: Ecology and Environmental Biology

Biology · Work through your exam, one topic at a time.

All guide sections

Ecology and Environmental Biology

Ecology and Environmental Biology tests population dynamics, species interactions, community structure, energy and nutrient flow, and conservation. Expect graph interpretation and questions that ask you to separate short-term change from long-term equilibrium.

Core rules

  • Population growth: exponential growth is dN/dt = rN; logistic growth is dN/dt = rN(1 − N/K), where K is carrying capacity. Growth is fastest at K/2 in the logistic model.
  • Life history: r-selected species favor rapid reproduction and many offspring; K-selected species favor fewer, better-provisioned offspring and competitive ability.
  • Interactions: competition (−/−), predation and herbivory (+/−), mutualism (+/+), commensalism (+/0), and parasitism (+/−). A keystone species has an effect disproportionate to its abundance.
  • Energy flow: primary productivity converts light or chemical energy into organic matter; only about 10% of energy transfers between trophic levels, so food chains are short.
  • Communities and ecosystems: succession changes species composition over time; nutrient cycles move elements among living and nonliving reservoirs; island biogeography links species richness to area and isolation.

Worked example

A mark–recapture study marks 80 animals, releases them, and later captures 100 animals, of which 20 are marked. The Lincoln–Petersen estimate is N = (80 × 100)/20 = 400 animals. The reasoning assumes a closed population, equal catchability, no mark loss, and no effect of marking on survival. If marked animals become trap-shy, recapture is too low and the estimate is too high; if they become trap-happy, the estimate is too low.

Traps

  • Reversing the 10% energy rule or applying it to nutrients, which cycle rather than flow directionally.
  • Confusing density-dependent factors, such as disease and competition, with density-independent factors, such as fire and drought.
  • Calling a dominant species a keystone species; dominance is about abundance, keystone status about disproportionate effect.
  • Using r as a fixed maximum when conditions change; rmax is an ideal-condition value.
  • Assuming logistic growth immediately follows a drop in resources; time lags can cause overshoot.