Joohee Kim IB Biology Sec. II Population Dynamics Joohee Kim IB Biology Sec. II

Population Group of organisms of the same species Live in the same area at the same time Collection of inter-breeding organisms

Population Dynamics Short- and long-term changes in the size and age composition of populations Biological and environmental processes influencing changes

History Dominant branch of mathematical biology History of more than 210 years First principle: exponential law of Malthus, modeled by Malthusian growth model

Malthusian Growth Model Simple exponential growth model Based on constant rate of compound interest Formula: P(t) = P0ert where P0 = Initial Population r = growth rate, t = time

Changes to Size of Population There are four ways in which the size of a population can change: 1. Natality: birth 2. Mortality: death 3. Immigration: move into area 4. Emigration: move out of area

Population Change Populations are often affected by all four of these things and the overall change can be calculated using an equation: Population Change = (natality + immigration) – (mortality + emigration)

Population Growth Curves If the size of a population is measured regularly, a curve can be plotted. When a species spreads into a new area, the population growth curve is often sigmoid (S-shaped).

3 Phases of Curve The three phases of the curve are explained by changes in natality and mortality. 1. Exponential Phase 2. Transitional Phase 3. Plateau Phase

Exponential Phase Population increases exponentially Natality Rate > Mortality Rate Resources needed by the population are abundant Diseases and predators are rare

Transitional Phase Natality rate starts to fall Mortality rate starts to rise Still, Natality Rate > Mortality Rate Rise less rapidly

Plateau Phase Natality = Mortality Constant population size Reached carrying capacity (max population size that can be supported by environment)

Limiting Factors of Population Shortage of resources (food) More predators More disease or parasites Limit population increase because they become more intense/crowded as the population rises Reduce natality rate Increase mortality rate