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Point-of-View • The root cause of nearly every environmental issue is human overpopulation. • Do you agree with this opinion? – Why or why not? © 2011 Pearson Education, Inc. Overview: Counting Sheep • A small population of Soay sheep were introduced to Hirta Island in 1932 • They provide an ideal opportunity to study changes in population size on an isolated island with abundant food and no predators © 2011 Pearson Education, Inc. Figure 53.1 Big Ideas • 2D.1: All biological systems from cells to ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and energy. – – – – – Symbiosis Predator/Prey interactions (AKA food chain) Availability of nutrients and energy Population density and dispersion (Figure 53.4) Species diversity Big Ideas • 4A.5: Communities are composed of populations of organisms that interact in complex ways. – – – – Predator/prey interactions (Lynx/Snowshoe hare; Fig. 53.18) Carrying capacity (Figure 53.9) Exponential v. Logistic growth (Sections 53.2-3) Density-dependent and density independent factors (Section 53.5) Key Points • #1: Population density, dispersion, and demographics are dynamic (ever changing). • #2: In ideal, unlimited environments, populations grow exponentially • #3: As populations near the carrying capacity, population growth is described as logistic. • #4: Many factors that regulate population growth are density dependent © 2011 Pearson Education, Inc. Population ecology •study of populations in relation to their environment •Influenced by i. ii. iii. iv. Density Distribution Age structure Population size © 2011 Pearson Education, Inc. Key Point #1: Population density, dispersion, and demographics are dynamic (ever changing). • A population is a group of individuals of a single species living in the same general area • Populations are described by their boundaries and size © 2011 Pearson Education, Inc. Population ecology •study of populations in relation to their environment •Influenced by i. ii. iii. iv. Density Distribution Age structure Population size © 2011 Pearson Education, Inc. Density and Dispersion • Density is the number of individuals per unit area or volume • Dispersion is the pattern of spacing among individuals within the boundaries of the population © 2011 Pearson Education, Inc. Population Density Tokyo: 9,499 persons/mi2 5.1 Population Growth Notes Seattle: 6,715 persons/mi2 Unit #5: Interconnected 22 US Population Density 5.1 Population Growth Notes Unit #5: Interconnected 23 Density: A Dynamic Perspective • In most cases, it is impractical or impossible to count all individuals in a population • Sampling techniques can be used to estimate densities and total population sizes APPLICATION Hector’s dolphins © 2011 Pearson Education, Inc. • Density is the result of an interplay between processes that add individuals to a population and those that remove individuals • Immigration is the influx of new individuals from other areas • Emigration is the movement of individuals out of a population © 2011 Pearson Education, Inc. Figure 53.3 Births Births and immigration add individuals to a population. Immigration Deaths Deaths and emigration remove individuals from a population. Emigration Figure 53.3 Births Births and immigration add individuals to a population. Immigration Deaths Deaths and emigration remove individuals from a population. Emigration Population ecology •study of populations in relation to their environment •Influenced by i. ii. iii. iv. Density Distribution Age structure Population size © 2011 Pearson Education, Inc. Figure 53.UN01 Patterns of dispersion Clumped Uniform Random Patterns of Dispersion (a) Clumped • Individuals gather in patches • May be influenced by resource availability and behavior (b) Uniform • • Even distribution of individuals May be influenced by social interactions such as territoriality (c) Random • • Even distribution of individuals May be influenced by social interactions such as territoriality Demographics • Demography – study of the vital statistics of a population – Study of the change in populations over time vs. © 2011 Pearson Education, Inc. Survivorship Curves • A survivorship curve is a graphic way of representing the data in a life table • The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate © 2011 Pearson Education, Inc. Figure 53.5 Number of survivors (log scale) 1,000 100 Females 10 Males 1 0 2 4 6 Age (years) 8 10 • Survivorship curves can be classified into three general types – Type I – Type II – Type III © 2011 Pearson Education, Inc. Survivorship Curves Type I • low death rates during early and middle life • an increase in death rates among older age groups © 2011 Pearson Education, Inc. Type II Type III Number of survivors (log scale) Figure 53.6 1,000 I 100 10 1 0 50 Percentage of maximum life span 100 Survivorship Curves Type I • low death rates during early and middle life • an increase in death rates among older age groups © 2011 Pearson Education, Inc. Type II • a constant death rate over the organism’s life span Type III Number of survivors (log scale) Figure 53.6 1,000 100 II 10 1 0 50 Percentage of maximum life span 100 Survivorship Curves Type I • low death rates during early and middle life • an increase in death rates among older age groups © 2011 Pearson Education, Inc. Type II Type III • a constant death rate over the organism’s life span • high death rates for the young • lower death rate for survivors Number of survivors (log scale) Figure 53.6 1,000 100 10 III 1 0 50 Percentage of maximum life span 100 Reproductive Rates • For species with sexual reproduction, demographers often concentrate on females in a population © 2011 Pearson Education, Inc. Key Points • #1: Population density, dispersion, and demographics are dynamic (ever changing). • #2: In ideal, unlimited environments, populations grow exponentially • #3: As populations near the carrying capacity, population growth is described as logistic. • #4: Many factors that regulate population growth are density dependent © 2011 Pearson Education, Inc. Key Concept #2: In ideal, unlimited environments, populations grow exponentially • It is useful to study population growth in an idealized situation • Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth © 2011 Pearson Education, Inc. Per Capita Rate of Increase Change in population size Immigrants Emigrants Births entering Deaths leaving population population • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate © 2011 Pearson Education, Inc. Figure 53.3 Births Births and immigration add individuals to a population. Immigration Deaths Deaths and emigration remove individuals from a population. Emigration Figure 53.3 If the initial population were 110 sparrows, how will The factors below change the population size? Births Deaths 58 70 Deaths and emigration remove individuals from a population. Births and immigration add individuals to a population. Immigration 32 12 Emigration A Little Population Math • 1) The population growth rate can be expressed mathematically as: r=(births-deaths) N -N is the initial population size - r is the reproductive growth rate © 2011 Pearson Education, Inc. Growth Pattern #1: Exponential Growth • Exponential population growth is population increase under idealized conditions • Under these conditions, the rate of increase is at its maximum, denoted as rmax • The equation of exponential population growth is dN dt rmaxN • Exponential population growth results in a Jshaped curve © 2011 Pearson Education, Inc. Population size (N) Figure 53.UN02 dN = rmax N dt Number of generations Figure 53.7 2,000 Population size (N) dN = 1.0N dt 1,500 dN = 0.5N dt 1,000 500 0 5 10 Number of generations 15 • The J-shaped curve of exponential growth characterizes some rebounding populations – For example, the elephant population in Kruger National Park, South Africa, grew exponentially after hunting was banned © 2011 Pearson Education, Inc. Figure 53.8 Elephant population 8,000 6,000 4,000 2,000 0 1900 1910 1920 1930 1940 Year 1950 1960 1970 Key Points • #1: Population density, dispersion, and demographics are dynamic (ever changing). • #2: In ideal, unlimited environments, populations grow exponentially • #3: As populations near the carrying capacity, population growth is described as logistic. • #4: Many factors that regulate population growth are density dependent © 2011 Pearson Education, Inc. Key Concept #3: As populations near the carrying capacity, population growth is described as logistic. • Exponential growth cannot be sustained for long in any population • A more realistic population model limits growth by incorporating carrying capacity • Carrying capacity (K) is the maximum population size the environment can support • Carrying capacity varies with the abundance of limiting resources © 2011 Pearson Education, Inc. Growth Pattern #2: Logistic Growth • In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached • The logistic model starts with the exponential model and adds an expression that reduces the per capita rate of increase as N approaches K (K N) dN rmax N dt K • The logistic model of population growth produces a sigmoid (S-shaped) curve © 2011 Pearson Education, Inc. Population size (N) Figure 53.UN03 K = carrying capacity K–N dN = rmax N K dt ( Number of generations ) Figure 53.9 Exponential growth dN = 1.0N dt Population size (N) 2,000 1,500 K = 1,500 Logistic growth 1,500 – N dN = 1.0N 1,500 dt ( 1,000 Population growth begins slowing here. 500 0 0 5 10 Number of generations 15 ) The Logistic Model and Real Populations • The growth of laboratory populations of paramecia fits an S-shaped curve • These organisms are grown in a constant environment lacking predators and competitors • Some populations overshoot K before settling down to a relatively stable density © 2011 Pearson Education, Inc. Number of Daphnia/50 mL Number of Paramecium/mL Figure 53.10 1,000 800 600 400 200 0 0 5 10 Time (days) 15 (a) A Paramecium population in the lab 180 150 120 90 60 30 0 0 20 40 60 80 100 120 140 160 Time (days) (b) A Daphnia population in the lab Figure 53.11 Key Points • #1: Population density, dispersion, and demographics are dynamic (ever changing). • #2: In ideal, unlimited environments, populations grow exponentially • #3: As populations near the carrying capacity, population growth is described as logistic. • #4: Many factors that regulate population growth are density dependent © 2011 Pearson Education, Inc. Key Concept #4: Many factors that regulate population growth are density dependent • There are two general questions about regulation of population growth – What environmental factors stop a population from growing indefinitely? – Why do some populations show radical fluctuations in size over time, while others remain stable? © 2011 Pearson Education, Inc. Population Change and Population Density Abiotic Factors •Weather •Fire •Floods •Wind and storms •Oxygen supply Biotic Factors •Predation •Infection •Competition •Toxic build-up •Available mates © 2011 Pearson Education, Inc. 1) Competition for Resources • In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate © 2011 Pearson Education, Inc. Figure 53.17a 2) Toxic Wastes • Accumulation of toxic wastes can contribute to density-dependent regulation of population size © 2011 Pearson Education, Inc. Figure 53.17c 5 m 3) Predation • As a prey population builds up, predators may feed preferentially on that species © 2011 Pearson Education, Inc. Figure 53.17b 4) Intrinsic Factors • For some populations, intrinsic (physiological) factors appear to regulate population size © 2011 Pearson Education, Inc. Figure 53.17d 5) Territoriality • In many vertebrates and some invertebrates, competition for territory may limit density © 2011 Pearson Education, Inc. Figure 53.17e 6) Disease • Population density can influence the health and survival of organisms • In dense populations, pathogens can spread more rapidly © 2011 Pearson Education, Inc. Figure 53.17f Population Dynamics • The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size © 2011 Pearson Education, Inc. Stability and Fluctuation • Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time • Both weather and predator population can affect population size over time – For example, the moose population on Isle Royale collapsed during a harsh winter, and when wolf numbers peaked © 2011 Pearson Education, Inc. Figure 53.18 2,500 Wolves Moose 40 2,000 30 1,500 20 1,000 10 500 0 1955 0 1965 1975 1985 Year 1995 2005 Number of moose Number of wolves 50 Paramecium Competition 120 100 80 P. aurelia grown alone, cells/mL P. caudatum grown alone, cells/mL 60 P. aurelia grown in mixed culture, cells/ mL P caudatum grown in mixed culture, cells/mL 40 20 0 Day O Day 2 Day 4 Day 6 Day 8 Day 10 Day 12 Day 14 Day 16 Figure 53.19a Figure 53.19 Snowshoe hare 120 9 Lynx 80 6 40 3 0 0 1850 1875 1900 Year 1925 Number of lynx (thousands) Number of hares (thousands) 160 Lynx v. Rabbits: Predation 120 Population Size 100 80 60 Rabbits Left Total Lynx 40 20 0 1 3 5 7 9 11 13 15 17 19 21 23 25 Generations Key Points • #1: Population density, dispersion, and demographics are dynamic (ever changing) • #2: In ideal, unlimited environments, populations grow exponentially • #3: As populations near the carrying capacity, population growth is described as logistic. • #4: Many factors that regulate population growth are density dependent © 2011 Pearson Education, Inc. Review Questions 1. Explain why a population that fits the logistic growth model increases more rapidly at intermediate population size than at relatively small and large population sizes. 2. Where is exponential growth by a plant population more likely- in an area where a forest was destroyed by fire or in a mature, undisturbed forest? Why? 3. Describe three attributes of a habitat that could affect (+ or-) the population density (or dispersion) and rates of emigration and immigration. © 2011 Pearson Education, Inc. Review Questions: Answered! 1. Explain why a population that fits the logistic growth model increases more rapidly at intermediate population size than at relatively small and large population sizes. When N (population size) is small, there are relatively few individuals producing offspring. When N is large, near the carrying capacity, the per capita growth rate (r) is relatively small because it is limited by available resources. © 2011 Pearson Education, Inc. Review Questions: Answered! 2. Where is exponential growth by a plant population more likely- in an area where a forest was destroyed by fire or in a mature, undisturbed forest? Why? Exponential growth is more likely in the area where a forest was destroyed by fire. The first plants that found suitable habitat there would encounter an abundance of space, nutrients, and light. In the undisturbed forest, competition among plants for these resources would be intense. © 2011 Pearson Education, Inc. Review Questions: Answered! 3. Describe three attributes of a habitat that could affect (+ or-) the population density (or dispersion) and rates of emigration and immigration. Three attributes are the size, quality, and isolation of the patches. A patch that is larger or of higher quality is more likely to attract individuals and to be a source of individuals for other patches. A patch that is relatively isolated will undergo less exchange of individuals with other patches. © 2011 Pearson Education, Inc.