- “Study of the growth, abundance, and distribution of populations”
- Population abundance and distribution are described by…

**Size**(*N*)**:**total number of individuals in the population**Density:**total number of individuals per area occupied

- Adding = thru birth or immigration; Removing = deaths or emigration
- Resource availability affects population density
- More food → population can become denser → higher reproductive rates & limited space

**Dispersion:**how individuals in a population are distributed

- May be clumped (like humans in cities), uniform (like trees in an orchard), or random (trees in a forest)

**Age structure:**description of individuals of each age

- Rapidly growing population is indicated when the large proportion is young
- Pyramid-shaped = rapidly growing populations
- Tiers of equal width = stable populations with little/no population growth (
**zero population growth**)

**Survivorship curves:**Describes mortality rates of individuals in a species

**Type I**: species in which most individuals survive to middle age and after that age mortality is high- Ex: humans

**Type II**: die randomly (any age)- Ex: many rodents

**Type III**: most individuals die young; only a few survive to reproductive age

6.** Biotic potential**: maximum growth rate of a population under ideal conditions (unlimited resources and no growth restrictions) → **rMax**

- Ex: some bacteria can divide every 20 minutes

- Factors that contribute to the biotic potential & reproductive success of a species

- Age at reproductive maturity
**Clutch size**(number of offspring produced at each reproductive event)- Frequency of reproduction
- Reproductive lifetime
- Survivorship of offspring to reproductive maturity

**Carrying capacity:**max number of individuals of a population that can be sustained by a particular habitat

- Largely contributed by competition for resources

**Limiting factors:**factors that prevent a population from attaining its biotic potential and determine carrying capacity → can be**density-dependent**or**density-independent factors**

- Energy, shelter, nutrient & water availability can all be limiting factors
- Limited quantities of these resources result in intraspecific competition

**Density-dependent factors:**abiotic and biotic factors whose limiting effects become more intense as the population density increases- Ex: competition for resources, territoriality, disease, predation, waste
- Process that maintains a stable population
- Factors are a negative feedback which stops population growth by reducing birth rates or increasing death rates

**Density-independent factors:**occur independently of the density of the population- Ex: Natural disasters, pollution, and extremes of climate

**Population Growths **

__Equation for Growth of Population__

**Growth of Populations: **

- R is the reproductive rate (or growth rate), N is the population size, numerator is net increase in individuals
- When deaths exceed births,
*r*will be negative and population size will decrease **Intrinsic rate of growth:**when reproductive rate (r) is at maximum (biotic potential)

__Patterns of Population Growth __

**Exponential growth**: occurs whenever the reproductive rate is greater than one. Results from reproduction w/o constraints

- A population will usually exhibit exponential growth when they have more resources and space that they need
- Formula:
*ΔN*is the change in population size → add to original N to find total population*ΔT*is change in time- A population’s growth rate stays the same regardless of population size

**Logistic growth:** when limiting factors restrict size of the population to the carrying capacity of the habitat

- Cause for logistic growth: density-dependent factor becomes limiting and then population stabilizes (population at carrying capacity = stable)
- Population will usually switch to logistic growth bcuz of competition for resources (food, space, mates)

- Formula:

- K = carrying capacity.
- If question gives you max population, use logistic formula

- When N = K, the reproductive rate is 0 & the population is at its carrying capacity (stable)
- A population’s growth rate gets smaller as population reaches K
- A population is stable when it is not growing/at carrying capacity

**Population cycles**: fluctuations in population size in response to varying effects of limiting factors- Population may grow exponentially beyond carrying capacity of the habitat before limiting factors inhibit growth
- Population increases above carrying capacity →
*N*decreases → factors ease → renew population grow- Other times new, lower carrying capacity may be established (perhaps because the habitat was damaged by the excessively large population)

- When the population size is small, limiting factors exert little negative feedback → population growth
- Ex: since many limiting factors are density-dependent, will have a greater effect when population size is large

**Life History**of an organism is its strategy for maximum fitness.

**R-selected species**exhibit rapid (exponential) growth

- Opportunistic species that quickly invade a habitat, reproduce and die
- Produce many offspring that are small, mature quickly, and require little parental care

**K-selected species**exhibit logistic growth as they remain at K

- Species (ex: humans) produce a small number or large offering that require extensive parental care until they mature

Key Idea: What maintains a stable population ?

- Density-dependent factors, stability of the environment and climate