1. Populations A group of individuals of the same species that live in the same area

2. Populations  Three important characteristics of populations are:  Geographic location: the area inhabited by the poulation  Density  Growth rate

3. Population Density  The number of individuals per unit area  Varies depending on species and ecosystem

4. Population Growth  Three factors affect the size of a population:  Number of births  Number of deaths  Number of individuals that enter or leave the population  Populations grow if more individuals are born than die  If the birthrate and the death rate are equal the population stays about the same  If the death rate is more than the birthrate, the population shrinks

5. Population growth  Immigration:  The movement of individuals into an area  Affects population growth by increasing it  Emigration:  The movement of individuals out of a population  Can cause population to decrease  Causes:  Shortage of food  Finding a mate and establishing new territory

6. Exponential growth  Growth of a population when the individuals reproduce at a constant rate.  Increases slowly at first, over time becomes larger and larger  Occurs with unlimited resources  Example: bacterial in a petri dish

7. Logistic Growth  Growth pattern in which a population’s growth rate slows or stops following a period of exponential growth  Occurs as resources become less available  Produces a graph with an S shaped curve  Carrying Capacity:  The largest number of individuals an environment can support  At this point the average growth rate is zero

8. Exponential growth

9. Limits to population growth  Limiting factor:  A factor that causes population growth to decrease  Examples include  Competition *  Predation *  Parasitism and disease *  Drought and other extremes **  Human disturbances **  * Affect population growth more strongly as it grows  ** Limit population growth regardless of size

10. Limits to Growth  Density-dependent limiting factor:  Limiting factor that depends on the size of the population  Factors become limiting ONLY when the population density reaches a certain level  Include  Competition  Predation  Parasitism  Disease

11. Competition  Occurs when populations become crowded  Compete for food, water, space, sunlight, nutrients  Can occur between member s of the same species or different species  The more individuals in an area, the sooner the resources will be used up.  Over time one species may evolve to occupy new niche  Competitive exclusion principle

12. Predation  Predator-Prey relationship: the regulation of a population by predation, the balance of predators and prey in an area  Example: Sea otter and Sea urchins  Increase in sea otters results in decrease in sea urchins which eventually will result in a decrease in sea otters  Cycle can be repeated indefinitely

13. Parasitism and disease  Like predation because one organism is weakened or killed while the other receives nourishment  Example: Sphinx moth caterpillars and wasp

14. Density Independent factors  Affects all populations in similar ways, regardless of size  Include:  Unusual weather (severe hot and cold, droughts and floods)  Natural disasters  Seasonal cycles  Certain human activities (damming rivers, cutting trees)  Causes crash in population size