Population Growth and Carrying Capacity

1. Population Growth Population = all members of same species (interbreeding organisms) within an ecosystem.

2. Populations can grow exponentially... ...If each organism has multiple offspring. For Example • 1 fly lays about 120 eggs • In one year, that one fly has about 5 x 1012 great, great, great, great grandchildren. • This fly population is meeting its Biotic Potential because it is increasing at the maximum rate possible.

3. BioticPotential • Maximum rate at which a population can increase in ideal conditions. Biotic Potential is affected by the organism’s • Lifespan • Age at first reproduction • Frequency of reproduction • Clutch size (how many offspring produced) • Length of reproductive capability

4. Exponential growth of 2 organisms with different biotic potentials • Bacteria divide every 20 minutes; it takes 220 minutes to reach a population of about 2000. • Eagles reproduce once a year starting at age 4 (red) or age 6 (green). It takes about 23 years (red) or 32 years (green) to reach 2000 individuals.

5. Reality Check: There are limits to exponential growth ! Population growth is limited by“environmental resistance” Density - Dependent Factors (tend to be biotic) • Limited resources (food, space, light for photosynthesizers) • Competition • Predation - increased prey means increased predation • Parasitism - spread more easily in high density pops Density- Independent Factors (tend to be abiotic) • Weather (e.g. plants, insects sensitive to extreme hot, cold) • Natural disasters - fire, hurricanes, earthquake, volcanos

6. Some populations expand until they reach equilibrium at their limit • Exponential growth under favorable conditions: food, space available, little to no predation, parasitism or competition. • Once the population size matches the carrying capacity of the ecosystem, its growth slows and reaches equilibrium.

7. Carrying Capacity • Is the maximum population size that can be supported by an • ecosystem over the long term • Is typically limited by the resources available in that ecosystem

8. What happens if a population exceeds carrying capacity? • Some populations grow too fast… • Population overshoots resources… • Population crashes • E.g. Gypsy Moth caterpillars can defoliate the trees they live on so quickly thattheir larvae have nothing to feed on!

9. Sometimes they overshoot but are able to stabilize

10. Predator - Prey relationships can cause cyclical population curves • When prey populations increase, more predation occurs because- (1) predators encounter prey more often and (2) more prey support a bigger predator population. • When predators get too numerous, they reduce the prey population, thus depleting their food supply. • A change in the prey population illicits change in the predator population and vice versa.

11. Predator-Prey population trends Hare and Lynx Weavil and Wasp

12. Human Population Growth:When will we hit carrying capacity? Advances have increased the earth’s carrying capacity and pop size