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Ecology of Populations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Population ecology is the study of populations in relation to environment – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Dynamic biological processes influence population density, dispersion, and demography • A population is a group of individuals of a single species living in the same general area Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Four main factors affecting population size • Natality = number of births • • Immigration = number of individuals arriving from other places • Emigration = number of individuals leaving the population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The exponential model describes population growth in an idealized, unlimited environment • It is useful to study population growth in an idealized situation – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Exponential Growth • Exponential population growth – • Under these conditions – The rate of reproduction is at its maximum, called the intrinsic rate of increase (rmax) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Exponential population growth – Results in a J-shaped curve Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The J-shaped curve of exponential growth – Is characteristic of some populations that are rebounding Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Environmental factors that restrict population growth are called population-limiting factors • Environmental factors limit the growth of populations, preventing exponential growth • • Produces an S-shaped or logistic growth curve Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • • Exponential growth – Cannot be sustained for long in any population • A more realistic population model – Limits growth by incorporating carrying capacity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Carrying capacity (K) – Is the maximum population size the environment can support with no net increase or decrease – The value of K varies, depending on species and habitat – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Carrying capacity is defined by limiting factors such as: – Availability of resources – Build up of wastes, CO2 – Predation – Disease Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The logistic model of population growth – Produces a sigmoid (S-shaped) curve Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sigmoid or Logistic Growth Curve 1. Exponential phase 2. Transitional phase 3. Plateau phase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Causes of each phase • Exponential phase - under ideal conditions, population doubles on a regular basis, due to: – Plentiful resources – – Favorable abiotic factors – Little or no predation or disease Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Causes of each phase • Transitional phase - population growth rate slows because of : – Increased competition for resources – More predators – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Causes of each phase • Plateau phase - population size has stabilized, no more growth, this is due to: – – Predators and disease increase mortality and growth curve levels off – Births + immigration = deaths + emigrations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Logistic Model and Real Populations • The growth of laboratory populations of paramecia – Fits an S-shaped curve Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some populations overshoot K – Before settling down to a relatively stable density Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some populations – Fluctuate greatly around K Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The logistic model fits few real populations – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Logistic Model and Life Histories • Life history traits favored by natural selection – May vary with population density and environmental conditions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • K-selection, or density-dependent selection – • r-selection, or density-independent selection – Selects for life history traits that maximize reproduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Survivorship Curves • A survivorship curve is a graphic way of representing the data in a life table that shows the differential mortality rates in relation to age Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The survivorship curve for Belding’s ground squirrels – Shows that the death rate is relatively constant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Survivorship curves can be classified into three general types – Type I, Type II, and Type III Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Type I curve shows low infant mortality – These species produce few offspring but provide them with a high degree of parental care. This increases the likelihood that they will survive to maturity – – Show a low rmax Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Type II curve mortality is more constant over the life span Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Type III curve have high death rates for very young. Death rates drop as individuals survive to increased ages • Species produce very large numbers of offspring but provide little or no care for them • • Have a high rmax Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An organisms life history influences the growth rate of a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Opportunistic life history or r-strategists • Small bodied species, reproduce when young, produce many offspring, populations tend to grow exponentially when conditions are favorable • Such populations typically live in unstable or disrupted environments and are controlled by densityindependent factors • • Exhibit type III survivorship curves • Use a tactic called r-selection • Insects and other invertebrates Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Equilibrial life history or K-strategists • Larger bodied species, produce few slowly maturing offspring but provide care for their young (nurturing) • • Occurs in stable, balanced environments • Exhibit type I survivorship curve • Use tactic called K-selection • Large animals like mammals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some plants produce a large number of small seeds – Ensuring that at least some of them will grow and eventually reproduce Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Other types of plants produce a moderate number of large seeds – That provide a large store of energy that will help seedlings become established Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Cycles • Many populations – Undergo regular boom-and-bust cycles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors • • Prey populations may be influenced by predation, and by fluctuations in the availability of the plants they feed on Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Population ecology is the study of populations in relation to environment – Including environmental influences on population density and distribution, age structure, and variations in population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Dynamic biological processes influence population density, dispersion, and demography • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Demographics of Populations • Demography is the statistical study of a population, which includes its density, distribution, rate of growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Density and Distribution of Populations • Population Density - Number of individuals per unit area or volume • Population Distribution - Pattern of dispersal of individuals within a space of interest – Ecologists analyze what causes the spatial and temporal “patchiness” of organisms – Affected by the availability of resources • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Density: A Dynamic Perspective • Determining the density of natural populations – • In most cases – It is impractical or impossible to count all individuals in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Density is the result of a dynamic interplay – Between processes that add individuals to a population and those that remove individuals from it Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Patterns of Dispersion • Environmental and social factors – Influence the spacing of individuals in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A clumped dispersion – Is one in which individuals aggregate in patches – May be influenced by resource availability and behavior Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A uniform dispersion – Is one in which individuals are evenly distributed – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A random dispersion – Is one in which the position of each individual is independent of other individuals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Growth • Exponential Growth – • Biotic Potential – Maximum population growth that can possibly occur under ideal circumstances • Environmental Resistance – All environmental conditions that prevent populations from achieving biotic potential • Biotic potential is having it’s full effect and birthrate is at it’s maximum during exponential growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Zero population growth – Occurs when the birth rate equals the death rate • The population growth equation can be expressed as dN rN dt Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G = rN dN dt rN G = growth rate of population N = population size r = intrinsic rate of increase If r is constant, then the rate at which population grows depends on the number of individuals already in the population (N), value of r depends on the kind of organism G = dN/dt (∆N/∆t) = change in # of individuals over a given time r = (b-d) = birth rate - death rate G = rN or Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ∆N/∆t = (b-d)N • The equation of exponential population growth is dN dt rmaxN Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The logistic growth equation – Includes K, the carrying capacity (K N) dN rmax N dt K Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Environmental factors that restrict population growth are called population-limiting factors • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Dynamics • The study of population dynamics – Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Change and Population Density • In density-independent populations – • In density-dependent populations – Birth rates fall and death rates rise with population density Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Density-dependent factors • Population limiting factors whose effects depend on population density • As the number of individuals increases, so does the percentage of individuals affected • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Competition for Resources • In crowded populations, increasing population density – Intensifies intraspecific competition for resources Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Competition • Intraspecific competition is competition between members of the same species for resources that are limited supply (food, shelter, mates) • • Interspecific competition is competition between members of different species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Food supply • Individuals in a large population have a smaller share of the limited food supply Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Territoriality • In many vertebrates and some invertebrates – Territoriality may limit density Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cheetahs are highly territorial – Using chemical communication to warn other cheetahs of their boundaries Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Oceanic birds – Exhibit territoriality in nesting behavior Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Health • Population density can influence the health and survival of organisms • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Predation • As a prey population builds up predators may feed preferentially on that species • The major limiting factor for predator populations is the availability of its prey • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Toxic Wastes • The accumulation of toxic wastes – Can contribute to density-dependent regulation of population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intrinsic Factors • For some populations – Intrinsic (physiological, anatomical, behavioral) factors appear to regulate population size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Emigration – • Mutualism - two species acting in a mutually beneficial fashion may increase survival of both species involved Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Density-independent factors • Population limiting factors whose occurrence is not affected by population density • • Limit population size well before resources or other density-dependent factors become important • Can cause rapid crashes in populations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Most populations are probably regulated by a mixture of density-independent and density dependent factors • Many populations are fairly stable and near carrying capacity that is regulated by densitydependent factors • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Logistic Model and Life Histories • Life history traits favored by natural selection • Populations vary on factors such as number of births per reproduction, age of reproduction, life span of individuals, probability of living entire life spans (survivorship curves) • These factors are part of a species life history • Natural selection shapes the life histories of species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Population • Human population growth has slowed after centuries of exponential increase • – And humans are no exception Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Global Human Population • The human population – Increased relatively slowly until about 1650 and then began to grow exponentially Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Though the global population is still growing – The rate of growth began to slow approximately 40 years ago Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Population Growth • Human population has an exponential growth pattern • Doubling time currently estimated at 53 years • Population Size – 1800 1 Billion – 1930 2 Billion – 1960 3 Billion – 2000 6 Billion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Country Development • More-Developed Countries (MDCs) – Slow population growth – High standard of living – Completed Demographic Transition (decreased deathrate followed by decreased birthrate) – • Less-Developed Countries (LDCs) – Rapid population growth – Low standard of living • Latin America • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Infant Mortality and Life Expectancy • Infant mortality and life expectancy at birth – Vary widely among developed and developing countries but do not capture the wide range of the human condition Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Age Distributions • • At least three structures possible – Increasing (pyramid-shaped) - prereproductive group is largest of three groups – Stable (bell-shaped) - reproductive group equals size of prereproductive group – Decreasing (urn-shaped) - prereproductive group becomes smaller than reproductive group, postreproductive group is the largest Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Age Structure • One important demographic factor in present and future growth trends – – Divide populations into three age groups • Pre-Reproductive • Reproductive • Post-Reproductive Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Age structure – Is commonly represented in pyramids Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Age structure diagrams – – Can illuminate social conditions and help us plan for the future Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Environmental Impact • Environmental impact of a population is measured in terms of: – Population size – – Resultant pollution caused by population size and increased resource consumption Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Population growth is putting extreme pressure on each country’s social organization, the Earth’s resources, and the Biosphere • Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Global Carrying Capacity • Just how many humans can the biosphere support? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Estimates of Carrying Capacity • The carrying capacity of Earth for humans is uncertain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings U.S.Population • 281.4 M (4/00) – Most people are Caucasian (69.1%) – Hispanic (12.5%) - 60% from Mexico – Asian (3.7%) - ~36% live in California – Black (12.1%) • 40% of population under 18 belong to minority group - minorities increasing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • U.S. – 73 people/mi2 • Diamond Bar – 14.9 mi2 / 38.8 km2 • Population – 58, 763 • Density – 1,515 people/km2 or 3,944 people/mi2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mumbai, India - World’s Most Densely Populated City • 14.35 million people/484 km2 • Density = 29,650 people/km2 – (DB-1,515/km2) 75,925 people/mi2 (DB-3,944/km2) • 29,650 people/km2 (density-Mumbai) X 38.8 km2 (D.B.) = 1,150,420 people (DB-58,763) • Others: – Shenzhen, China (5th) - 466 km2 / 8 million people (17,150 people/km2) – Seoul, South Korea - 16,700 people/km2 – Taipei, Taiwan - 15,200 people/km2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings