Population Ecology

Population ecology studies how biotic (living) and abiotic (non-living) environmental factors influence population abundance, dispersion, and age structure.
Abiotic factors, like temperature, are non-living environmental components that affect populations.

A population is a group of individuals of a single species in a specific geographic area, characterized by its population size ( $N$ ).

Density is the number of individuals per unit area or volume. It changes due to births, deaths, immigration (influx), and emigration (outflux).

Dispersion is the spatial pattern of individuals in a population.
Three types are recognized:
- Clumped: Individuals gathered in patches, often due to resource distribution or social behavior.
- Uniform: Evenly spaced individuals, often due to territoriality or competition.
- Random: Individuals positioned independently, occurring in the absence of strong attractions or repulsions or in uniform environments.

Demography is the study of population vital statistics (birth rates, death rates, migration rates) and their changes over time.

Life tables track survival and reproductive rates of a cohort (same-aged group) through its life.

Survivorship curves graph the proportion of a cohort surviving at each age, categorized into three types:
- Type I: Low early/middle death rates, high older death rates (e.g., large mammals, humans).
- Type II: Constant death rate throughout life (e.g., some rodents).
- Type III: High death rates for the young, lower for survivors (e.g., oysters, many fish).

Population size is estimated using sampling techniques like:
- Extrapolation from small samples.
- Index of population size (e.g., nests, tracks).
- Mark-Recapture Method: For mobile species, where $N = \frac{sn}{x}$ (N = estimated population size, s = marked in first sample, n = total in second sample, x = marked in second sample).

Occurs under ideal conditions (unlimited resources), leading to a J-shaped growth curve.
The instantaneous rate of change is described by $\frac{dN}{dt} = rN$ , where $r$ is the instantaneous per capita growth rate and $N$ is the current population size.

Accounts for limited resources; populations cannot grow exponentially forever.
Carrying Capacity ( $K$ ): The maximum population size an environment can sustain.
Results in an S-shaped growth curve; growth slows as $N$ approaches $K$ .
Formula: $\frac{dN}{dt} = rN\frac{(K-N)}{K}$ , where $K$ is carrying capacity.
When $N = K$ , population growth stops ( $\frac{dN}{dt} = 0$ ).
Comparison: Exponential growth assumes unlimited resources (J-curve); Logistic growth accounts for limits and carrying capacity (S-curve).

Birth rates fall and death rates rise with increasing population density, acting as negative feedback.
Examples: Competition for resources, disease spread, predation, territoriality, toxic wastes, and intrinsic physiological factors.

Only density-dependent factors regulate population size, causing it to stabilize or decrease if thresholds are exceeded.

An organism's life history comprises traits characterizing its reproduction and survival schedule, shaped by natural selection.

Critical components include:
1. Age at first reproduction.
2. Frequency of reproduction.
3. Number of offspring per reproductive episode.