Biology 2 · Ecology and the Biosphere
Population Interbreeding group of one species in an area Full entry → ecology studies groups of individuals of the same species living in the same area and how their size and structure change over time. Populations grow exponentially when resources are unlimited but logistically as they approach a carrying capacity set by limiting factors. Density-dependent factors (disease, competition) regulate populations, while density-independent factors (fires, floods) affect them regardless of size.
Population ecology underpins public health and resource planning. Epidemiology uses demographic models to track how diseases spread through human and animal populations, where Density Individuals per unit area or volume Full entry → and contact rates influence transmission. Age-structure diagrams inform health systems about future demand — an aging population shifts needs toward chronic care, while a young, growing population increases demand for maternal and child health services. In conservation and wildlife management, carrying capacity and density-dependent factors guide harvest limits, habitat protection, and invasive-species control. These are conceptual tools that support, but do not replace, professional training in epidemiology, medicine, or conservation.
A population is a group of individuals of the same species in the same area that interbreed. Ecologists describe it by density (individuals per unit area or volume), Dispersion Spacing of individuals (clumped, uniform, random) Full entry → (how individuals are spaced — clumped, uniform, or random), and demography (birth and death rates and age structure). Dispersion reflects resources and behavior: clumped is most common (animals cluster around food or social groups), uniform often results from territoriality or competition, and random occurs when spacing is independent of others.
Survivorship curves plot the proportion of a cohort surviving to each age:
Exponential growth Growth proportional to current size (J-curve) Full entry → occurs when resources are effectively unlimited. The change in population size equals the per capita growth rate (r) times the current size (dN/dt = rN). Because growth is proportional to size already present, the population forms a J-shaped curve. The intrinsic rate of increase (r-max) is the maximum growth rate under ideal conditions.
Logistic growth Growth that slows near K (S-curve) Full entry → adds a limit. As the population (N) approaches the Carrying capacity (K) Maximum population the environment can sustain Full entry → — the maximum number the environment can sustain — growth declines, producing an S-shaped curve: dN/dt = rN((K − N)/K). When N is small, growth is nearly exponential; when N equals K, growth stops.
Density-dependent factors change in effect as density changes: competition for resources, predation, disease, and waste accumulation. They regulate populations around a stable size because their impact grows as the population grows. Density-independent factors — fires, floods, storms, temperature extremes — affect a population regardless of density and can crash one of any size.
These pressures shape life-history strategies. r-selected species reproduce early, produce many small offspring, and provide little care, thriving in unstable or disturbed environments (weeds, insects). K-selected species reproduce later, produce few large offspring, and invest heavily in care, thriving in stable environments (elephants, whales). Most species fall between these extremes.
Logistic growth, step by step:
| Do not confuse | With | Difference |
|---|---|---|
| Exponential growth | Logistic growth | Unlimited J-curve growth vs. S-curve growth slowed by carrying capacity |
| Carrying capacity | Population size | K is the environmental limit; N is the current number of individuals |
| Density-dependent factor | Density-independent factor | Effect depends on density vs. affects any population size |
| Type I survivorship | Type III survivorship | Most survive to old age vs. most die young |
| r-selected | K-selected | Many small offspring, little care vs. few large offspring, much care |
"K caps the J." Exponential growth makes a J, and the carrying capacity (K) flattens it into an S. For survivorship, remember "I, II, III = care, constant, crash" — Type I animals are cared for and live long, Type II have steady death rates, and Type III crash early but survivors persist.
Eli explains
Explain it like I’m 10
Think of a population like the number of people in a theater. When the theater is nearly empty, every new person finds a seat easily and the crowd grows fast — that is like exponential growth. But the theater has only so many seats; as it fills, it gets harder for newcomers to find a seat, and growth slows and stops at the number of seats available. That limit is the carrying capacity.
The seats are the resources: food, water, shelter, space. When resources are plentiful relative to the number of animals, the population grows; when they run short, growth can no longer continue.
The comparison stops being exact because a real environment's "number of seats" is not fixed — it changes with seasons, weather, and other species, and animals can sometimes find new resources or move. Still, the core idea — growth that speeds up, then slows as limits are reached — is exactly how ecologists describe real populations.
A few bacteria in a dish with plenty of food double again and again very quickly (exponential growth). As food runs out and waste builds up, growth slows and levels off (logistic growth) — the dish's carrying capacity.
5 questions here, of 12 in this lesson’s practice set. Answers stay hidden until you check.
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