A population is a group of individuals of the same species that live in the same geographic area at the same time and are capable of interbreeding. Population ecology studies how the size, density, and structure of populations change over time.
Species have evolved different strategies to maximize their reproductive success. These are often simplified into two main types: r-selected and K-selected.
| Characteristic | r-selected Species (Opportunistic) | K-selected Species (Equilibrium) |
|---|---|---|
| Environment | Unstable, unpredictable | Stable, predictable |
| Reproduction | Many offspring, one reproductive event | Few offspring, multiple reproductive events |
| Parental Care | Little or none | High |
| Body Size | Small | Large |
| Lifespan | Short | Long |
| Population Growth | Rapid (exponential), J-curve | Slow, levels off at K (S-curve) |
| Examples | Insects, bacteria, algae, weeds | Elephants, whales, humans, large trees |
Populations have unique properties that individuals do not. These are used to describe and measure a population.
A life table is an age-specific summary of the survival and reproductive patterns of a population. It follows a cohort (a group of individuals born at the same time) from birth until all are dead.
A survivorship curve is a graph that plots the proportion of a cohort still alive at each age.
These are mathematical models that describe how populations change in size.
Used for populations with discrete (non-overlapping) breeding seasons (e.g., annual plants, insects that breed once a year). The population grows in steps.
Describes population growth in an ideal, unlimited environment (unlimited resources, no predators). The population grows at its maximum rate (intrinsic rate of increase, 'r').
Formula: dN/dt = rN
- dN/dt = the change in population size over time
- r = intrinsic rate of increase
- N = population size
This results in a J-shaped curve. It cannot be sustained forever.
This is a more realistic model. It incorporates limiting factors and the concept of carrying capacity (K)—the maximum population size an environment can sustainably support.
As the population (N) approaches K, growth slows down due to density-dependent factors (e.g., competition for food, increased disease).
Formula: dN/dt = rN * [(K - N) / K]
- (K - N) / K is the "logistic term."
- When N is small, this term is close to 1, and growth is exponential.
- When N = K, this term is 0, and growth stops (dN/dt = 0).
This results in a sigmoidal or S-shaped curve.