Preview
IB DP Biology — C: Interaction and Interdependence | SL | C4.1 Populations and Communities
📋 What You Will Learn
- Identify the key features of exponential (J-shaped) and logistic (S-shaped) population growth curves and the conditions that produce each
- Explain the roles of biotic and abiotic limiting factors in regulating population size, including the concept of carrying capacity
- Analyse population data from graphs and tables to describe trends, calculate rates of change, and interpret deviations from expected models
- Apply the concepts of natality, mortality, immigration, and emigration to calculate net population change in unfamiliar contexts
- Evaluate the assumptions and limitations of mathematical growth models when applied to real populations
🔍 Guiding Question What factors determine the distribution and abundance of populations, and how do populations change over time in response to their environment?
🌍 Why This Matters In 1944, 29 reindeer were introduced to St. Matthew Island in Alaska — a pristine, predator-free environment with abundant lichen. By 1963, the population had exploded to 6,000 animals. By 1966, it had crashed to just 42. No predators. No disease. Just a population that outgrew its food supply and collapsed. This is not an anomaly — it is a pattern written into the mathematics of life itself.
What Is a Population?
Before we can model how populations grow, we need to be precise about what a population actually is. Ecologists are not simply counting animals — they are measuring a dynamic system with inputs, outputs, and internal structure. Every number in a population is the result of four processes happening simultaneously.
📖 Key Definition — Population A population is a group of organisms of the same species living in the same area at the same time, capable of interbreeding and sharing a common gene pool.
📖 Key Definition — Population Size (N) The total number of individuals of a species within a defined area at a given time. Population size changes as a result of four processes: natality, mortality, immigration, and emigration.
The size of any population at any moment is the net result of births adding individuals, deaths removing them, immigrants arriving from elsewhere, and emigrants leaving. These four rates interact continuously. Understanding their balance is the foundation of all population ecology.
Population Change Equation
\[ \Delta N = (B + I) - (D + E) \] Where \(\Delta N\) = change in population size, \(B\) = births (natality), \(I\) = immigration, \(D\) = deaths (mortality), \(E\) = emigration.
Worked substitution: A rabbit population of 500 individuals experiences 120 births, 80 deaths, 20 immigrants, and 10 emigrants in one season. Then: \(\Delta N = (120 + 20) - (80 + 10) = 140 - 90 = +50\). The population grows to 550.
📖 Key Definition — Natality Natality is the rate at which new individuals are added to a population through reproduction. It is the primary driver of population increase.
📖 Key Definition — Mortality Mortality is the rate at which individuals are lost from a population through death. Together with natality, it determines the intrinsic rate of natural increase.
The Two Growth Models
Here is the central tension of population ecology: every species has the biological capacity to increase without limit, yet no population ever does. The story of population dynamics is the story of that gap — between potential and reality. Two mathematical models capture the two ends of this story.
Model 1 — Exponential Growth (J-Shaped Curve)
Imagine a population of bacteria introduced into a flask of unlimited