Unit 2: Ecology of populations
Table of Contents
Concept of Population
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.
Life History Strategies (r- and K-selection)
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 |
Characteristics of Population
Populations have unique properties that individuals do not. These are used to describe and measure a population.
- Density: The number of individuals per unit area (e.g., 100 trees per hectare) or volume (e.g., 500 algae per mL of water).
- Dispersion: The spatial pattern of individuals within an area.
- Clumped: Individuals are grouped in patches (most common).
- Uniform: Individuals are evenly spaced (e.g., territorial birds).
- Random: Individuals are spaced in an unpredictable way (rare).
- Natality: The birth rate; the number of new individuals produced per unit of time.
- Mortality: The death rate; the number of individuals that die per unit of time.
- Age Structure: The proportion of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive). This helps predict future population growth. [Image of the three types of age structure pyramids]
Life Tables and Survivorship Curves
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.
- Type I (Convex): High survival in early and middle life, followed by a rapid decline in old age. (e.g., humans, elephants). [Associated with K-selection]
- Type II (Linear): Constant mortality rate throughout life. (e.g., many birds, small mammals, lizards).
- Type III (Concave): Very high mortality for the young, but high survival for those who live past this stage. (e.g., oysters, fish, trees, most r-strategists).
Population Growth Models
These are mathematical models that describe how populations change in size.
1. Geometric Growth
Used for populations with discrete (non-overlapping) breeding seasons (e.g., annual plants, insects that breed once a year). The population grows in steps.
2. Exponential Growth (J-Shaped Curve)
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.
3. Logistic Growth (S-Shaped Curve)
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.