Unit 3: How Do Ecological Systems Change?
Core concepts
Unit 3 investigates change in ecosystems over time and in response to disturbance. You must understand succession (primary and secondary), climax communities, resilience and resistance, and population dynamics including exponential and logistic growth, carrying capacity, and factors affecting population size (density-dependent and density-independent). Study biodiversity loss, invasive species, habitat fragmentation and ecosystem services (provisioning, regulating, supporting, cultural). Understand monitoring and environmental indicators, experimental design (controls, replication, variables), and how to interpret data and evaluate management strategies such as protected areas, restoration and biological control.
Traps
Students mix up resilience (ability to recover) with resistance (ability to withstand change). Another trap is assuming succession always ends in a forest; the climax depends on climate and disturbance. In experimental questions, students fail to identify the independent and dependent variables or ignore confounding factors. Data interpretation questions reward quoting specific trends and identifying limitations of the evidence.
Worked example / rule of thumb
For a logistic growth curve, population growth is fastest at about half the carrying capacity and slows as it approaches the carrying capacity. Rule of thumb: when evaluating a management strategy, state the aim, the evidence of success, and at least one limitation or risk.