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Population Ecology Chapter 52 Study of populations in relation to environment – Environmental influences on: • population density and distribution • age structure • variations in population size Definition of Population: • Group of individuals of a single species living in the same general area 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 Density: A Dynamic Perspective • Determining the density of natural populations is possible, but difficult to accomplish • In most cases it is impractical or impossible to count all individuals in a population – How do wildlife biologists approximate populations? Estimating Wildlife Population Size Defined Populations Undefined Populations • Density is the result of a dynamic interaction of processes that add individuals to a population and those that remove individuals from it Births and immigration add individuals to a population. Births Immigration How do these factors Contribute to Population Size?? PopuIation size • • • • Emigration Deaths Deaths and emigration remove individuals from a population. Figure 52.2 Births Deaths Immigration Emigration Patterns of Dispersion • Environmental and social factors influence the spacing of individuals in a population 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 • Position of each individual is independent of other individuals (c) Random. Dandelions grow from windblown seeds that land at random and later germinate. Life history traits are products of natural selection • Life history traits are evolutionary outcomes – Reflected in the development, physiology, and behavior of an organism Semelparity: Big Bang – Reproduce a single time and die Figure 52.6 Iteroparity – Repeated Reproduction – Produce offspring repeatedly over time “Trade-offs” and Life Histories • Organisms have finite resources – Which may lead to trade-offs between survival and reproduction Kestrels: • Produce a few eggs? – • Can invest more into each, ensuring greater survival Produce many eggs? – Costly but if all survive, fitness is better Male Female Parents surviving the following winter (%) – 100 80 RESULTS 60 40 20 0 Reduced brood size Normal brood size Enlarged brood size CONCLUSION The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. More is Better? • Some plants produce a large number of small seeds – Ensuring that at least some of them will grow and eventually reproduce (a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves. Figure 52.8a Fewer is Better? • Other types of plants produce a moderate number of large seeds – That provide a large store of energy that will help seedlings become established (b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring. Figure 52.8b Demography • Study of the vital statistics of a population – And how they change over time • Death rates and birth rates • Zero population growth – Occurs when the birth rate equals the death rate Exponential Population Growth Population increase under idealized conditions No limits on growth • Under these conditions – The rate of reproduction is at its maximum, called the intrinsic rate of increase Example-understanding growth Question: I offer you a job for 1 cent/day and your pay will double every day. You will be hired for 30 days. Will you take my job offer? Answer: If you said YES, you will have made $~21 million dollars for 30 days of work. How is this possible????? 1ST DAY OF WORK: 1 cent pay/day 30TH DAY OF WORK: ~10.2 million/day Amount of Pay/Day How is this possible????? # of Days Exponential Growth Model *Idealized population in an unlimited environment *Very rapid doubling time; steep J curve *r=N=(b-d)N t r=instrinsic rate of growth dN dt rmaxN Exponential Growth in the Real World • Characteristic of some populations that are rebounding 8,000 Elephant population 6,000 4,000 2,000 0 1900 1920 1940 Year 1960 1980 –Cannot be sustained for long in any population Logistic Population Growth • A more realistic population model – Limits growth by incorporating carrying capacity Logistic Population Growth • Carrying capacity (K) – Is the maximum population size the environment can support • In the logistic population growth model – The per capita rate of increase declines as carrying capacity is reached Logistic Growth Equation – Includes K, the carrying capacity (K N) dN rmax N dt K Logistic Population Growth – Produces a sigmoid (S-shaped) curve 2,000 dN dt Population size (N) 1,500 1.0N Exponential growth K 1,500 Logistic growth 1,000 dN dt 1.0N 1,500 N 1,500 500 (K N) dN rmax N dt K 0 0 5 10 Number of generations Figure 52.12 15 The Logistic Model and Real Populations • The growth of laboratory populations of paramecia – Fits an S-shaped curve Number of Paramecium/ml 1,000 Figure 52.13a 800 600 400 200 0 0 5 10 Time (days) 15 (a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment. Logistic Growth and The Real World • Some populations overshoot K Before settling down to a relatively stable density Number of Daphnia/50 ml – 180 150 120 90 60 30 0 0 20 40 60 80 100 120 140 160 Time (days) Figure 52.13b (b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size. Logistic Growth and the Real World • Some populations – Fluctuate greatly around K Number of females 80 60 40 20 0 1975 1980 1985 1990 1995 2000 Time (years) Figure 52.13c (c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model. The Logistic Model and Life Histories • Life history traits favored by natural selection – May vary with population density and environmental conditions Life History and Logistic Growth • K-selection, or density-dependent selection – Selects for life history traits that are sensitive to population density • Reproduce slowly, small litters • r-selection, or density-independent selection – Selects for life history traits that maximize reproduction • Reproduce rapidly, large litters Natural selection (diverse reproductive strategies) a) Relatively few, large offspring (K selected species) b) Many, small offspring (r selected species) (K selected species) (r selected species) Human Populations • No population can grow indefinitely and humans are no exception 6 4 3 2 The Plague 1 0 Figure 52.22 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 0 1000 A.D. 2000 A.D. Human population (billions) 5 Global Carrying Capacity • Just how many humans can the biosphere support? • Carrying capacity of earth is unknown…. http://www.youtube.com/watch?v=9_9SutNmfFk http://www.youtube.com/watch?v=UUOEcNomakw&feature=rec -LGOUT-exp_fresh+div-1r-8-HM http://www.youtube.com/watch?v=4B2xOvKFFz4&feature=relat ed Populations Regulated Biotic and Abiotic Factors Two general questions we can ask about regulation of population growth 1. What environmental factors stop a population from growing? 2. Why do some populations show radical fluctuations in size over time, while others remain stable? Population Change and Population Density • In density-independent populations – Birth rate and death rate do not change with population density • In density-dependent populations – Birth rates fall and death rates rise with population density Density-Dependent Population Regulation • Density-dependent birth and death rates – Are an example of negative feedback that regulates population growth – Are affected by many different mechanisms Competition for Resources • In crowded populations, increasing population density – Intensifies intraspecific competition for resources 4.0 3.8 Average clutch size Average number of seeds per reproducing individual (log scale) 10,000 1,000 100 3.4 3.2 3.0 2.8 0 0 10 100 Seeds planted per m2 (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. Figure 52.15a,b 3.6 0 10 20 30 40 50 60 70 Density of females (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply. 80 Territoriality • In many vertebrates and some invertebrates – Territoriality may limit density Territoriality Example: Cheetas • Cheetahs are highly territorial – Using chemical communication to warn other cheetahs of their boundaries Figure 52.16 Territoriality: Ocean birds – Figure 52.17 Exhibit territoriality in nesting behavior Health • Population density – Can influence the health and survival of organisms • In dense populations – Pathogens can spread more rapidly Predation • As a prey population builds up – Predators may feed preferentially on that species Intrinsic Factors • For some populations – Intrinsic (physiological) factors appear to regulate population size Population Dynamics • The study of population dynamics – Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size Fluctuations in Population Size • Extreme fluctuations in population size – Are typically more common in invertebrates than in large mammals Commercial catch (kg) of male crabs (log scale) 730,000 100,000 10,000 1950 Figure 52.19 1960 1970 Year 1980 1990 Metapopulations and Immigration • Metapopulations – Groups of populations linked by immigration and emigration Immigration- Movement Into a Population • High levels of immigration combined with higher survival can result in greater stability in populations 60 Number of breeding females 50 40 Mandarte island 30 20 10 Small islands 0 1988 Figure 52.20 1989 1990 Year 1991 Population Cycles 160 Snowshoe hare 120 Lynx 9 80 6 40 3 0 1850 0 1875 1900 Year • Influenced by complex interactions between biotic and abiotic factors 1925 Lynx population size (thousands) Hare population size (thousands) • Many populations undergo regular boom-and-bust cycles The Global Human Population • The human population increased relatively slowly until about 1650 and then began to grow exponentially 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 Age Structure • One important demographic factor in present and future growth trends – Is a country’s age structure, the relative number of individuals at each age • Age structure is commonly represented in pyramids Rapid growth Afghanistan Male Female 8 6 4 2 0 2 4 6 8 Percent of population Figure 52.25 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 Slow growth United States Female Male 8 6 4 2 0 2 4 6 8 Percent of population 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 Decrease Italy Female Male 8 6 4 2 0 2 4 6 8 Percent of population Infant Mortality and Life Expectancy • Infant mortality and life expectancy at birth 50 Life expectancy (years) Infant mortality (deaths per 1,000 births) – Vary widely among developed and developing countries but do not capture the wide range of the 60 80 human condition 40 30 20 40 20 10 0 0 Developed countries Figure 52.26 60 Developing countries Developed countries Developing countries