Marine Ecology and Biodiversity
Core concepts
Understand how populations, communities and ecosystems are structured. Key ideas: population dynamics (births, deaths, immigration, emigration, carrying capacity, density-dependent and density-independent factors); species interactions (predation, competition, mutualism, parasitism); trophic structure and food webs (producers, consumers, decomposers, trophic levels, energy transfer and the ~10% rule); biodiversity (species richness, evenness, genetic and ecosystem diversity); and ecosystem function (productivity, nutrient cycling, habitat provision, resilience). Know how disturbances and keystone species alter community structure, and how energy and biomass differ across trophic levels.
Traps to watch
- Energy vs biomass pyramids. Energy always decreases up trophic levels; biomass can invert (e.g. phytoplankton turnover is fast, so standing biomass may be low while energy flow is high).
- Confusing abundance with diversity. A community can be abundant but low in richness if one species dominates.
- Keystone vs dominant species. A keystone species has disproportionate influence relative to abundance; a dominant species is simply the most abundant.
- Density dependence. Competition and disease intensify with density; storms and temperature extremes are usually density-independent.
- Misreading food webs. Removing a top predator can cause trophic cascades; do not assume removal only affects its direct prey.
Rule of thumb
For any ecological scenario, trace the energy and nutrient pathway first, then ask which interaction or limiting factor controls population size. If the question asks about stability, consider diversity, redundancy and connectivity.
Worked example
A reef loses its herbivorous fish. Predict the outcome. Grazing pressure falls, macroalgae overgrow corals, coral cover and habitat complexity decline, and biodiversity drops - a trophic cascade. The herbivores act as a functional group, and their loss reduces ecosystem resilience.