UNIT 2: ECOLOGY POPULATIONS How Populations Grow 5-1 Limits to Growth 5-2 Human Population Growth 5-3

1. How Populations Grow Characteristics of Populations Three important characteristics of a population are: Geographic Distribution Density Growth Rate

Geographic Distribution (Range) = the area inhabited by a population

Population Density = the number of individuals per unit area

Population size (blue) Population Growth Populations can stay the same (=), grow (+), or decrease (-) in size from year to year Three factors can affect population size: The number of births The number of deaths The number of individuals that enter or leave the population Population Size = # of Births - # of Deaths Births (olive) Deaths (red) Population size (blue) ? ?

Characteristics of a Population Population - individuals inhabiting the same area at the same time Population Dynamics: Population change due to Population Size - number of individuals Population Density - population size in a certain space at a given time Population Dispersion - spatial pattern in habitat Age Structure - proportion of individuals in each age group in population

Population Size Natality Mortality Number of individuals added through reproduction Crude Birth Rate - Births per 1000 Total Fertility Rate – Average number of children born alive per woman in her lifetime Mortality Number of individuals removed through death Crude Death Rate Deaths per 1000

Population Density Population Density (or ecological population density) is the amount of individuals in a population per unit habitat area Some species exist in high densities - Mice Some species exist in low densities - Mountain lions Density depends upon social/population structure mating relationships time of year

Immigration = the movement of individuals into an area Emigration = the movement of individuals out of an area Which causes an increase in population size? Decrease?  

Exponential Growth Exponential Growth = occurs when the individuals in a population reproduce at a constant rate Under ideal conditions with unlimited resources “J-shaped curve “

E. coli Reproduce once/20 minutes 20 minutes = 2 2n 40 minutes = 4 n = # times reproduced 60 minutes = 8 80 minutes = 16 100 minutes = 32 120 minutes = 64 140 minutes = 128 160 minutes = 256 180 minutes = 512 200 minutes = 1024 220 minutes = 2048 240 minutes = 4096 4 hours? 5 hours = 32,768 6 hours = 262,144 8 hours = 2,097,152 24 hours? 274 What if “Unchecked?

Logistic Growth Logistic Growth = occurs when a populations growth slows or stops following a period of exponential growth What factors decrease? What factors increase? Birthrate and Immigration Death rate and Emigration “S-shaped” Curve

Carrying Capacity (K) Exponential curve is not realistic due to carrying capacity of area Carrying capacity is maximum number of individuals a habitat can support over a given period of time due to environmental resistance (sustainability)

Carrying Capacity = the largest number of individuals (within a species) that a given environment can support More Natural Growth rate equals zero

Age Structure The age structure of a population is usually shown graphically The age structure of a population dictates whether it will grow, shrink, or stay the same size

Age Structure Diagrams Positive Growth Zero Growth Negative Growth (ZPG) Pyramid Shape Vertical Edges Inverted Pyramid

2. Limits to Growth Limiting Factors Limiting Factor = a factor that causes population growth to decrease Primary Productivity Limiting Nutrients

Density-Dependent Factors Density-Dependent Limiting Factor = a limiting factor that depend on population size These factors operate most strongly when a population is large and dense

Competition When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials

Environmental Resistance Biotic Potential factors allow a population to increase under ideal conditions, potentially leading to exponential growth Environmental Resistance affect the young more than the elderly in a population, thereby affecting recruitment (survival to reproductive age)

Exponential and Logistic Population Growth: J-Curves and S-Curves Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4

Exponential and Logistic Population Growth: J-Curves and S-Curves As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4

Predator- Prey Relationship “Inverse” relationship

Parasitism

Density-Independent Factors Density-Independent Limiting Factors = a limiting factor that does not depend on the density of a population They affect all populations in similar ways, regardless of the population size Pop < then >

Natural Disasters Human Disturbance

3. HUMAN POPULATION GROWTH

Population Growth Population growth depends upon birth rates death rates immigration rates (into area) emigration rates (exit area) Pop = Pop0 + (b + i) - (d + e)

5-3 Human Population Growth Demography- the study of human populations.

Historical Overview Like the populations of many other living organisms, the size of the human population tends to increase with time For most of human existence, the population grew slowly Life was harsh, and limiting factors kept population sizes low Food was scarce and diseases were rampant Until fairly recently, only half the children in the world survived to adulthood Families would have many children to offset the death rate About 500 year ago, the human population began growing more rapidly Why?

Case Study: Fertility and Birth Rates in the United States In 2006, the total fertility rate in the United States was slightly > 2.0 Figure 9-5

Replacement Level Baby boom (1946–64) Births per woman Replacement Level Baby boom (1946–64) Figure 9.5 Total fertility rates for the United States between 1917 and 2006. Use this figure to trace changes in total fertility rates during your lifetime. QUESTION: How many children do you plan to have? (Data from Population Reference Bureau and U.S. Census Bureau) Year Fig. 9-5, p. 175

Case Study: Fertility and Birth Rates in the United States The baby bust that followed the baby boom was largely due to delayed marriage, contraception, and abortion. Figure 9-6

Agriculture and industry Improved sanitation, medicine, and health Death Rate? Birth Rate? Experienced exponential growth

1650 = ~ 500 million 1850 (200 years) = doubled  1 billion 1930 (80 years) = doubled  2 billion 1975 (45 yeas) = doubled  4 billion Today = + 6 billion Increasing ~80 million/year 214,000 people/day = Amarillo population Weekly = + San Antonio 3 years = + United States 2025 = 7.8 billion Indefinite Growth?

Limited Resources The question is, when and how will our population growth slow?

Patterns of Population Growth Demography = the scientific study of human populations Examines the characteristics of human populations and attempts to explain how those populations will change over time Examine: birthrates, death rates, and the age structure of a population people per km²

Over the past century, population growth in the U.S., Japan, and much of Europe has slowed dramatically (Developed Countries) Hypothesis: These countries have completed the demographic transition

Throughout most of history, human societies have had high death rates and equally high birthrates With advances in nutrition, sanitation, and medicine, more children survive to adulthood and more adults live to old age These changes lower death rates and begin the demographic transition

5-3 The shift from high birth and death rates to low birth and death rates is called the demographic transition.

The demographic transition has three stages The demographic transition has three stages. Write this in off to the side. In stage 1, there are high death rates and high birthrates. In stage 2, the death rate drops, while the birthrate remains high. The population increases rapidly. In stage 3, the birthrate decreases, causing population growth to slow.

Demographic transition graph

Demographic Transition = a dramatic change in birth and death rates BR = DR

5-3 Stage III Japan Most European countries United States Stage II African countries South and Central American countries Many Asian countries

Much of the population growth today is contributed by only 10 countries, with India and China in the lead, where birthrates remain high

Age Structure: Age-Structure diagrams (Population Profiles) = graph the numbers of people in different age groups in the population Used to predict future growth

Future Population Growth Current projections suggest that by 2050, the world’s population may reach more than 9 billion people

Most ecologists suggest that if this growth does not slow down even more, there could be serious damage to the environment as well as to the global economy On the other hand, many economists assert that science, technology, and changes in society will control those negative impacts on the environment and economy