Unit 4- Interaction of Living Things Population Dynamics Population = all members of same species (interbreeding organisms) within an ecosystem.

Population Dynamics is the study of the factors which affect population growth Population growth is the change in size of a population with time. Population size is the number of individuals in a population. Population density is the number of individuals per unit of volume or area.

Population Growth Depends upon four variables 1. Natality (birthrate) = number of organisms born into a population per unit of time. 2. Mortality (death rate) = number of organisms which die per unit of time

Population Growth 3. Immigration is the movement of new organism into a population. 4. Emigration is the movement of organisms out of a population Population Growth: (Natality + Immigration) – (Mortality + Emigration)

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.

Biotic Potential Biotic Potential is affected by the organism’s 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

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 4-6. It takes about 23-32 years to reach 2000 individuals.

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 – non-living things) Weather (e.g. plants, insects sensitive to extreme hot, cold) Natural disasters - fire, hurricanes, earthquake, volcanos Crash Course Biology

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.

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

What happens if a population exceeds carrying capacity? Some populations grow too fast… Population overshoots resources… Population crashes Ex: Gypsy Moth caterpillars can defoliate the trees they live on so quickly that their larvae have nothing to feed on!

Sometimes they overshoot but are able to stabilize

Population Growth Rate Curves It is a graph of the rate of increase in size of a population with time. Examples of population growth curves: (A) exponential growth curve (B) S-shaped or logistic curve (C) predator - prey relationship

Exponential growth curve or J - shaped curve This graph shows population growth in an unlimited environment. The curve rises slowly at first (i.e. Slow growth or lag phase) Then it increases rapidly (i.e. Rapid growth or exponential phase) This graph increases indefinitely

Logistic growth curve or S - shaped curve This graph shows population change that occurs in limited environments. The population increases slowly at first (i.e. slow growth or lag phase) It increases quickly (i.e. rapid growth or exponential phase) Eventually the population becomes so large that it stops growing altogether (i.e. no growth or equilibrium phase — the birth rate and death rate are equal).

Predator - prey relationship This shows a population cycle (i.e. alternating periods of high and low populations) In this type of relationship, one population gains at the expense of the other.

When prey populations increase, more predation occurs because: (1) predators encounter prey more often (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 leads to change in the predator population and vice versa.

Predator-Prey population trends Hare and Lynx Weavil and Wasp

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