Population Ecology Notes

Population changes are influenced by:
- Births
- Deaths
- Immigration
- Emigration

Ecology is the study of the patterns of distribution and abundance of organisms on Earth.
It depends on both biotic (living) and abiotic (non-living) factors.

Populations are groups of individuals of the same species living in a particular area at a particular time.
Species can reproduce in nature to produce viable, fertile offspring.
Demography is the study of populations.

Population size is the total number of organisms.
Population density (D) is the number of individuals of the same species that occur per unit area or volume.
Formula: D = N / S, where N is the total numbers counted, and S is the space occupied by the population.

Calculate population density of 480 moose in a 600-hectare region:
- D = 480 \text{ moose} / 600 \text{ ha} = 0.8 \text{ moose/ha}
Smaller organisms usually have higher population densities than larger organisms (e.g., ant colony).
Population density can be deceiving due to unused or unusable space within a habitat, which may need to be removed from calculations.

Crude density: Number of individuals of the same species within the total area of the entire habitat.
Ecological density: Number of individuals of the same species per unit area or volume actually used by the individuals.
Environmental conditions and suitable niches differ throughout a population's geographic range; ecological density takes this into account.

Calculate the population density of 480 moose living in a 600 hectare (ha) region of Algonquin Park. However, moose don’t utilize open water, and 70 of those hectares consist of open water.
- Crude density = 0.8 \text{ moose/ha}
- Ecological density = 480 \text{ moose} / (600 – 70) \text{ha} = 0.9 \text{ moose/ha}

Population dispersion is the general pattern in which individuals of a population are distributed through a specific area.
It can be:
- Clumped
- Uniform
- Random

Dispersion pattern in which individuals in a population are more concentrated in certain parts of a habitat.
Organisms are densely grouped in areas with favorable conditions for survival, where resources are not evenly distributed.
May result from social behavior.
E.g., fish schools, flocks of birds.

Individuals are equally spaced throughout the habitat.
May result from competition between individuals that set up territories.
Occurs when resources are evenly distributed across the area and are in short supply.
E.g., King Penguins.

Individuals are spread throughout a habitat in an unpredictable and patternless manner.
Occurs when individuals are minimally influenced by interactions with other individuals or when habitat conditions are virtually uniform with abundant resources.
E.g., trees in a tropical rainforest.

Populations are dynamic; their numbers and geographic locations change over time.
Often impractical to precisely count populations.
Indirect indicators can be used, such as:
- Nests
- Fecal droppings
- Number of tracks

Biologists use a variety of sampling techniques to estimate population sizes and densities.
Accurate estimations of population sizes are essential in determining allowable harvest rates while maintaining healthy viable populations.

Commonly used for stationary or small organisms such as plants or insects.
Used to isolate a small area (quadrat).
Population size and density are measured in each quadrat.
Average estimates of population size can be extrapolated based on these calculations.
N/A = N1/A1

Ragweed plants occupy a field measuring 100 m x 100 m. A student places three 2.0 m x 2.0 m quadrats in the field. Estimate the population density and size if she finds 18, 11, and 24 ragweed plants in the three quadrats.

Sampling technique for estimating the size and density of mobile wildlife populations.
A small sample of animals is captured, marked, and then released.
After a period of time that allows marked individuals to mix randomly with unmarked animals, researchers capture a second sample.
Estimates are made by comparing the second sample to the amount of tagged animals.
Calculations are based on the assumption that the ratio of marked individuals to the size of the total population equals the ratio of marked individuals recaptured in the second sample to the size of the second sample.

Accuracy of mark-recapture method depends on the following assumptions:
- Every organism in a population has an equal opportunity of being captured.
- During the time period between the initial marking and the subsequent recapture, the proportion of marked to unmarked individuals remains the same.
- The population size does not increase or decrease during the sampling study.

Formula: \frac{\text{Total # marked (M)}}{\text{Total population (N)}} = \frac{\text{# of recaptures (m)}}{\text{size of second sample (n)}}
E.g., In a fish population of unknown size, 26 individuals are captured, marked, and released. A second sample of 21 individuals is caught, in which 3 individuals are marked. \frac{26}{N} = \frac{3}{21}, therefore N = \frac{26 \times 21}{3} = 182. Estimated population size is 182.

Estimating population sizes, behavior, and migration patterns can be tracked using radio collars, satellite-linked devices, and Geographic Information Systems.