population ecology exponential growth occurs when resources are not limiting. logistic growth occurs...

Population ecology

Exponential growth occurs when resources are not limiting.

Logistic growth occurs when resources become more and more limiting as population size increases.

• Population growth

Population ecology

– Does population growth continue without limits?• Number of resources usually prevent populations from

growing exponentially • Carrying capacity (K) = maximum number of individual

that an environment can support– When population reaches carrying capacity

» birth rate = death rate

» population growth rate = 0

Population ecology

– Logistic growth model• Population growth rates decreases as population

approaches its carrying capacity

dN

dtrN

K NK

Population growth rate

Per capita growth rate

Population size

Adjustment for limited resources

Population ecology

• Logistic growth produces S-shaped curve; population growth rate decreases as N approaches K

Population size

(N)

Time (t)

K

Population ecology

– Examples of logistic growth

Population ecology

• When N is very small (imagine N is 1 and K is 1000)...K NK

is close to 1, so population grows exponentially

Population size

(N)

Time (t)

dN

dtrN

K NK

(1)

– How does logistic growth model work?

dN

dtrN

K NK

Population ecology

• When N approaches K (imagine N is 500, 600, ...900 and K is 1000)...K NK

Gets closer and closer to 0, so population growth

slowly approaches 0

Population size

(N)

Time (t)

dN

dtrN

K NK

Population ecology

• When N equals K (imagine N is 1000 and K is 1000)...K NK

is 0, so population growth is 0

Population size

(N)

Time (t)

dN

dtrN

K NK

• Population growth

Population ecology

– Density-dependent population regulation• As populations near carrying capacity…population

growth rate declines – Per capita birth rates decrease (fewer resources available for

production of offspring)

– Per capita death rates increase (fewer resources for survival, predators focus attention on common prey)

» fewer resources available for production of offspring

» fewer resources for survival

» predators focus attention on common prey

• Population growth

Population ecology

– Example of density-dependence

• As population size increases, fecundity decreases

• As population size increases, mortality increases

• Population growth

Population ecology

– Density-dependent factors include:• disease• predators• competition for resources

• Allee effect = population growth rate reduced at low population density

– Difficulty finding mates could reduce birth rates at low population size

– Not all density-dependent factors result in reduced population growth rates as population size increase

• Population growth

Population ecology

– Sometimes population regulated by density-independent factors

• Birth rates decrease and death rates increase regardless of population size

– Extremely cold winter

– drought

– fires

• Type of population regulation may influence life-history traits:

Population ecology

– Species regulated by density-dependent factors are selected to be good competitors (populations are often close to carrying capacity)

• Such species invest heavily in survival• But at a cost of reduced reproductive potential (i.e. a life

history trade-off)• Called K-selected species• Examples: elephants, oak trees

• Type of population regulation may influence life-history traits:

Population ecology

– Species regulated by density-independent factors are selected to be good reproducers (populations are often below carrying capacity)

• Such species invest heavily in reproductive output• But at a cost of reduced survival (i.e. a life history trade-off)• Called r-selected species• Examples: cockroaches, birch trees

• r- and K-selected life history traits (ends of continuum)

Population ecology

trait r-selected K-selected – age at 1st

reproduction early late – lifespan short long – Survivorship low (type III) high

(type I)– Fecundity high low– Parity semelparity

iteroparity– Offspring size small large– Parental care none lots