Analyze how limiting factors and carrying capacity affect population size, and interpret population graphs and predator-prey relationships (TEKS Biology, Reporting Category 5; cause and effect; stability and change).

What this topic is asking

The Biology TEKS ask you to analyze how limiting factors and carrying capacity affect population size, and to interpret population graphs and predator-prey relationships. For STAAR Reporting Category 5 you need to read a growth curve, identify carrying capacity, name limiting factors, and explain why predator and prey populations cycle. This is a cause-and-effect and stability and change topic, and it is heavily graph-based.

Limiting factors

Limiting factors are often divided into density-dependent factors (those that act more strongly as the population gets crowded, such as food shortage, disease, and predators) and density-independent factors (those that act regardless of population size, such as drought, fire, or extreme cold). STAAR usually tests the general idea: as a population grows, resources run short, and growth slows.

Carrying capacity

On a typical population graph, this produces an S-shaped (logistic) curve: rapid growth, then a slowing, then a plateau at the carrying capacity. Reading where the curve levels off tells you the carrying capacity, and a change in resources (more food, less space) shifts it. If a population temporarily overshoots its carrying capacity, the shortage of resources causes deaths until it falls back.

Predator-prey relationships

Predator and prey populations affect each other in a repeating cycle:

1. When prey are plentiful, predators have plenty of food, so more survive and reproduce and the predator population rises.
2. The increased predators eat more prey, so the prey population falls.
3. With less food, the predator population then falls.
4. With fewer predators, the prey population recovers, and the cycle repeats.

This linked rise and fall is a classic cause-and-effect pattern, and a graph of the two populations shows the predator peaks lagging just behind the prey peaks. It is also an example of a feedback that keeps both populations in a rough balance over time.

Stability and change

These dynamics show the stability and change theme. Limiting factors and carrying capacity tend to keep a population in a dynamic balance, fluctuating around a level rather than growing without limit or crashing to zero. But a change (a new disease, a lost food source, a removed predator, or a human disturbance) can shift the carrying capacity or break the balance, causing the population to rise or fall. STAAR often asks you to predict how a population will respond to such a change.

Try this

Q1. Define carrying capacity and name two limiting factors. [2]

• Cue. The maximum population size an environment can support over time; limiting factors include food, water, space, predators, and disease (any two).

Q2. Explain why a predator population tends to fall after the prey population falls. [2]

• Cue. Fewer prey means less food for the predators, so fewer predators survive and reproduce, and the predator population declines.