Population Ecology Notes

Population ecology explores populations and their interactions with the environment.
It considers both abiotic and biotic factors.
These factors influence population variations, including:
- Density
- Distribution
- Size
- Dispersal
- Dispersion
- Age structure

A population is defined as a group of individuals of a single species living in the same general area.
The definition of the area must be very precise.
- It could be small (e.g., black ants in a backyard).
- Or it could be large (e.g., humans in Tacoma).

Once a population is defined, its trends can be explored based on various factors.
Comparisons can be made with other populations of the same species.
Example: Loggerhead turtles
- Their survival to adulthood is influenced by both biotic and abiotic factors.

Density is the number of individuals per unit area.
- Example: Number of humans per square kilometer in Tacoma
- Example: Number of plants of a particular species in a back 40 area
Density = \frac{Number \ of \ Individuals}{Unit \ Area}

Counting individuals in a large population area can be impractical or impossible.
Sampling techniques are used to estimate densities and total population numbers.

Commonly used with animals.
Capture and trap a number of individuals.
Mark them without affecting their survival.
Recapture a sample later.
Estimate population size based on the proportion of marked individuals recaptured.
Formulas exist to calculate population numbers.

Dispersal refers to processes that add or subtract individuals from a population.
Factors increasing population numbers:
- Births
- Immigration
Factors decreasing population numbers:
- Deaths
- Emigration
Analyzing these factors over time reveals population changes.
Resample the population later to see how it has changed.
Estimate future population changes based on birth, immigration, death, and emigration rates.
Most populations are dynamic due to dispersal.

No predictable pattern.
The position of each individual is independent of others.
Found when there's no strong attraction or repulsion between individuals.
Example: Dandelions in a field, where spacing is due to resource availability rather than social factors.

Demographics is the study of vital statistics in a population over time.
Includes age demographics:
- What percentage of the population falls into different age groups?
- Probability of survival to reproductive age.
- Follow a cohort of individuals of similar age over time.

Track a population over time to understand age relationships.
Identify who is reproducing, who is not, and survival likelihood.
Example: Life table for ground squirrels in the Sierra Nevada.
- Separate males and females into age groups.
- Calculate average life expectancy and probability of survival.
Shows how many males and females there are within each age group.
What can you determine right off the bat by looking at this?
Based on the population, the number of individuals at each age group, they have sort of given an average life expectancy of how the probability of these organisms living to the next age group, and how many of each type of individual you have being born and dying. And so this is a pretty common type of life table for this type of organism.

Graphs showing the likelihood of survival based on factors like sex and age.
Example: Ground squirrel data shows that females tend to live longer than males.
Reveal death rates and maximum lifespans.

Type II Curve: Constant death rate over the lifespan.
Type I Curve: Low death rates in early and middle life, with high death rates at the end of life.
- Corresponds to organisms with high parental care and medical treatment (e.g., humans).
Type III Curve: High death rates for the young, with higher survival for those that reach a certain age.
- Corresponds to organisms producing many offspring with little parental care.
Real-life populations may fall between these extreme examples.

Studying populations over time reveals growth or decline.
Calculate growth rate with delta N / delta t = B - D
Change in population size calculation:
- {Change = (Original \ Population \ Size + Births + Immigration) - (Deaths + Emigration)}
Populations can be:
- Static (stable).
- Growing.
- Decreasing.
Growth rates can be exponential or have endpoints.

Produces a J-shaped curve.
Depends on the rate of exponent for population size and number of generations.
dN/dT = r_{max}N
Where {dN/dT} is the rate of population increase, {r_{max}} is the intrinsic rate of increase, and N is the population size.
{r_{max}} is the per capita rate at which an exponentially growing population increases in size at each instant in time.
Not typically seen permanently in living populations.
May occur temporarily with abundant resources, mates, and no predators or pathogens.
Example: Rebounding populations after a drastic decline, such as elephants in Kruger National Park.

Real-life populations eventually face limiting factors.
These include environmental constraints.
Resources become scarce.
Competition increases.
Predators take more advantage of available prey.
Carrying capacity (K) is the maximum population size an environment can support indefinitely.

Incorporates carrying capacity and limiting factors.
Produces an S-shaped curve.
As the population approaches carrying capacity, the growth rate declines.
Density-dependent factors contribute to this decline.
The equation for logistic growth is:
- dN/dT = r_{max}N (K-N)/K
- Where {dN/dT} is the rate of population increase, {r_{max}} is the intrinsic rate of increase, N is the population size, and K is the carrying capacity.
Real populations don't always follow idealized curves.

Environmental factors not caused by biotic interactions.
Examples: drought, famine, hurricanes, soil acidification, floods, harsh winters.
Affect food availability and survival.
Can have positive effects (e.g., more rainfall).