Ecology Chapter on Population Dynamics

Population sizes are dynamic, meaning populations constantly change – they increase, decrease, or stay stable.
The size of a population is affected by births, deaths, immigration, and emigration.
Population ecologists study characteristics like geographic range, density and distribution, growth rate, and age structure.

Populations can increase, decrease, or stay stable. Changes depend on births, deaths, immigration, and emigration.

Geographic Range: The area a population inhabits. The range can vary significantly depending on the species; organisms have different sized ranges (e.g., bacteria vs. Atlantic Cod).
Density and Distribution: The number of individuals per unit area and how they are spread out.
- Different species have different densities in the same area.
- Density can be clumped, uniform, or random.
Growth Rate: Determines how the population size changes. If the birth rate is higher than the death rate, the population grows; if the death rate is higher than the birth rate, the population shrinks. If they're equal, the growth rate is zero.
Age Structure: The number of males and females in each age group of the population.

Immigration: Causes a population to grow, because organisms are entering the population.
Emigration: Causes a population to shrink, because organisms are leaving the population.

The rate of exponential growth varies among different organisms.
Organisms reproduce at differing rates, based on biology or environment factors.
Organisms have different gestational periods; this affects the rate of population change. An example is given of rats growing exponentially due to short gestation. Given two rats, under ideal conditions they can grow to 1250 in a year.

Exponential Growth: A population grows faster and faster as time goes on. This occurs when there are unlimited resources. Graphing creates a J-shaped curve.
Logistic Growth: Exponential growth slows down and eventually stops as the population reaches carrying capacity. This occurs when resources become limited. Following exponential growth, the curve creates an S-shaped graph.

If a population has unlimited resources, the population will grow exponentially.
Factors such as food supply, space, disease, and predators cause growth to eventually limit.
What happens when a population has all its needs met, like unlimited resources, protection from predators, and access to space? In these perfect conditions, will it grow, shrink, or remain the same?

Carrying capacity is the maximum number of individuals of a species that an environment can support.
When growth stops, the population reaches carrying capacity, meaning it levels off and thus is stable.
The environment cannot sustain any more individuals.

These determine carrying capacity and control population growth.
Examples include resources, space, climate, and other factors.
- Limiting factors can act separately or together to limit carrying capacity. They limit how large a population can grow.
Limiting factors control populations, keeping them between overpopulation and extinction.
Over time, limiting factors lead to natural selection and evolution.

These factors depend on the density (number per unit area) of a population.
Examples include competition, predation, herbivory, parasitism, disease, stress from overcrowding.

These factors do not depend on the density of a population.
Examples include natural events like hurricanes, droughts, floods, or wildfires.
Human influence such as pesticide use also play a role.

Most populations do not grow exponentially forever.
Eventually they will reach a point that is controlled by growth limitations such as carrying capacity.
Factors like birth rate, death rate, immigration and emigration, space, resources, and disease all influence Logistic Growth, and may cause exponential growth to slow, stop and eventually stabilize.

Logistic growth occurs in three phases:

Phase 1: Exponential Growth – resources are unlimited. Death rate is low, birth rate is high
Phase 2: Growth Slows – Population still grows but slows down.
Phase 3: Growth Stops – The population reaches its carrying capacity. Birth rate and death rate are equal.

Limiting factors can be density-dependent or density-independent.
Density-dependent factors (competition, disease, predation, etc.) change with population density.
Density-independent factors (weather, natural disaster, etc.) affect populations regardless of density.

Competition is a density-dependent limiting factor; it increases with population density.
Individuals compete for resources such as food, water, territory, sunlight, and mates.

Populations of predators and prey cycle over time.
For example, when moose populations increase, wolves have more to eat, which causes wolf populations to grow. This, in turn, causes a decrease in the moose population as wolves eat more moose. The cycle continues over time as predator populations and prey populations fluctuate.

Herbivores (animals that eat plants) act as predators from a plant's perspective.
Plant and herbivore populations cycle like predator-prey relationships: When herbivore populations increase they eat more plants causing plant populations to decrease, which then causes herbivore populations to decrease.