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Population Ecology Chapter 45 Impacts, Issues The Numbers Game Once, thousands of people lived on Easter Island, clearing forests and planting crops – when resources ran out, there was no escape 45.1 Population Demographics Each population has characteristic demographics • Statistics that describe population size, density, distribution, and age structure, that are shaped by ecological factors, and may shift over time Population size • The number of individuals in a population Population Demographics Age structure is the number of individuals in each of several age categories • Sometimes grouped as pre-reproductive, reproductive, and post-reproductive Pre-reproductive and reproductive groups make up the population’s reproductive base Population Demographics Population density is the number of individuals in a specified portion of a habitat • Refers to how many individuals are in an area, not how they are dispersed Population distribution is the pattern in which individuals are dispersed in their habitat • Clumped, nearly uniform, or random Three Patterns of Population Distribution 45.2 Elusive Heads to Count Ecologists use capture-recapture methods to estimate population sizes of animals that move • Marked animals are released; the proportion recaptured represents proportions in the population Counting Stationary Organisms To estimate population size of organisms that don’t move, an area may be divided into equal areas (quadrats) and a random sample counted 45.1-45.2 Key Concepts The Vital Statistics Ecologists explain population growth in terms of population size, density, distribution, and number of individuals in different age categories Field studies allow ecologists to estimate population size and density 45.3 Population Size and Exponential Growth Populations are dynamic units, continually adding and losing individuals • Populations increase by births and immigration (arrival of individuals from other populations) • Populations decrease by deaths and emigration (individuals that take up residence elsewhere) Migration Migration • A recurring round-trip between regions, usually in response to environmental factors • May occur daily or seasonally • Changes in population size average out over time Changes in Population Size Per capita growth rate, r • A measure of the growth rate of a population per capita growth rate (r) = per capita birth rate (b) – per capita death rate (d) Zero population growth • Number of births balances number of deaths • Population size remains stable Population Growth (G) Population growth (G) for a given interval is calculated based on per capita growth rate (r) and the number of individuals (N) G=rxN Exponential Growth Exponential growth • Population size increases by the same proportion in every successive time interval as long as r remains constant and greater than zero • Though r is constant, the population grows faster and faster, with a characteristic doubling time • The plot of population size against time results in a characteristic J-shaped curve Exponential Growth in Mice Animation: Exponential growth Exponential Growth in Bacteria: Variations in r Size of r affects the speed of exponential growth Biotic Potential Biotic potential • The maximum possible per capita growth rate for a species under optimal conditions All species have a characteristic biotic potential • Bacteria: 100% every half hour • Humans: 2% to 5% per year 45.3 Key Concepts Exponential Rates of Growth A population’s size and reproductive base influence its rate of growth When the population is increasing at a rate proportional to its size, it is undergoing exponential growth 45.4 Limits on Population Growth Natural populations seldom grow unchecked; competition and crowding can slow growth Limiting factor • Many factors can limit population growth – such as food, mineral ions, refuge, safe nesting sites • The first essential resource to be in short supply is the limiting factor Carrying Capacity The sustainable supply of resources in an environment limits population size Carrying capacity • Maximum number of individuals of a population that an environment can sustain indefinitely Logistic Growth Logistic growth • A small population starts growing slowly in size, then it grows rapidly, then its size levels off as carrying capacity is reached • Graph of logistic growth yields an S-shaped curve When a change in environment lowers carrying capacity, population size declines Logistic Growth Equation Population growth per unit time = Maximum per capita growth rate x Number of individuals x Proportion of resources not yet used Logistic Growth initial carrying capacity new carrying capacity Fig. 45-8, p. 802 Unregulated Population Growth A population of reindeer introduced to St. Matthew island peaked, then collapsed Fig. 45-9a, p. 803 carrying capacity Fig. 45-9a, p. 803 Two Categories of Limiting Factors Density-dependent factors such as disease are related to crowding and competition • Control population size through negative feedback, resulting in logistic growth patterns Density-independent factors such as fire, storms, and other natural disasters are unrelated to crowding – no logistic growth pattern 45.4 Key Concepts Limits on Increases in Number Over time, an exponentially growing population typically overshoots the carrying capacity—the maximum number of individuals of a species that environmental resources can sustain Some populations stabilize after a big decline; others never recover 45.5 Life History Patterns Each species has a life history pattern • Life span, age at maturity, and number of offspring produced vary widely among organisms • Individuals at different stages of life require different resources • Natural selection influences life history traits Cohorts and Life Tables Cohort • Group of individuals born in the same time interval • Tracking a cohort from birth until the last one dies reveals patterns of reproduction, death, migration Data about cohorts are summarized in life tables • Help determine how harvesting or environmental changes may affect population numbers • Insurance companies use human life tables Life Table for an Annual Plant Cohort Life Table for Humans in the US Survivorship Curves Survivorship curve • A graph line that emerges when a cohort’s agespecific survival in its habitat is plotted • Reveals differences in age-specific survival among species or populations of the same species • Three types are common in nature Type I Survivorship Curve Type I: Survivorship is high until late in life Type II Survivorship Curve Type II: Death rate varies little with age Type III Survivorship Curve Type III: Death rate peaks early in life Reproductive Strategies Different environments and population densities can favor different reproductive strategies r-selection favors traits that maximize number of offspring K-selection favors traits that improve offspring quality 45.6 Natural Selection and Life Histories Predation can serve as a selection pressure that shapes life history patterns Predation on Guppies in Trinidad Predators are agents of natural selection • Killifish eat only small guppies • Cichlids eat only large guppies Guppies in streams with cichlids grow faster, reproduce earlier, are smaller at maturity, have more offspring at a time, breed more frequently • Traits can change rapidly if predators change Two Guppies and Guppy Eaters Experiment: Predation Pressures on Guppies Overfishing and Atlantic Cod Overfishing of mature Atlantic cod led to population collapse Early warning sign: rapid decline in age at first reproduction 45.5-45.6 Key Concepts Patterns of Survival and Reproduction Resource availability, disease, and predation are major factors that can restrict population growth These limiting factors differ among species and shape their life history patterns 45.7 Human Population Growth The size of the human population is at its highest level ever and is expected to increase Humans have temporarily evaded environmental resistance to growth through expansion into new habitats, cultural interventions, and technology Eventually, density-dependent controls will slow human population growth Human Population Growth Extraordinary Foundations for Growth Geographic expansion • Technology allows humans to occupy habitats that would not otherwise support them Increased carrying capacity • Agricultural productivity increased carrying capacity for human population Sidestepped limiting factors • Sanitation and medicine lowered death rates Unequal Distribution of Resources More than 1 billion people lack clean drinking water, more than 2 billion face fuel shortages, and more than 800 million are malnourished 45.8 Fertility Rates and Age Structure Worldwide population is expected to peak at 8.9 billion by 2050 – increasing problems with resources, food production, and pollution Risks posed by rising populations have led to increased family planning in almost every region Demographic Indicators for Countries with Different Levels of Development 298 million Population in 2006 188 million 132 million Population in 2025 (projected) Population under age 15 Population above age 65 349 million 211 million 206 million 20% 26% 42% 13% 6% 3% 2.1 1.9 Total fertility rate (TFR) 5.5 Infant mortality rate 6 per 1,000 live births 29 per 1,000 live births 97 per 1,000 live births Life expectancy 78 years 72 years 47 years Per capita income $43,740 $3,460 $560 Fig. 45-15, p. 810 Shifting Fertility Rates Total fertility rate (TFR) • The average number of children born to women of a population during their reproductive years Worldwide total fertility rate has been declining, but is still above replacement level Comparing Age Structure Diagrams More than one-third of the world population is in a broad pre-reproductive base Even if TFR declines to replacement level worldwide, population will continue to increase Age Structure Diagrams Fig. 45-16c, p. 811 Animation: Age structure diagrams Animation: U.S. age structure 45.9 Population Growth and Economic Effects Demographic transition model • Describes population growth rate changes over time as a country becomes economically developed • Four stages based on industrialization: Preindustrial, transitional, industrial, postindustrial The Demographic Transition Model Animation: Demographic transition model Growth and Resource Consumption Population growth and resource consumption are correlated with levels of economic development • The most developed countries have the slowest growth rates and use the most resources Growth rates are typically greatest during the transition to industrialization • As more countries become industrialized, pressure on Earth’s resources will increase Projected Effects of Population Growth 45.10 Rise of the Seniors In some developed countries, decreasing total fertility rate and increasing life expectancy have resulted in a high proportion of older adults In the US, the proportion of people over age 65 is projected to reach 20 percent by 2030 Social Implications In 2050, there could be 31 million Americans over age 85 45.7-45.10 Key Concepts The Human Population Human populations sidestepped limits to growth by way of global expansion into new habitats, cultural interventions, and innovative technology Even so, no population can continue to expand indefinitely Animation: Current and projected population sizes by region Animation: Distribution patterns Animation: Effect of death on growth Animation: Guppy charateristics Animation: Life history patterns Animation: Logistic growth Animation: Mark-recapture method ABC video: People's Explosion ABC video: Easter Island Video: The human touch Video: Pelican nesting colony