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BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor CHAPTER 35 Population Dynamics Modules 35.1 – 35.5 From PowerPoint® Lectures for Biology: Concepts & Connections Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Spread of Shakespeare's Starlings • In the 1800s and early 1900s, introducing foreign species of animals and plants to North America was a popular, unregulated activity • In 1890, a group of Shakespeare enthusiasts released about 120 starlings in New York's Central Park – It was part of a project to bring to America every bird species mentioned in Shakespeare’s works Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Today, the starling range extends from Mexico to Alaska • Their population is estimated at well over 100 million Current 1955 Current 1955 1945 1935 1925 1945 1905 1915 1935 1925 1925 1935 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Over 5 million starlings have been counted in a single roost • Starlings are omnivorous, aggressive, and tenacious • They cause destruction and often replace native bird species • Attempts to eradicate starlings have been unsuccessful Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The starling population in North America has some features in common with the global human population – Both are expanding and are virtually uncontrolled – Both are harming other species • Population ecology is concerned with changes in population size and the factors that regulate populations over time Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.1 Populations are defined in several ways • Ecologists define a population as a singlespecies group of individuals that use common resources and are regulated by the same environmental factors – Individuals in a population have a high likelihood of interacting and breeding with one another • Researchers must define a population by geographic boundaries appropriate to the questions being asked Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings POPULATION STRUCTURE AND DYNAMICS 35.2 Density and dispersion patterns are important population variables • Population density = # of individuals in a given area or volume • It is sometimes possible to count all the individuals in a population – More often, density is estimated by sampling Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • One useful sampling technique for estimating population density is the mark-recapture method Figure 35.2A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The dispersion pattern of a population refers to the way individuals are spaced within their area – Clumped, Uniform (even), Random Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Clumped dispersion = a pattern in which individuals are aggregated in patches – This is the most common dispersion pattern in nature – It often results from an unequal distribution of resources in the environment Figure 35.2B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • A uniform pattern of dispersion often results from interactions among individuals of a population – Territorial behavior and competition for water are examples of such interactions Figure 35.2C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Random dispersion is characterized by individuals in a population spaced in a patternless, unpredictable way – Example: clams living in a mudflat – Environmental conditions and social interactions make random dispersion rare Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How Does Population Size Change? • Several processes can change the size of populations – Birth and immigration add individuals to a population – Death and emigration remove individuals from the population How Does Population Size Change? • Change in population size = (births – deaths) + (immigrants – emigrants) How Does Population Size Change? • Ignoring migration, population size is determined by two opposing forces 1. Biotic potential: the maximum rate at which a population could increase when birth rate is maximal and death rate minimal 2. Environmental resistance: limits set by the living and nonliving environment that decrease birth rates and/or increase death rates (examples: food, space, and predation Population Growth • The growth rate (r) of a population is the change in the population size per individual over some time interval • Determined by Growth rate (r) = birth rate (b) – death rate (d) Ex. In a pop. of 1000, 150 births and 50 deaths r=0.15 – 0.05 r=0.1 or 10% growth per year Population Growth • Population growth per unit of time can be calculated by multiplying growth rate (r) by the original population size (N) Population growth (G) = rN • In the previous example, population growth = rN = 0.1(1000) = 100, so the population has grown by one hundred individuals 35.3 Idealized models help us understand population growth • Idealized models describe two kinds of population growth – exponential growth – logistic growth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Exponential growth = the accelerating increase that occurs during a time when growth is unregulated • A J-shaped growth curve, described by the equation G = rN, is typical of exponential growth – G = the population growth rate – r = the intrinsic rate of increase, or an organism's maximum capacity to reproduce – N = the population size Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 35.3A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Exponential Growth • Exponential growth cannot continue indefinitely • All populations that exhibit exponential growth must eventually stabilize or crash Exponential Growth • Exponential growth can be observed in populations that undergo boom-and-bust cycles – Periods of rapid growth followed by a sudden massive die-off Exponential Growth • Boom-and-bust cycles can be seen in short lived, rapidly reproducing species – Ideal conditions encourage rapid growth – Deteriorating conditions encourage massive die-off Exponential Growth • Example – Each year cyanobacteria in a lake may exhibit exponential growth when conditions are ideal, but crash when they have depleted their nutrient supply Cyanobacteria Abiotic factors may limit many natural populations Exponential Growth • Example – Lemming cycles are more complex and involve overgrazing of food supply, large migrations, and massive mortality caused by predators and starvation – About every 10 years, both hare and lynx populations have a rapid increase (a "boom") followed by a sharp decline (a "bust") • Population cycles may also result from a time lag in the response of predators to rising prey numbers Figure 35.5 Exponential Growth • Temporary exponential growth can occur when population-controlling factors are relaxed, such as – When food supply is increased – When predators are reduced Exponential Growth • When exotic species are introduced into a new ecosystem, population numbers may explode due to lack of natural predators Exponential Growth • When species are protected, e.g. the whooping crane population has grown exponentially since they were protected from hunting and human disturbance in 1940 Environmental Resistance • Many populations that exhibit exponential growth eventually stabilize • Environmental resistance limits population growth – As resources become depleted, reproduction slows Environmental Resistance • This growth pattern, where populations increase to the maximum number sustainable by their environment, is called logistic growth • When this growth pattern is plotted, it results in an S-shaped growth curve (or Scurve) Environmental Resistance • Carrying capacity (K) is the maximum population size that can be sustained by an ecosystem for an extended time without damage to the ecosystem Environmental Resistance • Logistic population growth can occur in nature when a species moves into a new habitat, e.g. barnacles colonizing bare rock along a rocky ocean shoreline • Initially, new settlers may find ideal conditions that allow their population to grow almost exponentially • As population density increases, individuals compete for space, energy, and nutrients Environmental Resistance • In nature, conditions are never completely stable, so both K and the population size will vary somewhat from year to year • However, environmental resistance ideally maintains populations at or below the carrying capacity of their environment Environmental Resistance • These forms of environmental resistance can reduce the reproductive rate and average life span and increase the death rate of young • As environmental resistance increases, population growth slows and eventually stops Environmental Resistance • If a population far exceeds the carrying capacity, excess demands decimate crucial resources • This can permanently and severely reduce K, causing the population to decline to a fraction of its former size or disappear entirely Environmental Resistance • Example: Pribilof Island reindeer populations Environmental Resistance • Environmental resistance can be classified into two broad categories – Density-independent factors – Density-dependent factors Density-Independent Factors • Density-independent factors limit populations regardless of their density – Examples: climate, weather, floods, fires, pesticide use, pollutant release, and overhunting Density-Independent Factors • Some species have evolved means of limiting their losses – Examples: seasonally migrating to a better climate or entering a period of dormancy when conditions deteriorate Density-Dependent Factors • Density-dependent factors become more effective as population density increases • Exert negative feedback effect on population size Density-Dependent Factors • Density-dependent factors can cause birth rates to drop and/or death rates to increase – Population growth slows resulting in an Sshaped growth curve (or S-curve) Density-Dependent Factors • At carrying capacity, each individual's share of resources is just enough to allow it to replace itself in the next generation • At carrying capacity birth rate (b) = death rate (d) Density-Dependent Factors • Carrying capacity is determined by the continuous availability of resources Density-Dependent Factors • Include community interactions – Predation – Parasitism – Competition Predation • Predation involves a predator killing a prey organism in order to eat it – Example: a pack of grey wolves hunting an elk Predation • Predators exert density-dependent controls on a population – Increased prey availability can increase birth rates and/or decrease death rates of predators • Prey population losses will increase Predation • There is often a lag between prey availability and changes in predator numbers – Overshoots in predator numbers may cause predator-prey population cycles – Predator and prey population numbers alternate cycles of growth and decline Predation • Predation may maintain prey populations near carrying capacity – “Surplus" animals are weakened or more exposed Predation • Predation can also maintain prey populations well below carrying capacity – Example: the cactus moth used to control exotic prickly pear in Australia Parasitism • Parasitism involves a parasite living on or in a host organism, feeding on it but not generally killing it – Examples: bacterium causing Lyme disease, some fungi, intestinal worms, ticks, and some protists Parasitism • While parasites seldom directly kill their hosts, they may weaken them enough that death due to other causes is more likely • Parasites spread more readily in large populations Competition for Resources • Competition – Describes the interaction among individuals who attempt to utilize a resource that is limited relative to the demand for it Competition for Resources • Competition intensifies as populations grow and near carrying capacity • For two organisms to compete, they must share the same resource(s) Competition for Resources • Competition may be divided into two groups based on the species identity of the competitors – Interspecific competition is between individuals of different species – Intraspecific competition is between individuals of the same species Competition for Resources • Intense local competition may drive organisms to emigrate – Example: swarming in locusts Factors Interact • The size of a population at any given time is the result of complex interactions between density-independent and densitydependent forms of environmental resistance • Logistic growth = slowed by populationlimiting factors – It tends to level off at carrying capacity – Carrying capacity = the maximum population size that an environment can support at a particular time with no degradation to the habitat Figure 35.3B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The equation G = rN(K - N)/K describes a logistic growth curve – K = carrying capacity – The term (K - N)/K accounts for the leveling off of the curve Figure 35.3C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The logistic growth model predicts that – a population's growth rate will be low when the population size is either small or large – a population’s growth rate will be highest when the population is at an intermediate level relative to the carrying capacity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.4 Multiple factors may limit population growth Review: • The regulation of growth in a natural population is determined by several factors – limited food supply (competition) – the buildup of toxic wastes – increased disease (parasitism) – predation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings LIFE HISTORIES AND THEIR EVOLUTION 35.6 Life tables track mortality and survivorship in populations • Life tables and survivorship curves predict an individual's statistical chance of dying or surviving during each interval in its life • Life tables predict how long, on average, an individual of a given age can expect to live Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings – This table was compiled using 1995 data from the U.S. Centers for Disease Control Table 35.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Survivorship curves plot the proportion of individuals alive at each age • Three types of survivorship curves reflect important species differences in life history Figure 35.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Survivorship in Populations • "Late loss" curves: seen in many animals with few offspring that receive substantial parental care; are convex in shape, with low mortality until individuals reach old age – Examples: humans and many large mammals Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Survivorship in Populations • "Constant loss" curves: an approximate straight line, indicates an equal chance of dying at any age – Example: some bird species Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Survivorship in Populations • "Early loss" curves: high early mortality as most offspring fail to become established; are concave in shape – Typical of most plants and many animals that do not receive parental care – Examples: most invertebrates and fish Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.7 Evolution shapes life histories • An organism's life history is the series of events from birth through reproduction to death • Life history traits (which influence the biotic potential) includes – the age at which reproduction first occurs – the frequency of reproduction – the number of offspring – the amount of parental care given – the energy cost of reproduction Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Biotic Potential (1) The age at which the organism first reproduces – Populations that have their offspring earlier in life tend to grow at a faster rate Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Biotic Potential (2) The frequency at which reproduction occurs Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Biotic Potential (3) The average number of offspring produced each time (4) The length of the organism's reproductive life span Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Biotic Potential (5) The death rate of individuals – Increased death rates can slow the rate of population growth significantly Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The effects of predation on life history traits of guppies has been tested by field experiments for several years Experimental transplant of guppies Predator: Killifish; preys mainly on small guppies Guppies: Larger at sexual maturity than those in “pike-cichlid” pools Predator: Pike-cichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in “killifish” pools Figure 35.7A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • In nature, every population has a particular life history adapted to its environment • The agave illustrates what ecologists call "big-bang reproduction" – It is able to store nutrients until environmental conditions favor reproductive success Figure 35.7B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Natural selection favors a combination of life history traits that maximizes an individual's output of viable, fertile offspring Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Selection for life history traits that maximize reproductive success in uncrowded, unpredictable environments is called r-selection – Such populations maximize r, the intrinsic rate of increase – Individuals of these populations mature early and produce a large number of offspring at a time – Many insect and weed species exhibit r-selection Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Selection for life history traits that maximize reproductive success in populations that live at densities close to the carrying capacity (K) of their environment is called K-selection – Individuals mature and reproduce at a later age and produce a few, well-cared-for offspring – Mammals exhibit K-selection Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings THE HUMAN POPULATION 35.8 Connection: The human population has been growing exponentially for centuries • The human population as a whole has doubled three times in the last three centuries • The human population now stands at about 6.1 billion and may reach 9.3 billion by the year 2050 • Most of the increase is due to improved health and technology – These have affected death rates Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The history of human population growth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The ecological footprint represents the amount of productive land needed to support a nation’s resource needs • The ecological capacity of the world may already be smaller than its ecological footprint Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Ecological footprint in relation to ecological capacity Figure 35.8B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The exponential growth of the human population is probably the greatest crisis ever faced by life on Earth Figure 35.8C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.9 Birth and death rates and age structure affect population growth • Population stability is achieved when there is zero population growth – Zero population growth is when birth rates equal death rates • There are two possible ways to reach zero population growth (ZPG) – ZPG = High birth rates - high death rates – ZPG = Low birth rates - low death rates Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The demographic transition is the shift from high birth and death rates to low birth and death rates – During this transition, populations may grow rapidly until birth rates decline Figure 35.9A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The age structure of a population is the proportion of individuals in different agegroups – Age structure affects population growth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings RAPID GROWTH SLOW GROWTH ZERO GROWTH/DECREASE Kenya United States Italy Male Female Male Female Ages 45+ Ages 45+ Ages 15–44 Ages 15–44 Under 15 Percent of population Male Female Under 15 Percent of population Percent of population Figure 35.9B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Age-structure diagrams not only reveal a population's growth trends – They also indicate social conditions • Increasing the status and education of women may help to reduce family size Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.10 Connection: Principles of population ecology have practical applications • Principles of population ecology may be used to – manage wildlife, fisheries, and forests for sustainable yield – reverse the decline of threatened or endangered species – reduce pest populations Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Renewable resource management is the harvesting of crops without damaging the resource – However, human economic and political pressures often outweigh ecological concerns – There is frequently insufficient scientific information Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The collapse of the northern cod fishery – Estimates of cod stocks were too high – The practice of discarding young cod (not of legal size) at sea caused a higher mortality rate than was predicted Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Collapse of northern cod fishery Figure 35.10A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • For species that are in decline or facing extinction, resource managers try to increase population size • Carrying capacity is usually increased by providing additional habitat or improving the quality of existing habitat Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Endangered species often have subtle habitat requirements – The red-cockaded woodpecker was recently recovered from near-extinction by protecting its pine habitat and using controlled fires to reduce undergrowth Figure 35.10B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Integrated pest management (IPM) uses a combination of biological, chemical, and cultural methods to control agricultural pests • IPM relies on knowledge of – the population ecology of the pest – its associated predators and parasites – crop growth dynamics • One objective of IPM is to minimize environmental and health risks by relying on natural biological control when possible Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings