L27. Population Growth and Limits

Ecosystems and Biodiversity

R-report

L27. Population Growth and Limits

If you put a few rabbits on a small meadow, will they keep increasing forever? What stops a population from growing without limit?

A simple start: what a population is

A population is all the individuals of one species living in the same place at the same time. You already know from "What Makes an Ecosystem?" that populations live inside habitats and interact with other species. For now, focus on the single-species view: a population's size changes because of births, deaths, and movement in or out (immigration and emigration). When more individuals are born than die, the population grows; when deaths exceed births, it shrinks. Scientists often use the letter N to mean population size.

How populations change over time

Population change often follows a common pattern when conditions are simple. We can think about growth as phases that describe how fast numbers change at different times.

• Lag phase: New or small populations may start slowly while individuals settle, find mates, or adapt. Exponential phase: If resources are plenty and threats are few, each generation can be bigger than the last and the population grows quickly. Deceleration phase: As more individuals use up resources or spread disease, growth slows. Stable / carrying capacity: Eventually the environment can support only so many individuals—this limit is called the carrying capacity (K). The population size tends to hover near K.

What limits growth: conditions and causes

Limits come from things that reduce births or increase deaths. Some limits depend on how crowded the population is; others happen no matter the crowding.

• Density-dependent limits (depend on N): competition for food or space, spread of disease, and more predation if predators respond to prey numbers. Density-independent limits (do not depend on N): storms, droughts, sudden temperature drops, or human activities like habitat destruction. Carrying capacity is not fixed: changes in food supply, water, shelter, or new predators can raise or lower K. Human actions often change limits quickly, so populations that seemed stable can fall or boom.

A short classroom investigation you can try

Use counters, paper rabbits, or a simple spreadsheet to model a meadow population. Start with 5 rabbits and a rule: each month, each pair produces 2 new rabbits, and 10% of the population dies from natural causes. Let space limit births when N passes 30 (reduce births by half). Track N for 12 months, plot a line graph, and watch the phases appear. Ask: when does growth slow and why? Discuss how changing the death rate, food supply, or introducing a predator would change the graph. Important: do not release animals; use models or data sets instead.

Connecting the ideas

Populations change through clear phases when conditions are simple: start slow, grow fast if resources allow, slow as limits appear, and settle near carrying capacity. Limits come from both crowding (density-dependent) and outside events (density-independent).

This lesson gives a small modeling route: predict, measure, and graph. Your next lessons will examine how other species—predators and competitors—affect these population limits, but here you learned to see growth and limits in one species first.