Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 40Population 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

• 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

Goose Troubles

• US Airways Flight 1549 floats in the Hudson River after collisions with geese incapacitated both of its engines

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

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

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

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

Mark-Recapture Sampling: Key Deer

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

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

Near-Uniform Distribution

• Individuals are more evenly spaced than would be expected by chance

• Found in breeding colonies, and with competition for resources

Random Distribution

• Individuals are distributed randomly when environmental resources are uniformly distributed, and proximity to others is neither beneficial nor harmful

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

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

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

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

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

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

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)

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

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

Example: Exponential Growth (G)

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

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

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

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

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

Nesting Cavities: A Limiting Factor for Wood Ducks

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

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

Logistic Growth Pattern

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

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

Example: Overshoot and Crash

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

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

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

Life Table for an Annual Plant Cohort

Life Table for Humans in the US

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

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

Type II Survivorship Curve

• Snowy egrets are type II population, with a fairly constant death rate

• Typical of lizards, small mammals, and large birds

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

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

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

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

ANIMATION: Life History Patterns

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