- “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