Chapter 20Chapter 20

Population Growth and Population Growth and InteractionsInteractions

Chapter OutcomesChapter Outcomes

Describe and apply models that represent Describe and apply models that represent population density and distribution of population density and distribution of individuals within populationsindividuals within populations

Describe the four main processes that Describe the four main processes that result in population change and explain result in population change and explain how these processes are relatedhow these processes are related

Analyze population data to determine Analyze population data to determine growth rate and per capita growth rategrowth rate and per capita growth rate

Chapter OutcomesChapter Outcomes

Describe how a population’s biotic Describe how a population’s biotic potential and the carrying capacity of potential and the carrying capacity of its habitat determine its pattern of its habitat determine its pattern of growthgrowth

Compare r-selected and K-selected Compare r-selected and K-selected reproductive strategies in terms of reproductive strategies in terms of life cycles and patterns of population life cycles and patterns of population growthgrowth

Chapter OutcomesChapter Outcomes

Describe the interactions among Describe the interactions among population members and among population members and among members of different populations members of different populations within a communitywithin a community

Explain how producer-consumer Explain how producer-consumer interactions affect population growthinteractions affect population growth

Describe defense mechanisms that Describe defense mechanisms that have evolved within populationshave evolved within populations

Chapter OutcomesChapter Outcomes

Understand that symbiosis includes Understand that symbiosis includes mutual, commensal, and parasitic mutual, commensal, and parasitic relationshipsrelationships

Distinguish between primary and Distinguish between primary and secondary successionsecondary succession

20.1 – Population Growth20.1 – Population Growth

Quantitative measurements of Quantitative measurements of populations are like snapshots of populations are like snapshots of moments in timemoments in time

Ecologists often rely on a number of Ecologists often rely on a number of measurements over a long period of measurements over a long period of time to make inferences about time to make inferences about population growthpopulation growth

Both the distribution and growth of a Both the distribution and growth of a population can be significant when population can be significant when studying populations and communitiesstudying populations and communities

Population DistributionsPopulation Distributions

habitat can play a role in how habitat can play a role in how populations are distributed populations are distributed

population distributions can follow population distributions can follow one of three general patterns:one of three general patterns:

Patterns of DistributionPatterns of Distribution

1.1. ClumpedClumped

Patterns of DistributionPatterns of Distribution

2.2. RandomRandom

Patterns of DistributionPatterns of Distribution

3.3. UniformUniform

Population Size and Population Size and DensityDensity

population size population size simply describes simply describes the number of organisms in an areathe number of organisms in an area

it is often more useful to compare it is often more useful to compare populations by describing populations by describing population densitypopulation density (the number of (the number of individuals in a given area or volume individuals in a given area or volume – for aquatic species)– for aquatic species)

Population Density FormulaPopulation Density Formula

D = N / A where:D = N / A where: D = density of organisms D = density of organisms

(organisms/unit)(organisms/unit) N = # of organismsN = # of organisms A = size of area in unitsA = size of area in units

Example:Example:

Ex: There are 200 lemmings in a 25 Ex: There are 200 lemmings in a 25 ha area. Determine the population ha area. Determine the population density of the lemmings:density of the lemmings:

Population ChangePopulation Change

4 factors determine population size:4 factors determine population size:

1.1. Natality –Natality –

2.2. Mortality –Mortality –

3.3. Immigration – Immigration –

4.4. Emigration – Emigration –

if all the factors remain the same if all the factors remain the same except for an increase in the birth except for an increase in the birth rate, population increasesrate, population increases

population change can be calculated population change can be calculated in the formula:in the formula:

∆∆N = N = (births + immigration) – (deaths + (births + immigration) – (deaths + emigration) emigration)

We can also calculate a We can also calculate a per capitaper capita population growth ratepopulation growth rate

cgr = cgr = (births + immigration) – (deaths + emigration)(births + immigration) – (deaths + emigration)

Initial number of organismsInitial number of organisms

Population Growth ExamplePopulation Growth Example

Ex: In a Banks Island Breeding site, Ex: In a Banks Island Breeding site, 40 cranes were born, and there were 40 cranes were born, and there were 55 deaths. There was no 55 deaths. There was no immigration or emigration of cranes. immigration or emigration of cranes. The original population was 200. The original population was 200. Calculate the population growth.Calculate the population growth.

Biotic PotentialBiotic Potential

biotic potential depends on a number of biotic potential depends on a number of factors:factors:

1.1. Offspring – the maximum number of Offspring – the maximum number of offspring per birthoffspring per birth

2.2. Capacity for Survival – the chances of Capacity for Survival – the chances of offspring reaching reproductive ageoffspring reaching reproductive age

3.3. Procreation – the number of times per Procreation – the number of times per year an organism reproducesyear an organism reproduces

4.4. Maturity – the age at which reproduction Maturity – the age at which reproduction beginsbegins

Carrying CapacityCarrying Capacity

Generally, growth in small Generally, growth in small populations begins slowly and then populations begins slowly and then the rate of growth increasesthe rate of growth increases

However, the growth must eventually However, the growth must eventually slow because there is a maximum slow because there is a maximum number of organisms that an number of organisms that an ecosystem can supportecosystem can support

Growth PhasesGrowth Phases1.1. LagLag

2.2. Log (or Log (or Exponential Exponential Growth)Growth)

3.3. StationaryStationary

4.4. DeathDeath

S-CurvesS-Curves

Many populations exhibit an S-Many populations exhibit an S-shaped (sigmoidal) growth curve shaped (sigmoidal) growth curve

This is also known as a logistic This is also known as a logistic growth patterngrowth pattern

The population number increases The population number increases until it reaches the carrying capacity until it reaches the carrying capacity of the ecosystemof the ecosystem

At this point, the population At this point, the population fluctuates near the carrying capacityfluctuates near the carrying capacity

J-CurvesJ-Curves

J – shaped curves are representative of quick J – shaped curves are representative of quick growth and then a sharp decline in the growth and then a sharp decline in the populationpopulation

this occurs when a population quickly this occurs when a population quickly outgrows the carrying capacity of an outgrows the carrying capacity of an ecosystemecosystem

as a result, there is a crash in the as a result, there is a crash in the population, which is followed by a relatively population, which is followed by a relatively stable stationary phasestable stationary phase

these J-curves are most often associated these J-curves are most often associated with organisms that can reproduce very with organisms that can reproduce very quickly (insects, bacteria, etc.)quickly (insects, bacteria, etc.)

Comparison of J & S-CurvesComparison of J & S-Curves

http://www.emc.maricopa.edu

Limiting Factors in Limiting Factors in PopulationsPopulations

if there are a number of substances if there are a number of substances required for growth, then the one with the required for growth, then the one with the least concentration will be a limiting factor least concentration will be a limiting factor for growth (for growth (law of the minimumlaw of the minimum))

as well, in some cases, too much of a as well, in some cases, too much of a substance can harm an organismsubstance can harm an organism

therefore, the greater an organisms’ range therefore, the greater an organisms’ range of tolerance for high and low concentrations of tolerance for high and low concentrations of nutrients, the greater its survival ability of nutrients, the greater its survival ability ((Shelford’s law of toleranceShelford’s law of tolerance))

the overall optimum ranges for abiotic the overall optimum ranges for abiotic factors for each species is different because factors for each species is different because each species reacts to each factor differentlyeach species reacts to each factor differently

any abiotic factors that are not affected by any abiotic factors that are not affected by population density are population density are density density independentindependent

such factors include temperature & climatesuch factors include temperature & climate factors that are dependent on the population factors that are dependent on the population

density are density are density dependentdensity dependent these are factors such as limits to food these are factors such as limits to food

supply, disease, and predationsupply, disease, and predation often problems involving density-dependent often problems involving density-dependent

factors are alleviated when a population factors are alleviated when a population density returns to lower levelsdensity returns to lower levels

r and K Selected r and K Selected PopulationsPopulations

K-selected populationsK-selected populations are: are:

r-selected populationsr-selected populations are: are:

20.2 – Interactions in 20.2 – Interactions in Ecological CommunitiesEcological Communities

An ecological community is a An ecological community is a collection of interacting populations collection of interacting populations within an areawithin an area

In any community, individuals must In any community, individuals must compete for limited resourcescompete for limited resources

The competition between populations The competition between populations is the driving force behind population is the driving force behind population dynamicsdynamics

Intraspecies and Intraspecies and Interspecies CompetitionInterspecies Competition

Gause’s PrincipleGause’s Principle states that if two states that if two populations occupy the same niche, one of populations occupy the same niche, one of them will be eliminatedthem will be eliminated

this principle would represent a worst-case this principle would represent a worst-case scenario in scenario in interspecies competitioninterspecies competition (the (the competition between two different species)competition between two different species)

there also exists there also exists intraspecies competitionintraspecies competition, , where members of the same species compete where members of the same species compete for resources such as food, space and matesfor resources such as food, space and mates

PredationPredation

predator and prey populations are often predator and prey populations are often closely tied to one another (for instance, closely tied to one another (for instance, if a prey population declines, it is likely if a prey population declines, it is likely that the predator species will as well)that the predator species will as well)

however, predators are important in however, predators are important in ecosystems as they reduce the number ecosystems as they reduce the number of primary consumers that are feeding of primary consumers that are feeding on producerson producers

Populations of Lynx & Hares Populations of Lynx & Hares

Predator-Prey CyclesPredator-Prey Cycles

Predator-prey cycles often depend on Predator-prey cycles often depend on a number of factors:a number of factors:

in some cases, animals develop in some cases, animals develop camouflage camouflage to escape detection to escape detection (either by predators or prey)(either by predators or prey)

other organisms produce other organisms produce physiological adaptations (such as physiological adaptations (such as plant toxins)plant toxins)

some animals will engage in some animals will engage in mimicrymimicry, where they will develop , where they will develop markings similar to those on a markings similar to those on a poisonous or dangerous animalpoisonous or dangerous animal

often predators and prey often predators and prey coevolvecoevolve in an attempt to gain an upper handin an attempt to gain an upper hand

SymbiosisSymbiosis

There are 3 types of symbiosis:There are 3 types of symbiosis:

1.1. CommensalismCommensalism

2.2. ParasitismParasitism

3.3. MutualismMutualism

SuccessionSuccession

succession succession is the slow, orderly replacement of is the slow, orderly replacement of one community by another through the one community by another through the development of vegetationdevelopment of vegetation

climax communitiesclimax communities are eventually formed are eventually formed through this processthrough this process

a climax community is a stable, mature communitya climax community is a stable, mature community primary successionprimary succession occurs where there occurs where there

previously was no community (on places such as previously was no community (on places such as barren volcanic islands)barren volcanic islands)

secondary successionsecondary succession occurs after the partial or occurs after the partial or complete destruction of a communitycomplete destruction of a community

Steps in Primary SuccessionSteps in Primary Succession

1.1. Bare land is formed.Bare land is formed.2.2. Pioneer species, such as mosses and grasses that Pioneer species, such as mosses and grasses that

are relatively hardy move in and decrease soil are relatively hardy move in and decrease soil temperature and evaporation, while increasing temperature and evaporation, while increasing soil fertility.soil fertility.

3.3. Small shrubs that tolerate full sunlight move in, Small shrubs that tolerate full sunlight move in, stabilizing and enriching the soil.stabilizing and enriching the soil.

4.4. Small, fast-growing trees replace the shrubs and Small, fast-growing trees replace the shrubs and deplete the soil of nutrients and sunlight.deplete the soil of nutrients and sunlight.

5.5. A climax community forms, produced from A climax community forms, produced from shade-tolerant trees which have a high sapling shade-tolerant trees which have a high sapling survival rate.survival rate.

Generalizations Regarding Generalizations Regarding SuccessionSuccession

1.1. Species composition changes more rapidly during Species composition changes more rapidly during the earlier stages of succession.the earlier stages of succession.

2.2. The total number of species increases The total number of species increases dramatically during the early stages of dramatically during the early stages of succession, levels off during the intermediary succession, levels off during the intermediary phases, and declines as the climax community is phases, and declines as the climax community is established.established.

3.3. Food webs become more complex and the Food webs become more complex and the relationships more clearly defined as succession relationships more clearly defined as succession proceeds.proceeds.

4.4. Both total biomass and nonliving organic matter Both total biomass and nonliving organic matter increase during succession and begin to level off increase during succession and begin to level off during the establishment of the climax during the establishment of the climax community.community.