Chapter 52 Earth’s Fluctuating Populations To understand human population growth we must consider the general principles of population ecology

Population A group of individuals of a single species living in the same general area

Density and Dispersion Density - the number of individuals per unit area or volume Dispersion - the pattern of spacing among individuals within the boundaries of the population

Population Dynamics Density is the result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it Immigration Births PopuIation size Emigration Deaths

Patterns of Dispersion Clumped dispersion individuals aggregate in patches may be influenced by resource availability and behavior Uniform dispersion individuals are evenly distributed may be influenced by social interactions such as territoriality Random dispersion the position of each individual is independent of other individuals

Life Table Age-specific summary of the survival pattern of a population

Survivorship Curve A graphic way of representing the data in a life table Figure 52.4 1000 100 10 1 Number of survivors (log scale) 2 4 6 8 Age (years) Males Females

Semelparity “Big-bang” reproduction Reproduce a single time and die Figure 52.6

Iteroparity Repeated reproduction Produce offspring repeatedly over time

“Trade-offs” and Life Histories Two types of reproductive strategies Many offspring with little energy put into each one. i.e. a plant that releases many small seeds Few offspring with more energy put into each one. i.e. a plant that releases less large seeds with a store of energy to help the seed surive and germinate

Exponential Growth The exponential model describes population growth in an idealized, unlimited environment

Exponential population growth results in a J-shaped curve Exponential growth is characteristic of some populations that are rebounding Figure 52.10 1900 1920 1940 1960 1980 Year 2,000 4,000 6,000 8,000 Elephant population

The Logistic Growth Model In the logistic population growth model the growth rate declines as carrying capacity is reached Carrying capacity (K) is the maximum population size the environment can support 800 600 400 200 Time (days) 5 10 15 1,000 Number of Paramecium/ml The logistic model of population growth produces a sigmoid (S-shaped) curve

Population Change and Population Density Density-independent Birth rate and death rate do not change with population density Density-dependent Birth rates fall and death rates rise with population density

Limiting Factors-factors that affect the carrying capacity Density-independent factors Weather (storms, cold, drought) Some diseases (DDT poisoning) Density-dependent factors Food or Predators Space or Shelter Other diseases (rabies)

Population Cycles Many populations Undergo regular boom-and-bust cycles Figure 52.21 Year 1850 1875 1900 1925 40 80 120 160 3 6 9 Lynx population size (thousands) Hare population size (thousands) Lynx Snowshoe hare Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors

The Global Human Population The human population increased relatively slowly until about 1650 and then began to grow exponentially Figure 52.22 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 1000 A.D. The Plague Human population (billions) 2000 A.D. 1 2 3 4 5 6

Though the global population is still growing The rate of growth began to slow approximately 40 years ago Figure 52.23 1950 1975 2000 2025 2050 Year 2003 Percent increase 2.2 2 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 1.8

Regional Patterns of Population Change To maintain population stability A regional human population can exist in one of two configurations Zero population growth = High birth rates – High death rates Zero population growth = Low birth rates – Low death rates

Population structure A population that is 75% adults, 25% juveniles is very different from a population of 25% adults and 75% juveniles.

Population structure Age structure – distribution of ages in a population. Size structure – distribution of sizes in a population.

Age structure is commonly represented in pyramids Figure 52.25 Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Age 8 6 4 2 Percent of population 80–84 85 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19

Infant mortality and life expectancy at birth vary widely among developed and developing countries but do not capture the wide range of the human condition Figure 52.26 Developed countries Developing countries Infant mortality (deaths per 1,000 births) Life expectancy (years) 60 50 40 30 20 10 80

Ecological footprints for 13 countries Show that the countries vary greatly in their footprint size and their available ecological capacity Figure 52.27 16 14 12 10 8 6 4 2 New Zealand Australia Canada Sweden World China India Available ecological capacity (ha per person) Spain UK Japan Germany Netherlands Norway USA Ecological footprint (ha per person)

At more than 6 billion people The world is already in ecological deficit No population can exponentially grow forever No exceptions!