Analyze how population size is determined by births, deaths, immigration, and emigration, and how limiting factors (biotic and abiotic) determine the carrying capacity of an environment (NGSSS SC.912.L.17.5; Reporting Category 3, Organisms, Populations, and Ecosystems).

What this topic is asking

The NGSSS benchmark SC.912.L.17.5 asks you to analyze how population size is determined by births, deaths, immigration, and emigration, and how limiting factors set the carrying capacity. For the Florida Biology 1 EOC you need the equation for population change, the meaning of carrying capacity, the kinds of limiting factors, and how to read a growth curve. Items test the births-and-immigration-versus-deaths-and-emigration logic and identifying carrying capacity on a graph.

What changes population size

So a population grows when births plus immigration exceed deaths plus emigration, shrinks when the reverse is true, and stays steady when they balance. The EOC tests this directly, so keep straight that immigration adds in and emigration exits.

Limiting factors

There are two kinds:

• Density-dependent factors depend on how crowded the population is, and have a bigger effect as the population grows. Examples: food, water, space, disease, and predators. When a population is dense, food runs short and disease spreads more easily.
• Density-independent factors affect a population regardless of its size. Examples: weather, natural disasters (fires, floods), and temperature. A flood kills a similar fraction whether the population is large or small.

Carrying capacity

On a graph, the carrying capacity is the level the population flattens out at. If a population overshoots its carrying capacity, deaths rise (from starvation or disease) and it falls back toward that level.

Two growth patterns

• Exponential growth (a J-shaped curve): the population grows faster and faster when resources are unlimited. This is uncommon and temporary, seen when a species enters a new, resource-rich environment.
• Logistic growth (an S-shaped curve): rapid growth at first, then slowing as limiting factors take effect, leveling off at the carrying capacity. This is the realistic pattern for most populations.

Try this

Q1. State the four processes that change the size of a population and whether each adds or removes individuals. [2]

• Cue. Births (add) and immigration (add); deaths (remove) and emigration (remove).

Q2. Define carrying capacity and give one limiting factor that helps set it. [2]

• Cue. The maximum population size an environment can support over time; a limiting factor such as food, water, space, disease, or predators.