Population Growth Exponential growth No population can continue to grow indefinitely. At high densities, growth becomes density-dependent. (Fig. 48.3) All populations eventually reach the carrying capacity of their habitat. (the max number that can be supported by available resources).

Figure 48.3 Carrying capacity Population size Time Figure: 48.3 Caption: This hypothetical curve illustrates density-dependent growth. The pattern often occurs when a small number of individuals colonizes an unoccupied habitat because competition for resources is initially low to nonexistent. Carrying capacity depends on the quality of the habitat. EXCERCISE Label the portion of the graph where growth rates begin to slow. This is the inflection point of the curve, which signals the end of exponential growth. Time

Case Studies Explaining How Population Size Changes Over Time Humans exhibiting density-dependent growth (Fig. 48.5a,b)

Human population (billions) Figure 48.5a Historical growth 6 1999: 6 billion 1900: 1.5 billion 5 1700: 600 million 4 1500: 400 million Human population (billions) 3 1 A.D.: 150–200 million 2 Figure: 48.5a Caption: (a) This graph is based on estimates from archaeological, historical, and census data. 4–5 million 1 10,000 B.C. 8000 6000 4000 2000 2000 A.D. Year

Human population size (billions) Figure 48.5b Recent growth 12 11 10 High 1998 Projections Medium 9 8 Low 7 Human population size (billions) 6 5 Figure: 48.5b Caption: (b) Each time the UN Population Division updates its database on population growth, it makes a 50-year projection based on high, medium, and low average-fertility rates over the interval. The projections shown were released in 1998 and were based on an average of 2.5, 2.1, or 1.7 children being born per woman. 4 3 2 1950 1970 1990 2010 2030 2050 Year

Projected population in 2050 Figure 48.5c 1992 Projections Projected population in 2050 Fertility rate High 12.5 billion Medium 10.15 billion Figure: 48.5c Caption: (b) Each time the UN Population Division updates its database on population growth, it makes a 50-year projection based on high, medium, and low average-fertility rates over the interval. The table summarizes projections made in 1992. Low 7.8 billion The 1992 projections for 2050 are higher than those from 1998 primarily because the earlier projections did not account for the impact of AIDS.

Population Structure Age structure Developed nations have an age distribution that tends to be even. (Fig. 48.9a) Developing nations have an age distribution that is bottom-heavy (mostly young individuals). (Fig. 48.9b)

More-developed countries Figure 48.9a More-developed countries 100 1998 data 95 90 85 2050 projections 80 75 70 65 60 55 50 45 40 35 30 Figure: 48.9a Caption: These age distributions show the number of males and females in 5-year age increments from 0 to 100. The horizontal bars represent data from 1998; the green lines show projections for 2050. 25 20 15 10 5 60 40 20 20 40 60 (In millions) Males Females

Less-developed countries Figure 48.9b Less-developed countries 100 95 1998 data 90 85 2050 projections 80 75 70 65 60 55 50 45 40 35 30 Figure: 48.9b Caption: These age distributions show the number of males and females in 5-year age increments from 0 to 100. The horizontal bars represent data from 1998; the green lines show projections for 2050. 25 20 15 10 5 300 200 100 100 200 300 (in millions) Males Females

Population Structure Geographic structure Many species exist as a metapopulation. Small, isolated populations, even those on nature reserves, are unlikely to survive over the long term. (Fig. 48.10a-c)

A metapopulation is made up of small, isolated populations. Figure 48.10a A metapopulation is made up of small, isolated populations. Individuals Figure: 48.10a Caption: The overall population size of a metapopulation stays relatively stable even if subpopulations go extinct. These populations may be restored by migration, or unoccupied habitats might be colonized. Habitat patches

Although some subpopulations go extinct over time... Figure 48.10b Although some subpopulations go extinct over time... Figure: 48.10b Caption: The overall population size of a metapopulation stays relatively stable even if subpopulations go extinct. These populations may be restored by migration, or unoccupied habitats might be colonized.

…migration can restore or establish subpopulations. Figure 48.10c …migration can restore or establish subpopulations. Figure: 48.10c Caption: The overall population size of a metapopulation stays relatively stable even if subpopulations go extinct. These populations may be restored by migration, or unoccupied habitats might be colonized.

Figure 48.11 Figure 48.11 The Glanville fritillary butterfly: This butterfly is extinct in much of its former range, and is now found only on the Åland islands.

Demography and Conservation Demography: the study of factors that determine the size and structure of populations through time.

Demography and Conservation Life tables Summarize the probability that an individual will survive and reproduce in any given year over the course of its lifetime. (Fig. 48.13a) Survivorship - Ix = Nx / N0 Fecundity: the number of female offspring produced by each female in a population.

Figure 48.13a Three general types of survivorship curves 1000 Type l High survivorship Type l 100 Type ll Low survivorship Number of survivors (Nx) Steady survivorship Low survivorship 10 Figure: 48.13a Caption: These generalized graphs show the number of survivors, on a logarithmic scale, plotted against age.  Exercise Fill in the graph in part (b) with data on survivorship of Lacerta vivipara given in Table 48.1. Compare the shape of the curve to the generalized graphs in part (a) 1 Type lll High survivorship 0.1 Age

Demography and Conservation Life tables Contain useful pieces of information, such as survivorship, fecundity, and net reproductive rate.

Figure 48.14a Life table Age (x) Survivorship (lx) Fecundity (mx) 0 (birth) 0.0 1 0.33 3.0 2 0.2 4.0 Figure: 48.14a Caption: (a) This life table provides age-specific survivorship and fecundity for a hypothetical population. Exercise Assume that all 4-year-old females die after producing 3 young. Fill in the 3rd and 4th years in the table. 3 0.2 5.0

Demography and Conservation Life tables Can be used to make population projections and guide conservation programs.

Demography and Conservation Population viability analysis (PVA) A model that estimates the likelihood that a population will avoid extinction for a given time period. Combine demographic models with geographic structure and rate and severity of habitat disturbance.

Demography and Conservation Population viability analysis (PVA) Populations are considered viable if they have a 95% probability of surviving for at least 100 years.

Demography and Conservation Population viability analysis (PVA) Currently being used by natural resource managers. (Fig. 48.15a,b)

Box 48.1, Figure 1 Box 48.1, Figure 1 Members of this cleanup crew used high-pressure hoses to spray hot water and clean oiled beaches after the Exxon Valdez spill.