Preview
IB DP Biology — Unit D: Continuity and Change | SL | D4.2 Population and Community Ecology
📋 What You Will Learn
- Identify the biotic and abiotic factors that limit population growth and determine carrying capacity
- Explain the difference between exponential and logistic population growth models, including the conditions that produce each
- Analyse survivorship curves and age-structure diagrams to deduce population trends and conservation status
- Describe the nature of interspecific relationships — predation, competition, mutualism, and parasitism — and their effects on community structure
- Explain the concept of ecological succession and how pioneer and climax communities differ
- Apply Simpson's Diversity Index to evaluate biodiversity within a community
🔍 Guiding Question How do interactions between organisms and their environment regulate population size and shape the structure of ecological communities?
🌍 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. By 1966, it had crashed to 42. No predators. No disease. Just a population that outgrew its resources and collapsed. This is not a cautionary tale — it is a perfect natural experiment in population ecology, and it plays out across every ecosystem on Earth.
Populations: The Units of Ecology
Before we can understand how ecosystems work, we need to understand their fundamental unit: the population. A population is not just a collection of individuals — it is a dynamic system with its own birth rates, death rates, age structure, and spatial distribution. These properties change over time, and understanding why they change is the central question of population ecology.
Every population exists within a community of other species, and that community is embedded in an abiotic environment. The interplay between these levels — organism, population, community, ecosystem — is what gives ecology its extraordinary complexity and beauty.
📖 Key Definition — Population A group of individuals of the same species living in the same area at the same time, capable of interbreeding and sharing a common gene pool.
📖 Key Definition — Community All the populations of different species living and interacting in the same area at the same time.
📖 Key Definition — Ecosystem A community of organisms together with the abiotic (non-living) environment with which they interact, functioning as a system through the flow of energy and cycling of matter.
Population Growth: Two Models, One Reality
Here is the problem every population faces: resources are finite, but reproduction is exponential. Left unchecked, even the slowest-reproducing species would eventually fill the universe. Something always stops this from happening. The question is what — and when.
Exponential Growth — The Unconstrained Model
When resources are unlimited, populations grow exponentially. Each individual reproduces, and those offspring reproduce, and the population size compounds over time like interest in a bank account. The rate of increase is proportional to the current population size — the more individuals there are, the faster the population grows. This produces a characteristic J-shaped curve when population size is plotted against time.
Exponential growth is not a theoretical abstraction. It occurs in nature whenever a species colonises a new environment with abundant resources and few competitors or predators — exactly the situation on St. Matthew Island in 1944. Bacterial cultures in fresh growth medium also show exponential growth during their log phase. The J-curve is real, but it never lasts.
Logistic Growth — The Constrained Reality
Real populations do not grow forever. As population size increases, resources become scarcer, waste accumulates, disease spreads more easily,