Population (Definition): A population is a group of individuals of the same species that live in the same geographic area at the same time, and have the capability of interbreeding.
Ecology at the population level studies how population size, density, and composition change over time and in different locations.
Meta-population (Definition): A meta-population is a "population of populations." It is a group of spatially separated populations of the same species that interact at some level (i.e., individuals move between them).
This concept was developed by Richard Levins. Imagine a group of islands, each with its own small population of butterflies. These populations are separate, but occasionally a butterfly from one island flies to another. This entire network of island populations is a meta-population.
This is a theory about life history strategies, describing two "extremes" of how organisms allocate resources to reproduction and survival. This concept relates directly to the logistic growth model (see Logistic Growth).
| Characteristic | r-selected species ("opportunists") | K-selected species ("competitors") |
|---|---|---|
| Mnemonic | r for rapid growth rate | K for Karrying Kapacity (capacity) |
| Environment | Unstable, unpredictable | Stable, predictable |
| Population Size | Variable, often below K, boom-and-bust cycles | Fairly constant, close to K |
| Reproduction | Many offspring, small size, early maturity, "big bang" (semelparity) | Few offspring, large size, late maturity, repeated (iteroparity) |
| Parental Care | Little or none | High |
| Survivorship | Type III (high early mortality) | Type I (high late mortality) |
| Examples | Insects, bacteria, algae, weeds, mice | Elephants, whales, humans, redwood trees |
Populations have unique characteristics that individuals do not, such as density, age structure, and growth rate.
Definition: Population density is the number of individuals of a species per unit area or volume.
Measurement Techniques:
Survivorship Curves: A graph showing the number of surviving individuals over time, from a cohort (group of individuals born at the same time).
The distribution of individuals among different age classes in a population. This is often visualized using an Age Pyramid, which plots the percentage of the population in pre-reproductive, reproductive, and post-reproductive age groups.
Age structure is a crucial predictor of future population growth.
Population size changes based on four factors: Births (B), Deaths (D), Immigration (I), and Emigration (E).
Change in N = (B + I) - (D + E)
For simplified models, we often ignore I and E and focus on births and deaths. The intrinsic rate of natural increase (r) is defined as r = b - d (per-capita birth rate minus per-capita death rate).
This model applies to populations with discrete breeding seasons (e.g., deer that mate once a year, annual plants). Growth is measured in "steps."
This model applies to populations with continuous breeding (e.g., bacteria, humans) living in an unlimited environment (infinite resources, no predators).
The rate of change in the population (dN/dt) is proportional to the population size (N) and the intrinsic rate of increase (r).
The resulting growth curve is J-shaped. Population growth starts slow but accelerates rapidly as the population base gets larger. This type of growth is unsustainable in the real world.
This is a more realistic model that incorporates environmental limits. As the population grows, resources become scarce, and growth slows down. The maximum population size an environment can sustainably support is called the Carrying Capacity (K).
The logistic model modifies the exponential model by adding a "braking" term: (K-N)/K.
The resulting growth curve is S-shaped (sigmoid). Growth is fastest when the population is at K/2 (half the carrying capacity). This point is called the maximum sustainable yield.
These are the environmental factors that stop a population from growing exponentially forever. The syllabus only lists "density-dependent" factors, but it's crucial to know both types.
Factors whose limiting effect becomes stronger as population density increases. These are usually biotic factors.
Density-dependent factors are what "push" a population towards its carrying capacity (K) in the logistic model.
Though not explicitly listed, understanding these is key. These factors limit a population's size regardless of its density. These are usually abiotic factors.
These factors often cause sudden "crashes" in population, rather than a gradual leveling off.