Overview: Counting Sheep Soay sheep were introduced to Hirta Island in 1932 Opportunity to study changes in population on isolated island with abundant food and no predators Population ecology: study of populations in relation to environment; influences on density, distribution, age structure, population size © 2011 Pearson Education, Inc.

Population Ecology Chapter 53

Population density, dispersion, and demographics Population: group of individuals of one species, living in an area Density: number of individuals per unit area or vol. Immigration and emigration Impractical to count all individuals Sampling techniques, as mark-recapture method © 2011 Pearson Education, Inc.

Deaths and emigration remove individuals from a population. Figure 53.3 Births Deaths Deaths and emigration remove individuals from a population. Births and immigration add individuals to a population. Figure 53.3 Population dynamics. Immigration Emigration 4

Patterns of Dispersion Dispersion: pattern of spacing among individuals Environmental and social factors influence spacing Clumped: resource availability, behavior Uniform: social interactions such as territoriality, defense of a space against other individuals Random: absence of strong attractions or repulsions © 2011 Pearson Education, Inc.

(a) Clumped (b) Uniform (c) Random Figure 53.4 (a) Clumped (b) Uniform Figure 53.4 Patterns of dispersion within a population’s geographic range. (c) Random 6

Demographics Demography: study of vital statistics of a population Death rates and birth rates Life table: age-specific summary of survival pattern; follows fate of a cohort (individual) © 2011 Pearson Education, Inc.

Example of a Life Table

Survivorship Curve for Belding’s ground squirrels Figure 53.5 Survivorship Curve for Belding’s ground squirrels 1,000 100 Number of survivors (log scale) Females 10 Males Figure 53.5 Survivorship curves for male and female Belding’s ground squirrels. 1 2 4 6 8 10 Age (years) 9

Survivorship Curves Type II – constant death rate over lifespan Type I – low death rates early, middle; death rates increase among old age Large mammals, Produce few offspring but give good care Type III – high death rate for young; few survive early dieoff Large numbers of offspring, little or no care of young Plants, fish, marine invertebrates Type II – constant death rate over lifespan Rodents, invertebrates, some lizards Many species don’t fall into any of these categories © 2011 Pearson Education, Inc.

Survivorship Curves are of three types Figure 53.6 Survivorship Curves are of three types 1,000 I 100 II Number of survivors (log scale) 10 Figure 53.6 Idealized survivorship curves: Types I, II, and III. III 1 50 100 Percentage of maximum life span 11

Reproductive Rates For species with sexual reproduction, demographers often concentrate on females Reproductive table or fertility schedule = age-specific summary of reproductive rates © 2011 Pearson Education, Inc.

Exponential model of population growth Useful to study population growth in an idealized situation Per capita rate of increase Often, migration is ignored © 2011 Pearson Education, Inc.

Expected number of births/deaths per year B  bN D  mN b = annual per capita birth rate m = per capita death rate N = population size © 2011 Pearson Education, Inc.

r = per capita rate of increase r  b  m Zero population growth (ZPG) when r  0 © 2011 Pearson Education, Inc.

Exponential Population Growth Population increase under ideal conditions Rate of increase at maximum – rmax Exponential population growth is Exponential growth results in J-shaped curve dN dt  rmaxN © 2011 Pearson Education, Inc.

2,000 dN dt = 1.0N 1,500 dN dt = 0.5N Population size (N) 1,000 500 5 Figure 53.7 Population growth predicted by the exponential model. 2,000 dN dt = 1.0N 1,500 dN dt = 0.5N Population size (N) 1,000 500 Figure 53.7 5 10 15 Number of generations 17

Exponential growth in the African elephant population Figure 53.8 Exponential growth in the African elephant population 8,000 6,000 Elephant population 4,000 2,000 Figure 53.8 of Kruger National Park, South Africa. 1900 1910 1920 1930 1940 1950 1960 1970 Year 18

Logistic model Describes how a population grows more slowly as it reaches its Carrying Capacity Exponential growth cannot be sustained More realistic model incorporates carrying capacity (K); produces sigmoid (S-shaped) curve Carrying capacity varies with abundance of resources Per capita rate of increase declines as carrying capacity reached (as N approaches K) © 2011 Pearson Education, Inc.

Table 53.3 Logistic Growth of a Hypothetical Population (K = 1,500) 20

( ) Exponential growth 2,000 dN dt = 1.0N 1,500 K = 1,500 Figure 53.9 Exponential growth Population growth predicted by the logistic model. 2,000 dN dt = 1.0N 1,500 K = 1,500 Logistic growth dN dt ( 1,500 – N 1,500 ) Population size (N) = 1.0N 1,000 Figure 53.9 Population growth begins slowing here. 500 5 10 15 Number of generations 21

Figure 53.10 In a constant environment lacking competitors and predators, how well do these populations fit the logistic growth model? 1,000 180 150 800 Number of Daphnia/50 mL 120 Number of Paramecium/mL 600 90 400 60 200 30 5 10 15 20 40 60 80 100 120 140 160 Figure 53.10 Time (days) Time (days) (a) A Paramecium population in the lab (b) A Daphnia population in the lab 22

Logistic Model and Real Populations Some populations overshoot K Some populations fluctuate greatly; difficult to define K Logistic model assumes instant adjustment to growth Logistic model can be used to estimate possible growth Conservation biologists use model to estimate critical size below which populations may become extinct © 2011 Pearson Education, Inc.

Life history traits are products of natural selection Life history comprises traits that affect schedule of reproduction and survival Age at which reproduction begins How often organism reproduces How many offspring are produced during each reproductive cycle Life history traits – evolutionary outcomes reflected in development, physiology, behavior © 2011 Pearson Education, Inc.

Evolution and Life History Diversity Species that exhibit semelparity, or big-bang reproduction, reproduce once and die Highly variable environments favor semelparity Century plant or Agave Other species show iteroparity (repeated reproduction) Dependable environments favor iteroparity Some Lizards Organisms have finite resources – may lead to trade-offs Trade-off between survival & paternal care in kestrels © 2011 Pearson Education, Inc.

Parents surviving the following winter (%) Figure 53.13 How does caring for offspring affect parental survival in kestrels? RESULTS 100 Male Female 80 60 Parents surviving the following winter (%) 40 Figure 53.13 Inquiry: 20 Reduced brood size Normal brood size Enlarged brood size 26

Trade offs between reproduction and survival Some plants produce large number of small seeds, ensuring that some will grow & eventually reproduce Other plants produce moderate number of large seeds that provide large store of energy that will help seedlings become established r-selection (density-independent) – selects for life history traits that maximize reproduction K-selection (density-dependent) – selects for life history traits sensitive to population density © 2011 Pearson Education, Inc.

(b) Brazil nut tree (right) and seeds in pod (above) Figure 53.14 (a) Dandelion Figure 53.14 Variation in the size of seed crops in plants. (b) Brazil nut tree (right) and seeds in pod (above) 28

Density-dependent factors Density-independent = birth and death rates do not change with population density Density- dependent = birth and death rates do change with population density © 2011 Pearson Education, Inc.

Density-dependent factors Density-dependent birth and death rates Negative feedback between population density and birth/death rates Affected by competition for resources, territoriality, disease, predation, toxic wastes © 2011 Pearson Education, Inc.

Birth or death rate per capita Figure 53.15 When population density is low, b > m. As a result, the population grows until the density reaches Q. When population density is high, m > b, and the population shrinks until the density reaches Q. Equilibrium density (Q) Birth or death rate per capita Density-independent death rate (m) Figure 53.15 Density-dependent birth rate (b) Determining equilibrium for population density. Population density 31

Decreased reproduction at high population densities. Figure 53.16 Decreased reproduction at high population densities. 100 80 60 % of young sheep producing lambs 40 Figure 53.16 20 200 300 400 500 600 Population size 32

Population Dynamics Focuses on complex interactions between biotic and abiotic factors that cause variation in population size Long-term studies have challenged hypothesis that populations of large mammals are relatively stable Weather and predators can affect population size Moose population on Isle Royale © 2011 Pearson Education, Inc.

Fluctuations in moose and wolf populations on Isle Royale, 1959–2008. Figure 53.18 50 40 30 20 10 2,500 2,000 1,500 1,000 500 Wolves Moose Number of wolves Number of moose Figure 53.18 1955 1965 1975 1985 1995 2005 Year Fluctuations in moose and wolf populations on Isle Royale, 1959–2008. 34

Population Cycles Some populations undergo regular boom-and-bust Lynx populations follow 10-year boom-and-bust cycle of hare populations Three hypotheses have been proposed © 2011 Pearson Education, Inc.

Number of hares (thousands) Number of lynx (thousands) Figure 53.19 Snowshoe hare 160 120 80 40 9 6 3 Number of hares (thousands) Lynx Number of lynx (thousands) Figure 53.19 Population cycles in the snowshoe hare and lynx. 1850 1875 1900 1925 Year 36

Hypothesis 1 Hare’s cycle follows cycle of winter food supply If hypothesis correct, then cycles should stop if food supply is increased Additional food was provided experimentally; whole population increased, but continued to cycle Rejected! © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Hypothesis 2 Hare’s cycle driven by pressure from other predators In study by field ecologists, 90% of hares were killed by predators Second hypothesis supported! © 2011 Pearson Education, Inc.

Hypothesis 3 Hare’s cycle linked to sunspot cycles Sunspot activity affects light quality, which in turn affects quality of hares’ food Good correlation between sunspot activity & population size © 2011 Pearson Education, Inc.

How does food availability affect emigration and Figure 53.20 How does food availability affect emigration and foraging in a cellular slime mold?` Immigration / Emigration – Dictyostelium amoebas can emigrate and forage better than individual amoebas EXPERIMENT Dictyostelium amoebas Topsoil Bacteria 200 m Figure 53.20 Inquiry: Dictyostelium discoideum slug Dictyostelium movement 40

Figure 53.21 Metapopulations – groups of populations linked by immigration and emigration Aland Islands ˚ EUROPE Figure 53.21 The Glanville fritillary: a metapopulation. Glanville fritillary Occupied patch Unoccupied patch 5 km 41

Human population Human population increased relatively slowly until about 1650, then began to grow exponentially Global population now ~7 billion people Though global population still growing, rate of growth began to slow during 1960s © 2011 Pearson Education, Inc.

Human population (billions) Figure 53.22 7 6 5 4 3 2 1 Human population (billions) The Plague Figure 53.22 Human population growth (data as of 2009). 8000 BCE 4000 BCE 3000 BCE 2000 BCE 1000 BCE 1000 CE 2000 CE 43

Annual percent increase in the global human population (as of 2009). Figure 53.23 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Annual percent increase in the global human population (as of 2009). 2009 Annual percent increase Projected data Figure 53.23 1950 1975 2000 2025 2050 Year 44

Regional Patterns of Population Change To maintain stability, regional human population can exist in one of two configurations ZPG = High birth rate – High death rate or ZPG = Low birth rate – Low death rate Demographic transition = move from first state to second state Associated with increase in quality of health care and improved access to education Most of current population growth concentrated in developing countries © 2011 Pearson Education, Inc.

Age Structure Important demographic factor in present and future growth trends Relative number of individuals at each age Diagrams can predict growth trends & help future planning © 2011 Pearson Education, Inc.

Figure 53.24 Age-structure pyramids for the human population of three countries (2009). Rapid growth Afghanistan Slow growth United States No growth Italy Male Female Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 Male Female Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 Male Female Figure 53.24 10 8 6 4 2 2 4 6 8 10 8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8 Percent of population Percent of population Percent of population 47

Infant Mortality and Life Expectancy Figure 53.25 Infant Mortality and Life Expectancy 60 50 40 30 20 10 80 60 40 20 Infant mortality (deaths per 1,000 births) Life expectancy (years) Figure 53.25 Infant mortality and life expectancy at birth in industrialized and less industrialized countries (data as of 2008). Indus- trialized countries Less indus- trialized countries Indus- trialized countries Less indus- trialized countries 48

Global Carrying Capacity Predicted population of 7.810.8 billion in 2050 Carrying capacity of Earth for humans is uncertain Average estimate is 10–15 billion Ecological footprint – aggregate land and water area needed to sustain people Countries vary greatly in footprint size and available ecological capacity © 2011 Pearson Education, Inc.

Average per capita energy use Figure 53.26 Average per capita energy use Gigajoules Figure 53.26 Annual per capita energy use around the world. > 300 150–300 50–150 10–50 < 10 50