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POPULATIONS Chapter 43 Populations intro What is a population? Why do humans study populations? Why would we want to influence the population size of different species? 43.1 Populations Are Patchy in Space and Dynamic over Time Abundance is measured by population density (number of individuals per unit area) and population size Populations are usually dispersed in habitat patches – “islands” of suitable habitat separated by areas of unsuitable habitat Populations are dynamic (always changing) and interdependent Life history—description of an organism’s life cycle Females emerge in spring, lay eggs, and die Life Cycle: Blacklegged ticks Survivors seek host Mated females overwinter Seek large mammal host & mate Winter dormancy (9/28) BR 1. Explain how the population shifts seen below demonstrate that the lynx and hare populations are interdependent. How does each influence the other? 2. 3. Explain how the three populations shown in the graph below are interdependent. Ticks are a vector for Lyme disease. How might the information depicted in the graph below be used to reduce the probability of getting Lyme disease? 43.3 Life Histories Determine Population Growth Rates Life tables shows ages at which individuals make life cycle transitions and how many individuals do so successfully survivorship—fraction of individuals that survive from birth to different life stages fecundity—average number of offspring each individual produces at those life stages or ages Survivorship curves • What do lines I, II, and III say about the survivorship of each species? I. Low mortality rate in early years II. constant mortality throughout life III. high mortality rate early on in life Characteristics of Three Types of Survivorship Curves: Type I : Low mortality rate in early years. Small clutch sizes and good care for young… humans, large mammals Type II: constant mortality rate throughout life…many animals Type III: high mortality rate early in life large clutch size, little care for young…fish, marine invertebrates 43.3 Life Histories Determine Population Growth Rates Principle of allocation—once an organism has acquired a unit of some resource, it can be used for only one function at a time: maintenance, foraging, growth, defense, OR reproduction In stressful conditions, more resources go to maintaining homeostasis Life-history tradeoffs—negative relationships among growth, reproduction, and survival Ex: investments in reproduction may be at the expense of survivorship or growth; age at first reproduction is younger in large clutch size reproducers because they invest less energy in their own growth and development. Principle of Allocation Figure 43.4 Resource Acquisition Increases with Resource Availability Figure 43.4 Environmental conditions affect per capita growth rates (r) and species distributions 43.2 Births Increase and Deaths Decrease Population Size Growth rate is dependent on birth rate and death rate (BD model) N = population size B = number of births t 1 t D = number of deaths N or N=B–D N BD 43.2 Births Increase and Deaths Decrease Population Size Birth rate (b = B/N) Can be estimated for large populations by keeping track of a sample of individuals called a cohort Death rate (d = D/N) Per capita growth rate (r) = average individual’s contribution to total population growth rate r = b – d or dN/dt = rN dN/dt is the change in population size per unit time (∆N/∆t) 43.4 Populations Grow Multiplicatively, but Not for Long Exponential (Multiplicative) growth — an ever-larger number of individuals is added in each successive time period (dN/dt = rmaxN) Additive growth — a constant number (rather than a constant multiple) is added in each time period Which line is which? Exponential growth Additive growth (9/28) BR 4. Populations never continue to grow indefinitely. If bacteria were able to reproduce uninhibited every 20 minutes, 1 bacterium would become 4,720,000,000,000,000, 000,000 in 24 hours. In 36 hours, the surface of the earth would be buried in a foot of bacteria. Why doesn’t this happen? 43.4 Populations Grow Multiplicatively, but Not for Long More individuals are produced than can possibly survive so there is a struggle for existence that drives natural selection and adaptations “Every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair…As more individuals are produced than can possibly survive, there must in every case be a struggle for existence.” -Charles Darwin Yellow star-thistle is a spiny annual plant native to the Mediterranean region. The species is a noxious weed that has invaded several regions of the U.S. It is unpalatable to bison. Rancher Jane discovers that 1 ha of her 128-ha pasture has been invaded by star-thistle. A year later the population has grown to cover 2 ha. How many hectares (ha) do you predict the star-thistle population will cover in 1, 2, and 3 more years if the population is growing additively? How many hectares if the population is growing multiplicatively? Additively: 3, 4, 5; Exponentially: 4, 8, 16 Imagine that Rancher Jane only discovers the star-thistle population after it has already covered 32 hectares of her pasture. How many years does she have until the weed completely covers the pasture if its population is growing additively? Multiplicatively? Additively: 96 years; Exponentially: 2 years 43.4 Populations Grow Multiplicatively, but Not for Long Exponential growth is temporary r is density dependent decreases as the population becomes more crowded Logistic growth – growth slows as r nears 0; population size stops increasing when it reaches its carrying capacity (K); rmaxN((K-N)/K) Which line is which? Exponential growth Logistic growth Examples of Logistic growth: K-selected species - likely to be living at carrying capacity, seen in areas with competition Effect of Population Density on Per Capita Growth Rate Effect of Population Density on Per Capita Growth Rate When population is small, r is at its maximum (rmax) When population density is below K, r is positive At carrying capacity, r = 0 If population exceeds K, r becomes negative and population decreases Exit: Population Practice Problem 1. 2. 3. 4. 5. B = 34 , N = 1000 Find b and apply that birth rate to a population of 500 to calculate the number of births. D = 16 , N = 1000 Find d and apply that death rate to a population of 500 to calculate the number of deaths. Calculate r. Is this population increasing or decreasing? How many individuals are added to the population of 500 in one year? dN/dt = rN; for this example dN/dt = ? (assume dt = 1 year) What is the population size after 1 year (starting size was 500)? 1. 2. 3. 4. 5. Answer: b = 34/1000 = .034, so B = bN ; (.034)(500) = 17 Answer: D = dN ; (.016)(500) = 8 r = b – d ; .034 - .016 = .018; r is positive, so increasing (slowly) Answer: .018(500) = dN/dt = 9 (could also use dN/dt = B – D) 500+ 9 = 509 (9/29) GROUP BR: Work in groups of 3-4! 1. 2. 3. 4. How has the human population typically changed throughout history? Has the growth rate changed in recent years? decades? centuries? Have we experienced logistic or exponential growth? Both? Neither? How much do you think the world population will change over the course of this class? (http://www.breathingearth.net) Starting population : 7,395,505,000 Limiting factors slow population growth. Some only limit growth when populations reach a certain density. List some examples of these density-dependent limiting factors. Other limiting factors limit growth regardless of population density. List some examples of these density-independent limiting factors. 43.4 Populations Grow Multiplicatively, but Not for Long Spatial variation in resource availability can result in variation of carrying capacity Temporal variation in environmental conditions may cause the population to fluctuate above and below the current carrying capacity Density-dependent limiting factors include competition for resources, predation, and disease Density-independent limiting factors include natural disasters and human activities 43.4 Populations Grow Multiplicatively, but Not for Long Human population has been growing exponentially 43.4 Populations Grow Multiplicatively, but Not for Long Some believe we have exceeded K due to: Dependency on fossil fuels Climate change and ecological degradation Ecological Footprint 43.5 Extinction and Recolonization Affect Population Dynamics Small subpopulations within a larger population are vulnerable to environmental disturbances and chance events are at greater risk of extinction Immigration (I) and emigration (E) may also impact populations Nt 1 Nt B I D E 43.6 Ecology Provides Tools for Managing Populations Life histories can be useful in managing other spp Fisheries Ex: rockfish (exhibit indeterminate growth, and larger, older females produce more eggs) Reducing disease risk Ex: ticks Conservation of endangered species Extinction prevention Continuous corridors of habitat can connect subpopulations and allow dispersal/migration (article) Habitat Corridors connect fragmented habitats EXIT World Population at start of class: 7,395,505,000 World Population at end of class: : 7,395,51 Population at end of class LAST YEAR: 7,320,072,720 Population on October 31st, 2013: 7,000,000,000 3. What is the carrying capacity of this population? What is the growth rate (r)? 4. How much will the population increase in one generation (dn/dt) if the starting population is 800 individuals? What about if the starting population is 1100? Show your work! Population pyramids are used to estimate how a population will change in the future 1. Identify the populations in the age-structure diagrams below as either expanding, stable, or contracting. 2. How did you think the “bulge” will affect population growth in the U.S.? Exit: Reflection Create 2 rectangles on your bellringer, and label one “Clear” and the other “Unclear.” In the “Clear” box, write down the information from CH 42 that you feel you understand. In the “Unclear” box, write down any information or concepts about which you feel confused.