2Chapter Overview Questions What are the major characteristics of populations?How do populations respond to changes in environmental conditions?How do species differ in their reproductive patterns?
3Updates OnlineThe latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at to access InfoTrac articles.InfoTrac: One Hatchling at a Time. Brownsville Herald (Brownsville, TX), July 2, 2006.InfoTrac: Where the Cattle Herds Roam, Ideally in Harmony With Their Neighbors. Jim Robbins. The New York Times, July 11, 2006 pF3(L).InfoTrac: A nudge for nature. Milwaukee Journal Sentinel, July 10, 2006.Earth Island InstituteEnvironmental Defense: Creating a Conservation Community in Oregon’s Williamette ValleyMarine Bio: Habitat Conservation
4Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.Figure 8-1
5Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? Sea otters are an important keystone species for sea urchins and other kelp-eating organisms.Figure 8-1
6POPULATION DYNAMICS AND CARRYING CAPACITY Most populations live in clumps although other patterns occur based on resource distribution.Figure 8-2
7(a) Clumped (elephants) Figure 8.2Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.(a) Clumped (elephants)Fig. 8-2a, p. 162
8(b) Uniform (creosote bush) Figure 8.2Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.(b) Uniform (creosote bush)Fig. 8-2b, p. 162
9(c) Random (dandelions) Figure 8.2Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.(c) Random (dandelions)Fig. 8-2c, p. 162
10Changes in Population Size: Entrances and Exits Populations increase through births and immigrationPopulations decrease through deaths and emigration
11Age Structure: Young Populations Can Grow Fast How fast a population grows or declines depends on its age structure.Prereproductive age: not mature enough to reproduce.Reproductive age: those capable of reproduction.Postreproductive age: those too old to reproduce.
12Limits on Population Growth: Biotic Potential vs Limits on Population Growth: Biotic Potential vs. Environmental ResistanceNo population can increase its size indefinitely.The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources.Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.
13Exponential and Logistic Population Growth: J-Curves and S-Curves Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.Figure 8-4
14Environmental Resistance Carrying capacity (K)Population size (N)BioticPotentialExponentialGrowthFigure 8.3Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers.Time (t)Fig. 8-3, p. 163
16Exponential and Logistic Population Growth: J-Curves and S-Curves As a population levels off, it often fluctuates slightly above and below the carrying capacity.Figure 8-4
17Number of sheep (millions) OvershootCarrying capacityNumber of sheep (millions)Figure 8.4Boom and bust: logistic growth of a sheep population on the island of Tasmania between 1800 and After sheep were introduced in 1800, their population grew exponentially thanks to an ample food supply. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep.YearFig. 8-4, p. 164
18Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.Figure 8-6
19Population overshoots carrying capacity Number of reindeer Population CrashesNumber of reindeerFigure 8.6Exponential growth, overshoot, and population crash of reindeer introduced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, with the herd size plummeting to only 8 reindeer by 1950.CarryingcapacityYearFig. 8-6, p. 165
20Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Over time species may increase their carrying capacity by developing adaptations.Some species maintain their carrying capacity by migrating to other areas.So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.
21How Would You Vote?Can we continue to expand the earth's carrying capacity for humans?a. No. Unless humans voluntarily control their population and conserve resources, nature will do it for us.b. Yes. New technologies and strategies will allow us to further delay exceeding the earth's carrying capacity.
22Population Density and Population Change: Effects of Crowding Population density: the number of individuals in a population found in a particular area or volume.A population’s density can affect how rapidly it can grow or decline.e.g. biotic factors like diseaseSome population control factors are not affected by population density.e.g. abiotic factors like weather
23Types of Population Change Curves in Nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.Stable: fluctuates slightly above and below carrying capacity.Irruptive: populations explode and then crash to a more stable level.Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles.Irregular: erratic changes possibly due to chaos or drastic change.
24Types of Population Change Curves in Nature Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.Figure 8-7
25Population size (thousands) HareLynxPopulation size (thousands)Figure 8.7Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich)YearFig. 8-7, p. 166
27Case Study: Exploding White-Tailed Deer Populations in the United States Since the 1930s the white-tailed deer population has exploded in the United States.Nearly extinct prior to their protection in 1920’s.Today million white-tailed deer in U.S. pose human interaction problems.Deer-vehicle collisions (1.5 million per year).Transmit disease (Lyme disease in deer ticks).
28REPRODUCTIVE PATTERNS Some species reproduce without having sex (asexual).Offspring are exact genetic copies (clones).Others reproduce by having sex (sexual).Genetic material is mixture of two individuals.Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly.Major advantages: genetic diversity, offspring protection.
29Sexual Reproduction: Courtship Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners.Figure 8-8
30Reproductive Patterns: Opportunists and Competitors Large number of smaller offspring with little parental care (r-selected species).Fewer, larger offspring with higher invested parental care (K-selected species).Figure 8-9
31Carrying capacity K K species; experience K selection Number of individualsFigure 8.9Positions of r-selected and K-selected species on the sigmoid (S-shaped) population growth curve.r species;experiencer selectionTimeFig. 8-9, p. 168
32Reproductive Patterns r-selected species tend to be opportunists while K-selected species tend to be competitors.Figure 8-10
33Little or no parental care and protection of offspring r-Selected SpeciesCockroachDandelionMany small offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carrying capacity (K)Generalist nicheLow ability to competeEarly successional speciesFigure 8.10Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes.Fig. 8-10a, p. 168
34Fewer, larger offspring High parental care and protection of offspring K-Selected SpeciesElephantSaguaroFewer, larger offspringHigh parental care and protectionof offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carrying capacity (K)Specialist nicheHigh ability to competeLate successional speciesFigure 8.10Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes.Fig. 8-10b, p. 168
35Survivorship Curves: Short to Long Lives The way to represent the age structure of a population is with a survivorship curve.Late loss population live to an old age.Constant loss population die at all ages.Most members of early loss population, die at young ages.
36Survivorship Curves: Short to Long Lives The populations of different species vary in how long individual members typically live.Figure 8-11
37Percentage surviving (log scale) Late lossConstant lossPercentage surviving (log scale)Figure 8.11When does death come? Survivorship curves for populations of different species, show the percentages of the members of a population surviving at different ages. Most members of a late loss population (such as elephants, rhinoceroses, and humans) live to an old age. Members of a constant loss population (such as many songbirds) die at all ages. In an early loss population (such as annual plants and many bony fish species), most members die at a young age. These generalized survivorship curves only approximate the realities of nature.Early lossAgeFig. 8-11, p. 169