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Ecology & Evolution 7 billion people and growing Logistic growth • Q 6, 7, 8 • Population growth • Add this question at the end: List three abiotic or biotic factors which set limits to population increase What sets a limit to population increase? • Density-dependent, biotic factors – mortality due to disease – mortality due to predation – intraspecific competition • Density-independent, abiotic factors – mortality due to weather – mortality due to natural disasters: fire, hurricane, floods, earthquakes Logistic growth curve • Exponential growth phase – Species doubles per unit time – No limiting factors: nutrients, oxygen, space in ample supply • Transitional phase – Population growth continues but at a slower rate – Limiting factors slow growth rate: nutrients, oxygen, space in short supply • Plateau phase – Population growth slows to zero – Limiting factors inhibit population growth: nutrients, oxygen, space in short supply • Carrying capacity (K) = maximum number of individuals in a species that can sustainably be supported in an environment Factors affecting population size • Natality: birth rate • Immigration: rate at which individuals move into a population • Mortality: death rate • Emigration: rate at which individuals move out of the population • P = (N + I) – (M + E) Logistic growth curve • Exponential growth phase –N+I>M+ E • Transitional phase –N+I>M+ E • Plateau phase –N+I=M+ E