Population EcologyPopulation Ecology

ObjectivesObjectives

• 5.3.1 – Outline how population size is affected by natality, immigration, mortality, and emigration.

• 5.3.2 – Draw and label a graph showing a sigmoid (S- shaped) population growth curve.

• G.1.3 – Describe one method of random sampling based on quadrat methods that is used to compare the population size of two plant or two animal species.

• G.1.4 – Outline the use of a transect to correlate the distribution of plant or animal species with an abiotic variable.

Characteristics of PopulationsCharacteristics of PopulationsAA populationpopulation is a group of individuals of a is a group of individuals of a

single species that simultaneously occupy single species that simultaneously occupy the same general area. the same general area. Characteristics of populations are shaped by the interactions between individuals and their environment.

Populations have size and geographic boundaries.

• Density is the number of individuals per unit area.

• Dispersion is the pattern of spacing among individuals within geographic boundaries.Darker color = greater density

Patterns of dispersionPatterns of dispersionDispersion patterns depend on variable Dispersion patterns depend on variable

population densities. population densities. Dispersion and density both depend on resource distribution.• Random dispersion, individual positions are

independent.• Uniform dispersion: individuals are evenly

spaced.• Clumped dispersion: individuals aggregate in

patches.

Random Uniform Clumped

Random samplingRandom samplingMeasuring population density is a difficult task. Measuring population density is a difficult task.

Do you …Do you …

• Count individuals - often impractical if individuals do not want to cooperate, or

• Use sampling techniques to estimate numbers.• The mark-recapture methodmark-recapture method: Individuals are

trapped, marked with a tag, recorded, then released and

captured a second time. The percentage recaptured gives an estimate of the true population.

• Quadrat methodsQuadrat methods: count all individuals within a pre- cisely outlined area, then extrapolate to the larger environment. ⇒ an estimate.

Quadrat sampling techniquesQuadrat sampling techniquesA quadraquadrat is a square of a

convenient size, perhaps 1 m x 1 m, in which species of interest are counted. • Later relate totals to the area

of interest – perhaps an entire forest.

Counting rule: Omit objects on the left and bottom edges.

Quadrat sampling techniquesQuadrat sampling techniques

DemographyDemographyDemography is the study of factors that is the study of factors that

affect the growth and decline of affect the growth and decline of populations.populations.

Additions occur through birth (natality) and im- migration; subtractions occur through death (mortality) & emigration.

Population Growth by County April, 2000, to July, 2003

DemographyDemographyRepresent a life table

data with a survivorship curve – which plots the number of indi- viduals in a cohort still alive at each age.

• Type I curve shows a low death rate early in life (humans).

• Type II curve shows constant mortality (squirrels).

• Type III curve shows high death rate early in life (oyster).

Population Growth Population Growth (ideal conditions) (ideal conditions)

Exponential growth Exponential growth increases faster and faster un-checked, like compound interest, because resources are unlimited. Growth becomes faster and faster.

Change in population size = Births - Deaths

• This model is best for single-celled life (bacteria).

a J-shaped curve

Population Growth Population Growth (ideal conditions) (ideal conditions)

No population can in- crease forever! Why?

• It eventually runs out of resources: • too little food, • too little space,• too many waste

products,• too many predators,• too much disease.

Population Growth Population Growth (typical conditions) (typical conditions)

The logistic growth logistic growth model shows a leveling-off because of environmental feedback. The carrying capacity (K) is the maximum population size that can be supported by available resources.

Typically, unlimited resources are rare,

so this graph is S-shaped.

Population GrowthPopulation GrowthStages of growth:

a)a) Lag phase: growth is slow as 1→2, then 2→4, 4→8, etc.

b)b) Exponential phase: eventually growth speeds up as 1 million → 2 million, etc.

c)c) Transitional phase: at some point the food begins to run out, and

the growth slows down.

d)d) Plateau phase: carrying capacity has been reached, and births

equal deaths.

Population GrowthPopulation GrowthThe logistic growth model does not fit all

situations!• Resource availability depends on the situation.• Natural selection can vary with density &

environment.• There are lag times that alter the S-curve.

Populations can grow large, then many may starve (birds, below).

Microbes in the lab fit the model well.

Higher organisms in nature are influenced by many factors.

Population-limiting FactorsPopulation-limiting FactorsWhy do all populations eventually stop

growing?What environmental factors stop a population

from growing?• Examine effects of increased population

density.

Population-limiting FactorsPopulation-limiting FactorsDensity-independent factors Density-independent factors act the same act the same

regard-less of population size.regard-less of population size.

• Natural disasters such as hurricanes, tsunamis, forest fires destroy all but the lucky individuals in large or small populations.

Population-limiting FactorsPopulation-limiting FactorsDensity-dependent factors Density-dependent factors ((negative negative

feedbackfeedback) ) increase their effect on increase their effect on populations as population density populations as population density increases.increases.

Negative feedback prevents unlim- ited population growth.• Intra-specific competition for

food, shelter, or mates can cause density-dependent behavior of populations.

• Resource limitation in crowded populations can stop growthpopulation by reducing

reproduction.

Population-limiting FactorsPopulation-limiting FactorsNegative feedback prevents unlimited population

growth.• Territoriality (defense of a space) may set a limit on

density.• Predation may cause mortality of prey species.• Waste accumulation can regulate population size.

• Ex: yeast make alco-hol during ferment-ation of grape juice

to wine, but yeast begin to die as alcohol content approaches

13%.

• Disease spreads much more rapidly in dense populations.

Population-limiting factorsPopulation-limiting factorsSome populations have regular boom-and-bust

cycles.• Lynx & hare populations in Alaska cycle on a 10-

year basis.1) More hares allow more baby lynxes to survive &

reproduce2) More lynxes eat more hares, then lynx population

crashes.3) Fewer lynxes allow more hares to survive - cycle

repeats.

Over-populationOver-populationThe human population has been growing almost

exponen-tially for three centuries but cannot do so indefinitely.

What might put a stop to this growth?

An apocalypse?An apocalypse?

Conquest, War, Famine, Disease