chapter 40 population ecology (sections 40.1 - 40.5)
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Chapter 40 Population Ecology (Sections 40.1 - 40.5). 40.1 A Honking Mess. Several different Canada goose populations spend time in the US – some migrate, some do not Nonmigratory populations devote more energy to producing young, and their numbers are increasing - PowerPoint PPT PresentationTRANSCRIPT
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Albia Dugger • Miami Dade College
Cecie StarrChristine EversLisa Starr
www.cengage.com/biology/starr
Chapter 40Population Ecology
(Sections 40.1 - 40.5)
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40.1 A Honking Mess
• Several different Canada goose populations spend time in the US – some migrate, some do not
• Nonmigratory populations devote more energy to producing young, and their numbers are increasing
• Wildlife mangers are looking for ways to reduce nonmigratory goose populations, without harming migratory birds
• population • Group of organisms of the same species that live in the
same area and interbreed
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Goose Troubles
• US Airways Flight 1549 floats in the Hudson River after collisions with geese incapacitated both of its engines
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40.2 Population Demographics
• Ecological factors affect the size, density, distribution, and age structure of a population
• Studying population ecology often involves the use of demographics, which often change over time
• demographics • Statistics that describe a population
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Population Size
• Biologists frequently use sampling techniques to estimate population size
• Plot sampling estimates the total number of individuals in an area based on direct counts in a small portion of that area
• Estimates from plot sampling are most accurate when the organisms counted are not very mobile and conditions across the area they occupy are more or less uniform
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Key Terms
• population size • Total number of individuals in a population
• plot sampling • Method of estimating population size of organisms that do
not move much by making counts in small plots, and extrapolating from this to the number in the larger area
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Population Size (cont.)
• Scientists use mark-recapture sampling to estimate the population size of mobile animals, such as Florida Key deer
• mark-recapture sampling • Method of estimating population size of mobile animals by
marking individuals, releasing them, then checking the proportion of marks among individuals recaptured at a later time
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Mark-Recapture Sampling: Key Deer
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Population Density and Distribution
• population density • Number of individuals per unit area or volume• Example: Number of dandelions per square meter of lawn
• population distribution • Describes how individuals are distributed• Individuals may be clumped, uniformly dispersed, or
randomly dispersed in an area
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Clumped Distribution
• Individuals are closer to one another than would be predicted by chance alone
• Due to resource distribution, limited dispersal availability, or asexual reproduction
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Near-Uniform Distribution
• Individuals are more evenly spaced than would be expected by chance
• Found in breeding colonies, and with competition for resources
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Random Distribution
• Individuals are distributed randomly when environmental resources are uniformly distributed, and proximity to others is neither beneficial nor harmful
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Age Structure
• Individuals in a population are frequently grouped as pre-reproductive, reproductive, or post-reproductive
• age structure• Of a population, the number of individuals in each of
several age categories
• reproductive base • Of a population, all individuals who are of reproductive age
or younger
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Effects of Scale and Timing
• The scale of the area sampled and the timing of a study can influence the observed demographics
• Example: Seabirds crowd together during the breeding season, but disperse when breeding is over
• Wildlife managers use demographic information to decide how best to manage populations
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Key Concepts
• The Vital Statistics• Ecologists explain population growth in terms of population
size, density, distribution, and number of individuals in different age categories
• They have methods of estimating population size and density in the field
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ANIMATION: Mark-Recapture Method
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40.3 Population Size and Exponential Growth
• The number of individuals in a population is increased by births and immigration, and decreased by deaths and emigration
• immigration • Movement of individuals into a population
• emigration • Movement of individuals out of a population
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From Zero to Exponential Growth
• Apart from immigration and emigration, an interval in which population size remains unchanged, with no net increase or decrease in the number of individuals, is called zero population growth
• zero population growth• Interval in which births equal deaths
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Growth Rate (r)
• We can measure births and deaths in terms of rates per individual, or per capita
• Per capita growth rate (r) = per capita bith rate (b) – per capita dreath rate (d)
• per capita growth rate (r)• For some interval, the added number of individuals divided
by the initial population size
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Exponential Growth (cont.)
• Example: 2,000 mice live in the same cornfield:• 1,000 mice are born each month• Birth rate is 0.5 births per mouse per month (1,000/2,000)• 200 mice die each month• Death rate is 0.1 deaths per mouse per month (200/2,000) • r is 0.4 per mouse per month (0.5 – 0.1)
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Exponential Growth
• As long as r remains constant and greater than zero, exponential growth will occur
• A population grows exponentially as long as birth rate (b) is greater than death rate (d)
• exponential growth • A population grows by a fixed percentage in successive
time intervals; the size of each increase is determined by the current population size
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Exponential Growth (cont.)
• We calculate population growth (G ) based on the per capita growth rate and the number of individuals (N ):
Population growth rate (G) =
per capita growth rate (r) X number of individuals (N)
• With exponential growth, a plot of population increases against time produces a J-shaped curve – number of new individuals increases each generation, although per capita growth rate stays the same
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Example: Exponential Growth (G)
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Example: Effect of Death Rates• Two populations of bacteria: Population 1 divides every half
hour; population 2 divides every half hour, with 25% dying between divisions – exponential growth continues
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What Is the Biotic Potential?
• Under ideal conditions (shelter, food, and other essential resources are unlimited, no predators or pathogens) a population’s growth rate reaches its biotic potential
• Microbes have high biotic potentials; large-bodied mammals have low biotic potentials
• biotic potential • Maximum possible population growth rate under optimal
conditions
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Key Concepts
• Exponential Rates of Growth• A population’s size and reproductive base influence its
rate of growth• As long as births exceed deaths, a population will grow
exponentially • Each generation will be larger than the preceding one
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ANIMATION: Patterns of Population growth
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40.4 Limits on Population Growth
• Populations seldom reach their biotic potential because of the effects of limiting factors
• Many complex interactions take place within and between populations in nature, and it is not always easy to identify all the factors that can restrict population growth
• limiting factor • A necessary resource, the depletion of which halts
population growth
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Environmental Limits on Growth
• Essential resources such as food, mineral ions, refuge from predators, and safe nesting sites are examples of limiting factors for population growth
• In any environment, one essential factor will run out first, and acts as the brake on population growth
• Supplying the first limiting factors simply substitutes one for another – all natural populations eventually encounter limits
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Nesting Cavities: A Limiting Factor for Wood Ducks
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Carrying Capacity
• A given environment can sustain only a certain number of individuals in a population indefinitely – ultimately, the sustainable supply of resources determines population size
• carrying capacity• Maximum number of individuals of a species that an
environment can sustain
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Logistic Growth
• A pattern of logistic growth shows how a small population starts growing slowly in size, then grows rapidly, then levels off as the carrying capacity is reached
• Logistic growth plots out as an S-shaped curve
• logistic growth• A population grows slowly, then increases rapidly until it
reaches carrying capacity and levels off
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Logistic Growth Pattern
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Two Categories of Limiting Factors
• Factors that affect population growth fall into two categories
• density-dependent factor • Factor that limits population growth and has a greater
effect in dense populations than less dense ones• Example: Pathogens and parasites
• density-independent factor • Factor that limits population growth and arises regardless
of population density• Example: Fires and earthquakes
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Example: Overshoot and Crash
• 1944: 29 reindeer were introduced to St. Matthew Island
• 1957: 1,350 well-fed reindeer munching on lichens
• 1963: 6,000 hungry reindeer (carry capacity exceeded)
• 1966: 42 live reindeer, and many bleached bones
• 1980s: No reindeer
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Example: Overshoot and Crash
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Key Concepts
• Limits on Increases in Size• Density dependent factors such as competition for
resources lead to logistic growth• A population grows exponentially at first, then growth
slows as the number approaches the environment’s carrying capacity
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ANIMATION: Effect of Death on Growth
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40.5 Life History Patterns
• Reproduction-related events that occur between birth and death make up a life history pattern
• life history pattern • A set of traits related to growth, survival, and reproduction
such as life span, age-specific mortality, age at first reproduction, and number of breeding events
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Patterns of Survival and Reproduction
• We study life history traits within a population by recording what happens to a specific cohort
• Human life-expectancy tables are usually based on information about current conditions rather than a real cohort
• cohort • Group of individuals born during the same interval
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Life Table for an Annual Plant Cohort
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Life Table for Humans in the US
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Patterns (cont.)
• Information about age-specific death rates can also be summarized by a survivorship curve, which shows how many members of a cohort remain alive over time
• Three types of survivorship curves are common
• survivorship curve • Graph showing the decline in numbers of a cohort over
time
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Type I Survivorship Curve
• Elephants have type I survivorship, with low mortality until old age
• Typical of large animals that bear one or few offspring at a time and provide extended parental care
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Type II Survivorship Curve
• Snowy egrets are type II population, with a fairly constant death rate
• Typical of lizards, small mammals, and large birds
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Type III Survivorship Curve
• Sea urchins are type III; mortality is high for larvae and in old age, but low in adults
• Typical of species that produce many small offspring and provide little or no parental care
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Allocating Reproductive Investment
• Natural selection influences the timing of reproduction and how much a parent invests in each offspring
• The most adaptive reproductive strategy is that which maximizes a parent’s lifetime reproductive success
• Reproduction involves trade-offs between offspring quality and quantity – the most effective reproductive strategy can vary with population density
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Allocating Reproductive Investment
• r-selection • Individuals who produce maximum number offspring as
quickly as possible have a selective advantage• Occurs when population density is low and resources are
abundant
• K-selection• Individuals who produce offspring that outcompete others
for limited resources have a selective advantage• Occurs when a population is near carrying capacity
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Key Concepts
• Patterns of Survival and Reproduction• Life history traits such as age at first reproduction and
number of offspring per reproductive event vary and are shaped by natural selection
• Adaptive life history traits are those that maximize an individual’s lifetime reproductive success
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ANIMATION: Life History Patterns
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