1. Population Biology Objectives: Explain how populations grow
Identify Factors that inhibit the growth of a populations.
Summarize forces behind and issues human population growth.

2. An increase in the size of a population over time.
Population Growth
An increase in the size of a population over time.

3. Researchers Study Populations
Geographic range
Density and distribution
Growth rate
Age Structure

4. Methods used to investigate population growth in organisms:
Lab-Place microorganisms, such as bacteria or yeast cells, into tube or bottle or nutrient solution and observe how rapidly the population grows.
Field-Introduce a plant or animal species into a new environment that contains abundant resources and then observe the population growth of that species.

5. Geographic Range Area inhabited by a population
Can be huge, depending on species
Bacteria on rotting pumpkin (smaller than cubic meter) to cod in Atlantic Ocean (Canada to North Carolina

6. the number of individuals per unit of area
PopulationDensity
the number of individuals per unit of area

7. Distribution
How individuals in a population are spaced out across the range of the population.
Randomly
Uniformly
Clumped

8. Difference between birthrate and death rate.
Growth Rate

9. Calculating Growth Rate—use “doubling time”
Time needed for any population to double its size.
Doubling Time (years) = 70/ Annual Percent Growth Rate

10. Age Structure: number of males and females of each age a population contains-can help predict growing rapidly, slowly or not at all.

12. Differences in environmental conditions and past history may cause populations to differ in their age distributions.
The future growth of a population depends on its current age distribution.
Population Age Structure
Differences in environmental conditions and past history may cause populations to differ in their age distributions. The future growth of a population will depend on its current age distribution if birth and death rates vary with age.
Reference:
Ricklefs, R.E. & Miller, G.L. (2000). Ecology ,(4th ed.). NY: WH Freeman and Co.
U.S. Census Bureau. (2003). International Data Base. Retrieved from
Image Reference:
Baylor College of Medicine, Center For Educational Outreach. (2004). Martha Young, Senior Graphic Designer.

13. Factors that can affect population size
Birthrate
Death rate
Rate at which individuals enter or leave the population

14. Birthrate and Death rate
Birthrate higher than death rate-populations grow
Death rate higher than birthrate-populations likely to shrink
Birthrate = death rate – population stays same size

15. United States Trends Birthrates
Death rates have greater effect on total populations
Life Expectancy increases almost every year
Birthrates
Fertility rate decreasing b/c people waiting longer to have kids
Fewer children

16. Mobility Immigration: Movement of individuals into a population
Emigration: Movement of people between countries
No effect on world population, but does affect national population.
Migration: Movement in or out of a population
Mobility

17. Exponential Growth Not Linear (straight line)
J-shaped –Initially growth is slow, then rapid growth due to total number of reproducing organisms (exponential growth)
As population gets larger, it grows faster
Unlimited resources
Exponential Growth

19. Life-history Patterns
Rapid Life-history patterns
Unpredictable and change rapidly
Typically small body size, mature rapidly, reproduce early and have short life span.
Populations increase rapidly and decline rapidly in unfavorable conditions.
e.g. mosquitoes, bacteria
Long Life-history patterns
Reproduce and mature slowly.
Maintain population size near carrying capacities.
e.g. elephants, humans, bears, whales, cacti, bristlecone pine

20. Organisms in new environments
Move in new place, populations grows exponentially over time
E.g. exotic species; European gypsy moth

21. Populations DO NOT grow indefinitely! (Level-off)
Limiting factors
Food
Space

22. Logistical Growth
When a population’s growth slows and then stops, following a period of exponential growth. (s curve)

23. Logistical Growth (s curve)
Phase 1-Exponential Growth
Population grows exponentially (J shaped curve)
Resources are unlimited
Growth and reproduction rapid
Population size and rate of growth are increasing more and more rapidly

24. Logistical Growth Phase 2-Growth Slows Down
Population growth slows down
Does not mean population size decreases
Rate of growth slows, population increasing more slowly

25. Logistical Growth Phase 3-Growth stops
Some point rate of growth drops to zero
Levels off
Under same conditions-population will remain near or at this size indefinately.

27. Number of organisms that an environment can support.
Carrying Capacity
Number of organisms that an environment can support.

28. Reproductive pattern is the most important factor that
Carrying Capacity Reached
Population Overshoots Carrying Capacity
Population Falls Below Carrying Capacity
Birth= Death
Deaths exceed births until carrying capacity reached
Population tends to decrease
Births Exceed Deaths until carrying capacity
Population tends to increase
Reproductive pattern is the most important factor that
determines population growth

30. Factor that controls growth of a population.
Limiting Factors
Factor that controls growth of a population.

31. Types of limiting factors
Competition
Predation
Parasitism and disease
Unusual weather
Natural disaster

32. Density-Dependent Limiting Factors
Operate strongly only when population density reaches certain levels.

33. Density Independent Factors
Factors that affect all populations in similar ways, regardless of population size and density.

34. Limits to Population Growth
Density Dependent
Disease, competition, parasites, predation, herbivory, parasitism, stress and overcrowding, and food
Increasing effect as population increases
e.g. more dense more quickly disease spreads.
Density Independent
Affect all populations, regardless of density
Abiotic factors-temperature, storms, floods, drought, habitat disruptions, pollution.
Human activity
Controlling Introduced Species-herbicides and mechanic removal
e.g. Cold temps kill mosquitoes

35. Acting separately or together, limiting factors determine the carrying capacity of an environment for a species

36. Organisms Interactions Limit Population Size
Predation-Prey-cycles of increases and decreases over time
Competition for Resources—Food, water and territory (Demand exceeds supply, populations decrease)—Density Dependent
Effects of Crowding and Stress (aggression, decrease in parental care, decreased fertility and decreased resistance to disease)

37. Predator-Prey Relationships

38. Human Population Growth

39. Historical Overview Human populations tend to increase
Rate of that increase has changed over time:
Then, Now and Future
Early human existence
Exponential Human Population Growth
Predictions of Malthus
World Population Growth Slows

40. Human Population Growth
For most of human existence growth was slow
Limiting factors kept death rates high; Harsh life; food hard to fine; predation and disease; many children did not survive to adulthood.

41. Human Population Growth
Exponential human population growth
Civilization advanced, life easier; human population growth began to grow more rapidly
1800 – industrial revolution
Improvement in nutrition, sanitation, medicine, and healthcare reduced death rates.

42. Historical Overview Continued
Predictions of Malthus
English Economist Thomas Malthus suggested that only war, famine and disease could limit human popultion growth.
Regulated by competition (war)
Limited resources (famine)
Parasitism (disease)
Other density-dependent factors

43. World Population Growth Slows
Exponential growth continued until the second half of the 20th century.
Human Growth rate reached its peak b/w , and then began to drop.
Size of global human population growing; rate slowing down.
1804 (1 billion) to 1927 (2 billion)- 123 years to double
33 years to double to 3 billion
Now takes longer for global human population to grow by 1 billion

44. Patterns of Human Population Growth

45. Demographic Trends
Demography- Study of human population growth characteristics.
Demographers study growth rate, birthrates, death rates, age structure, and geographic distribution. Help predict why some countries have high growth rates and others grow more slowly.
Humans reduce environmental effects by eliminating competing organisms, increasing food production, and controlling disease organisms.

46. Demographic Transition
Dramatic change from high birthrates and death rates to low birthrates and death rates.
US, Japan and European completed
3 Stages:
Stage 1- Birthrate and death rate = high
Stage 2 – Birthrates high, death rates low
Stage 3- Birthrates and death rates = low

48. Age Structure and Population Growth
Compares age structures in different countries
Us-nearly equal numbers of people in each age group-predicts slow, steady growth
Guatemala – more young children and teens –predicts rapid population growth

49. Future Population Growth
To predict future
Age structure for a country
Effects of disease on death rates
Current models- 9 billion by 2050
Data suggests-global human population will grow more slowly than it has been growting