Population ecology Exponential growth occurs when resources are

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 ecology • Population growth – 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 Adjustment for limited resources Population size Per capita growth rate Population growth rate

Population ecology • Logistic growth produces S-shaped curve; population growth rate decreases as N approaches K Population size K (N) Time (t)

Population ecology – Examples of logistic growth

Population ecology – How does logistic growth model work? • When N is very small (imagine N is 1 and K is 1000). . . is close to 1, so population grows exponentially (1) Population size (N) Time (t)

Population ecology • When N approaches K (imagine N is 500, 600, . . . 900 and K is 1000). . . Gets closer and closer to 0, so population growth slowly approaches 0 Population size (N) Time (t)

Population ecology • When N equals K (imagine N is 1000 and K is 1000). . . is 0, so population growth is 0 Population size (N) Time (t)

Population ecology • Population growth – 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) » fewer resources available for production of offspring – Per capita death rates increase (fewer resources for survival, predators focus attention on common prey) » fewer resources for survival » predators focus attention on common prey

Population ecology • Population growth – Example of densitydependence • As population size increases, fecundity decreases • As population size increases, mortality increases

Population ecology • Population growth – Density-dependent factors include: • disease • predators • competition for resources – Not all density-dependent factors result in reduced population growth rates as population size increase • Allee effect = population growth rate reduced at low population density – Difficulty finding mates could reduce birth rates at low population size

Population ecology • Population growth – Sometimes population regulated by densityindependent factors • Birth rates decrease and death rates increase regardless of population size – Extremely cold winter – drought – fires

Population ecology • Type of population regulation may influence lifehistory traits: – 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

Population ecology • Type of population regulation may influence lifehistory traits: – 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

Population ecology • r- and K-selected life history traits (ends of continuum) trait r-selected K-selected – age at 1 st 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