CHAPTER 53 POPULATION ECOLOGY
53.1 DYNAMIC BIOLOGICAL PROCESSES INFLUENCE POPULATION DENSITY, DISPERSION, AND DEMOGRAPHICS
POPULATION
• Population- a group of individuals of a single species living in the same general area• Three fundamental characteristics of a population
• Density• Dispersion• Demographics
DENSITY AND DISPERSION
• Scientists have begun investigating the boundaries of a population.• They may be natural or arbitrarily defined
• Density – the number of individuals per unit area or volume• Dispersion – the pattern of spacing among
individuals within the boundaries of the population
DENSITY
• Almost impossible to actually count the number of individuals• Some ecologists will estimate and extrapolate• Mark-Recapture method • Density is not a static property• Immigration- the movement of organisms into an area• Emigration- the movement of organisms out of the area
PATTERNS OF DISPERSION
• Clumped – sea stars grouping together where food is abundant• Uniform – penguins maintaining almost equal
spacing due to aggressive interactions between neighbors• Random – dandelions growing wherever the seeds
land and germinate
(a) Clumped
(b) Uniform
(c) Random
DEMOGRAPHICS
• Demography – the study of vital statistics of populations and how they change over time• Life tables• Survivorship curves
LIFE TABLES
• Age-specific summaries of the survival pattern of a population• Best way to construct one is to follow the fate of a
cohort from birth until death• A group of individuals of the same age
SURVIVORSHIP CURVES
• A graphing method of representing the data in a life table• Three different types of patterns• Type I- flat to start then drops steeply• Humans and other mammals
• Type II – steady decline• Squirrels
• Type III – drops sharply at the start• Oysters
Survivorship Curves
1,000
100
10
10 50 100
II
III
Percentage of maximum life span
Nu
mb
er
of
su
rviv
ors
(lo
g s
ca
le)
I
REPRODUCTIVE RATES
• Demographers typically ignore males and focus on females• A reproductive table (fertility schedule) is an
age specific summary of the reproductive rates in a population.• Tallies the number of female offspring produced
by each age group
53.2- LIFE HISTORY TRAITS ARE PRODUCTS OF NATURAL
SELECTION
• Natural selection favors traits of organisms that allow them to survive longer and reproduce• Life history- the trait that affects an organism’s
schedule of reproduction and survival• Start of reproduction• How often reproduction occurs• Amount of offspring per reproduction cycle
EVOLUTION AND LIFE HISTORY DIVERSITY
• One-shot reproduction• Semelparity- one big reproduction of offspring (big bang
reproduction)
• Iteroparity- offspring over many years
• 2 critical factors: • Survival rate of offspring• Likelihood hood that adults will live to reproduce
again
EVOLUTION AND LIFE HISTORY DIVERSITY
• Semelparity if offspring aren’t likely to survive long• Iteroparity if the environment is favorable to the
adults
“TRADE-OFFS” AND LIFE HISTORIES
• Natural selection cannot maximize all reproduction variables simultaneously• Time, energy, and nutrients limit reproduction of
organisms• Trade-offs between survival and reproduction• Selective pressures between number of offspring and size
of offspring• Parent care and learn through 1st year makes an impact
53.3- THE EXPONENTIAL MODEL DESCRIBES POPULATION GROWTH IN AN IDEALIZED,
UNLIMITED ENVIRONMENT
• Potential to expand if resources are right• Reveals capacity of species for increase and
conditions under which capacity may be expressed
PER CAPITA RATE OF INCREASE
• Population will increase with births and emigrations• Populations will decrease with deaths and
immigration• Change in population= (birth + immigration) –
(deaths + emigrations)
PER CAPITA RATE OF INCREASE
• Per capita birth rate- number of offspring produced by an average member of the population• Per capita death rate- expected number of deaths
per a unity of time • Most interested in the difference between the
death and birth rates
PER CAPITA RATE OF INCREASE
• R=b-d• R is the indication whether a given population is
growing or declining• Zero population growth (ZPG) the birth and death
rates equal zero
EXPONENTIAL GROWTH
• Exponential population growth- population increase under ideal conditions • J-shaped curved• Can mean the introduction to a new environment• Numbers that have been affected by a catastropic event
53.4 THE LOGISTIC MODEL DESCRIBES HOW A POPULATION GROWS MORE
SLOWLY AS IT NEARS ITS CARRY CAPACITY
• Exponential growth model assumes that resources are unlimited• Not the case in the real world
• Carrying capacity (K) – the maximum population size that a particular environment can sustain
THE LOGISTIC GROWTH MODEL
• The per capita rate of increase approaches zero as the carrying capacity is reached
• Will produce an s-shaped curve
2,000
1,500
1,000
500
00 5 10 15
Number of generations
Po
pu
lati
on
siz
e (N
)
Exponentialgrowth
1.0N=dN
dt
1.0N=dN
dt
K = 1,500
Logistic growth1,500 – N
1,500
1,000
800
600
400
200
00 5 10 15
Time (days)
Nu
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f Paramecium
/mL
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f Daphnia
/50
mL
0
30
60
90
180
150
120
0 20 40 60 80 100 120 140 160
Time (days)
(b) A Daphnia population in the lab(a) A Paramecium population in the lab
LOGISTIC MODEL AND REAL POPULATIONS
• The logistic model assumes that populations will adjust instantaneously• This is not typically the case
• This will cause a population to temporarily overshoot the carrying capacity• Allee effect- individuals may have a more difficult
time surviving or reproducing if the population size is too small
LOGISTIC MODEL AND LIFE HISTORIES
• K-selection- density dependent• Operates in populations living at a density near their
carrying capacity
• R-selection- density independent• Traits that maximize reproductive success in uncrowded
environments
53.5 MANY FACTORS THAT REGULATE POPULATION GROWTH ARE DENSITY
DEPENDENT
POPULATION CHANGE AND POPULATION DENSITY
• Density independent populations will have birth and death rates that will not change with density• Density dependent populations will have birth
and death rates that will rise and fall with density
DENSITY-DEPENDENT POPULATION REGULATION
• Without some type of negative feedback between population density and the rates of birth and death, a population will never stop growing.• Competition for resources• Increasing population density competing for declining
nutrients will lead to a lower birth rate
• Toxic Wastes• The accumulation of toxic waste can effect population
size
• Intrinsic Factors• In some cases the physiological factors rather than the
environmental factors will influence the population size
CONTINUED
• Territoriality• Territory spaces becomes a resource in which individuals
compete for.
• Disease• If the transmission rate of a certain disease depends on
the crowding in a population, density will be effected
• Predation• A predator encounters and captures more food as the
density of the prey increases
(a) Cheetah marking its territory
(b) Gannets
Territoriality
Wolves Moose
2,500
2,000
1,500
1,000
500
Nu
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0
Nu
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olv
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50
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01955 1965 1975 1985 1995 2005
Year
Predation
POPULATION DYNAMICS
• Focuses on the complex interactions between the biotic and abiotic factors that cause variation in the size of populations• Populations of large mammals were once thought
to remain stable over time, this is not the case
IMMIGRATION, EMIGRATION, AND METAPOPULATIONS
• Immigration and emigration can also affect populations• Metapopulation- a group of spatially separated
populations of one species that interact through immigration and emigration• Local populations can be thought of as occupying
small patches of suitable environment within a sea of unsuitable habitat• Patches will vary in size, quality, and isolation from other
patches• There are many factors that will influence how patches
interact
CHAPTER 54 COMMUNITY ECOLOGY
C O M M U N I T Y I N T E R A C T I O N S A R E C L A S S I F I E D B Y W H E T H E R T H E Y H E L P , H A R M , O R H A V E N O E F F E C T O N T H E S P E C I E S I N V O L V E D
54.1
COMPETITION
• Interspecific competition – has a negative effect on the survival and reproduction of the predator population and a negative effect on that of the prey population.• Occurs when individuals of different species
compete for a resource that limits their growth and survival.• ie. Weeds compete from soil nutrients and water.
Grasshoppers vs. bison for grass they both eat.
COMPETITION: COMPETITIVE EXCLUSION
• Two species competing for the same limiting resources cannot coexist in the same place. • Without disturbances, one species will use the
resource more efficiently and reproduce more rapidly than the other. • The slight reproductive advantage will eventually
lead to local elimination of the inferior competitor.• This is called competitive exclusion.
COMPETITION: ECOLOGICAL NICHES
• Ecological niche – the sum of a species’ use of the biotic and abiotic resources in its environment.• An organism’s niche is its ecological role , how it “fits
into” an ecosystem.• Two species cannot coexist permanently in a
community if their niches are identical. • However, ecologically similar species can coexist in a
community if there are more significant differences in their niches. Resource partitioning – the differentiation of niches that enables similar species to coexist in a community.
COMPETITION: ECOLOGICAL NICHES
• A species’ fundamental niche (the niche potentially occupied by that species) is often different from its realized niche (portion of its fundamental niche that it actually occupies).• The fundamental niche of a species can be
identified by testing the range of conditions in which it grows and reproduces without competitors.• Also test if a potential competitor limits a species’
realized niche by removing the competitor and seeing if the first species going into the newly available space.
COMPETITION: CHARACTER DISPLACEMENT
• Allopatric – geographically separate• Sympatric – geographically overlapping• In some cases, the allopatric populations of
species are morphologically similar and use similar resources.• In contrast, sympatric populations show
differences in body structures and in the resources they use. • This tendency for characteristics to diverge more
in sympatric populations of two species is character displacement.
PREDATION
• A positive predator population survival and reproduction activity with negative prey population survival and reproduction activity.• Both predators and prey have adaptations that
help eat, and help avoid being eaten such as claws, teeth, heat sensing organs, and poison while prey can hide, alert, flee, or form herds/schools.
PREDATION
• Animals display a variety of morpholocial and physiological defensive adaptations.• Cryptic coloration (camouflage) – makes prey difficult
to spot• Aposematic coloration (warning) – animals with
effective chemical defenses often show this.• Batesian mimicry – a palatable or harmless species
mimics a harmful model. • Mullerian mimicry – two or more unpalatable species
resemble each other. Each species gains and additional advantage because the more unpalatable prey they are, the more quickly avoid them as prey.
BATESIAN MIMICRY
MULLERIAN MIMICRY
CRYPTIC COLORATION
APOSEMATIC COLORATION
HERBIVORY
• Positive predator survival and reproduction with negative prey survival and reproduction.• Used for when an organism eats parts of a plant
or alga. • Specialized adaptations like chemical sensors
enabling them to distinguish toxic from nontoxic plants to eat. • Prey relies on toxins or spikes and thorns.
SYMBIOSIS
• Symbiosis – when individuals of two or more species live in direct and intimate contact with one another.
SYMBIOSIS: PARASITISM
• Positive predator survival and reproduction and negative prey survival and reproduction.• A parasite derives its nourishment from another
organism, the host. • Endoparasites – parasites that live within the body of
their host.• Ectoparasites – parasites that fee on the external
surface of a host.• Parasites can significantly affect the survival,
reproduction, and density of their host population (directly or indirectly).
SYMBIOSIS: MUTUALISM
• Benefits both species.• Sometimes involve the evolution of related
adaptations in both species, with changes in either species likely to affect the survival and reproduction of the other.
SYMBIOSIS: COMMENSALISM
• Benefits one species but neither harms nor helps the other. • Difficult to document in nature because any close
association between species likely affects both species if only slightly. • Some associations that are possibly commensal
involve one species obtaining food that is inadvertently exposed by another.
D O M I N A N T A N D K E Y S T O N E S P E C I E S E X E R T S T R O N G C O N T R O L S O N C O M M U N I T Y S T R U C T U R E
54.2
SPECIES DIVERSITY
• The variety of different kinds of organisms that make up the community is the species diversity of a community. • One component is species richness – number of
different components.• Other is relative abundance – the proportion each
species represents of all individuals in the community. • Often calculate an index of diversity based on species
richness and relative abundance. (Shannon diversity)
TROPHIC STRUCTURE
• The structure and dynamics of a community depend to a large extent on the feeding relationships between organisms (trophic structure). • Food chain – transfer of food energy up the
trophic levels.
TROPHIC STRUCTURE: FOOD WEBS
• Arrows linking species according to who eats whom. • A given species may weave into the web at more
than one trophic level. • Species are grouped with similar trophic
relationships in a given community into broad functional groups.
TROPHIC STRUCTURE: LIMITS ON FOOD CHAIN LENGTH
• Energetic hypothesis – the length of a food chain is limited by the inefficiency of energy transfer along the chain. • Only about 10% of the energy stored in the organic
matter of each trophic level is converted to organic matter at the next trophic level. • Dynamic stability hypothesis – long food chains are less
stable than short chains. The longer a food chain is, the more slowly top predators can recover from environmental setbacks so food chains should be shorter in unpredictable environments.• The size of a carnivore and its feeding mechanism put
some upper limit on the size of food it can take which can limit the food chain length limit.
SPECIES WITH A LARGE IMPACT: DOMINANT SPECIES
• Dominant species – species in a community that are most abundant or that collectively have the highest biomass.• Exert a powerful control over the occurrence and
distribution of other species.• Dominance could be a result of exploiting limited
resources, or successfully avoiding predation. • One way to discover the impact of a dominant
species is to remove it form the community.
SPECIES WITH A LARGE IMPACT: KEYSTONE SPECIES
• Keystone species- not necessarily abundant in a community. • Exert strong control on community structure but
not by numerical might but by their pivotal ecological niches.• One way to look at the impact is to remove the
keystone species. Highlights the importance of a keystone species in maintaining the diversity of a community.
SPECIES WITH A LARGE IMPACT: FOUNDATION SPECIES (ECOSYSTEM “ENGINEERS”)
• Some organisms exert their influence on a community by causing physical changes in the environment.
• Such organisms may alter the environment through their behavior or their large collective biomass.
• The effects of foundation species can be positive or negative on other species depending on the needs of the other species.
• By altering the structure or dynamics of the environment, foundation species sometimes act like facilitators.
• They have positive effects on the survival and reproduction of other species in the community.
D I S T U R B A N C E I N F L U E N C E S S P E C I E S D I V E R S I T Y A N D C O M P O S I T I O N
54.3
• A disturbance is an event that changes a community by removing organisms from it or altering resource availability. • (flood, hurricane, fire, drought, overgrazing, or
human activity)• The emphasis on change has produced the
nonequilibrium model (most communities are constantly changing after being affected by disturbances).
CHARACTERIZING DISTURBANCE
• Types of disturbances and their frequency and severity vary from community to community. • The intermediate disturbance hypothesis states
the moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance. • At the low end, low levels of disturbance can
reduce species diversity by allowing competitively dominent species to exclude less competitive species.
ECOLOGICAL SUCCESSION
• Ecological succession – the disturbed area may be colonized by a variety of species, gradually replaced by another species who are then also gradually replaced. • Primary succession – process begins in a virtually
lifeless area. Often only life-forms present are autotrophic prokaryotes and heterotrophic prokaryotes and protists. • Secondary succession – when an existing
community has been cleared by some disturbance that leaves the soil intact.
HUMAN DISTURBANCE
• Humans have the greatest impact on biological communities worldwide. • Reduces species diversity in many communities. • Human disturbance is often severe.• Agriculture development, ocean trawling, cutting
a forest, and cattle grazing.
54.4- BIOGEOGRAPHIC FACTORS AFFECT COMMUNITY BIODIVERSITY
• Influences on a diverse community:• Species interaction• Dominate species• Types of disturbances
• Biogeographic factors have the largest impact range
LATITUDINAL GRADIENT
• Plants and animals are most diverse in the topics• 2 key factors: evolutionary history and climate• Species diversity may increase as more
speciation occurs in a community• Climate is likely the primary cause• Solar radiation and water availability
LATITUDINAL GRADIENT
• Evapotranspiration- evaporation of water from the soil as well as the transpiration of water by plants• Potential evapotranspiration is a measure of
potential water loss • Determined by solar radiation and temperature
• Species richness correlates with both types evapotranspiration
AREA EFFECTS
• Species-area curve- biodiversity pattern; shows that the larger the geographic area of a community is, the more species it has• Proposed by Alexander von Humboldt
• Slope indicates the richness increase
ISLAND EQUILIBRIUM MODEL
• Islands are isolated and have limited sizes• New colonization• Rate of immigration• Rate of extinction
• Affects on immigration/extinction: distance from mainland and size• Equilibrium will eventually be met between
extinction and immigration
54.5- COMMUNITY ECOLOGY IS USEFUL FOR UNDERSTANDING PATHOGEN LIFE CYCLE AND
CONTROLLING HUMAN DISEASE
• Pathogens- disease-causing microorganisms, viruses, viroid, or prions• Pathogens have universal affects on ecosystems• Alter community structure quickly
PATHOGENS AND COMMUNITY STRUCTURE
• Coral reefs are easily affected• Terrestrial ecosystems are also affected by
pathogens• A reason for studying: human transportation via
activities• Can be transferred around the world
COMMUNITY ECOLOGY AND ZOONOTIC DISEASES
• Zoonotic pathogens- pathogens transferred from other animals to humans• Direct contact• Intermediate species called vector
• Need to understand parasite life cycle and be able to track the spread of zoonotic diseases
COMMUNITY ECOLOGY AND ZOONOTIC DISEASES
• Community interactions are the underlying sources between pathogen/host interactions• Changes in environment have an affect on
pathogen interactions
ESSAYS
• Name and describe the types of symbiosis. Give examples.• Name and explain the two models of growth.
Use examples of both• Draw type I, II, and III survivorship curves on a
graph with labeled axes. Explain why the growth rate of species with a type I survivorship curve depends primarily on fertility rates. Explain why the growth rate of species with a type III survivorship curve is extremely sensitive to changes in adult survivorship.
