Explain population dynamics, including carrying capacity, limiting factors, growth curves, and density-dependent and density-independent factors (Virginia 2018 Biology SOL BIO.8.a).

What this topic is asking

Virginia Biology SOL standard BIO.8 is about how populations and ecosystems are dynamic, and substandard BIO.8.a focuses on interactions within and among populations, including carrying capacity, limiting factors, and growth curves. The EOC expects you to read population graphs, to know the difference between exponential and logistic growth, to define carrying capacity, and to sort limiting factors into density-dependent and density-independent. A common item shows a growth curve or a predator-prey graph and asks you to interpret it.

Population growth curves

Reading these curves is a frequent task: identify which shape is shown, and recognize that the flat top of the S-curve marks the carrying capacity.

Carrying capacity and limiting factors

As a population approaches its carrying capacity, limiting factors intensify: there is less food and space per individual, disease spreads more easily, and competition increases, so the death rate rises and the birth rate falls until growth stops. A population may fluctuate slightly above and below the carrying capacity rather than sitting exactly on it.

Density-dependent and density-independent factors

Limiting factors fall into two groups, and telling them apart is a common EOC point:

• Density-dependent factors have a greater effect as the population becomes denser (more crowded). They include competition for food, water, and space; disease; predation; and the buildup of wastes. The more crowded the population, the stronger their effect.
• Density-independent factors affect a population regardless of its density, often suddenly. They include weather, natural disasters, fire, drought, and floods. A hard freeze kills the same fraction of a population whether it is crowded or sparse.

A quick test: if crowding makes the factor worse, it is density-dependent; if it would hit a large or a small population equally, it is density-independent.

Predator-prey relationships

Populations interact. In a predator-prey relationship, the two populations rise and fall in linked cycles: when prey are plentiful, predators have lots of food and increase; the growing predator population then eats more prey, so the prey decline; with less food, the predators decline; the prey then recover, and the cycle repeats. On a graph the predator peak comes slightly after the prey peak. Predation is a density-dependent limiting factor that helps keep both populations in check.

Try this

Q1. Define carrying capacity, and state what shape of growth curve shows it. [2]

• Cue. Carrying capacity is the maximum population size an environment can support over time; it is shown by the leveling-off top of a logistic (S-shaped) growth curve.

Q2. Classify each as density-dependent or density-independent: (a) a disease that spreads faster when animals are crowded; (b) a sudden flood. [2]

• Cue. (a) density-dependent (its effect grows with crowding); (b) density-independent (it affects the population regardless of density).