Biological Science: Concepts of Population Biology with Emphasis on Ecology
This ecology domain tests populations, species interactions, communities, and ecosystems - mostly as models and predictions rather than memorized numbers.
Population growth
Exponential growth: dN/dt = rN, unlimited resources, J-shaped curve. Logistic growth: dN/dt = rN(1 − N/K), where K is carrying capacity; S-shaped, slowing as N approaches K, and fastest at K/2. Density-dependent factors (food, disease, predation) intensify with crowding; density-independent factors (fire, storm, drought) act regardless of density. r-selected species produce many small offspring with little care; K-selected species produce few with heavy investment.
Population structure
Age-structure diagrams predict growth: a broad base means an expanding population. Survivorship curves: Type I (late loss, humans), Type II (constant, birds), Type III (early loss, fish and insects). Metapopulations are habitat patches linked by dispersal.
Species interactions
Competition (−/−) leads to competitive exclusion or resource partitioning. Predation and parasitism are +/−, mutualism +/+, commensalism +/0. Keystone species exert influence disproportionate to abundance; removing one can collapse the community.
Communities and ecosystems
Energy passes between trophic levels with roughly 10% transfer, so chains rarely exceed four or five links; biomass pyramids invert in some aquatic systems. Biomagnification concentrates persistent toxins at higher levels. Primary productivity is limited by light and nutrients, often nitrogen or phosphorus. Primary succession begins on bare rock (lichens, mosses); secondary succession follows disturbance with soil intact. Nutrient cycles include nitrogen fixation, nitrification, and denitrification.
Worked example
A population of 500 rabbits has r = 0.40 per year and K = 2000. dN/dt = 0.40 × 500 × (1 − 500/2000) = 0.40 × 500 × 0.75 = 150 rabbits per year. At N = 1000 the rate is 200; at N = 2000 it is zero. Reasoning: logistic growth peaks at K/2, so the absolute rate rises before falling.
Traps
Using exponential growth when a carrying capacity is stated; assuming energy or biomass always increases up a pyramid; mixing density-independent weather with density-dependent disease; assuming competition always ends in extinction.