POPULATIONSCHAPTER 19

POPULATIONSPopulation-all of the individuals of a

species that live together in one place at one time.

Demography-the statistical study of populations. It is used to predict how the size of a population will change.

KEY FEATURES OF KEY FEATURES OF POPULATIONSPOPULATIONS

1. 1. Population sizePopulation size – is the number of individuals in a population. – has an important effect on the ability of the population

to survive.

A small population is more likely to become extinct: -in the case of random events or natural disaster -due to inbreeding where the population is more

genetically alike. Recessive traits are more likely to appear.

-with reduced variability it is harder to adapt to changes.

KEY FEATURES OF KEY FEATURES OF POPULATIONS, con’tPOPULATIONS, con’t

2. 2. Population densityPopulation density – – the number of individuals in a given area.the number of individuals in a given area. – – if they are too far apart they may only rarely if they are too far apart they may only rarely

encounter one another resulting in little encounter one another resulting in little reproduction.reproduction.

KEY FEATURES OF POPULATIONS, con’tKEY FEATURES OF POPULATIONS, con’tPopulation size is limited by:Population size is limited by:

density-dependent density-dependent factorsfactors

DiseaseDisease CompetitionCompetition PredatorsPredators ParasitesParasites Food Food CrowdingCrowding

The greater the The greater the population, the greater population, the greater effect these factors have.effect these factors have.

Ex. Black plague in the Ex. Black plague in the Middle Ages – more Middle Ages – more deaths in citiesdeaths in cities

density-independent density-independent factorsfactors

Volcanic eruptionsVolcanic eruptions TemperatureTemperature StormsStorms FloodsFloods DroughtDrought Chemical pesticidesChemical pesticides Major habitat disruption Major habitat disruption

(as in the New Orleans (as in the New Orleans flooding)flooding)

Most are abiotic factorsMost are abiotic factors

KEY FEATURES OF KEY FEATURES OF POPULATIONS, con’tPOPULATIONS, con’t

3. Dispersion 3. Dispersion – – the way in which the individuals are the way in which the individuals are

arranged.arranged.

Most common

PREDICTING POPULATION PREDICTING POPULATION GROWTHGROWTH

Model:Model: A hypothetical population that has A hypothetical population that has

key characteristics of the real key characteristics of the real population being studied.population being studied.

Used by demographers to predict Used by demographers to predict how a population will grow.how a population will grow.

PREDICTING POPULATION PREDICTING POPULATION GROWTH, con’tGROWTH, con’t

Nearly all populations will tend to grow Nearly all populations will tend to grow exponentiallyexponentially as long as there are as long as there are resources available. resources available.

Two of the most basic factors that Two of the most basic factors that affect the rate of population growth are affect the rate of population growth are the birth rate, and the death rate. the birth rate, and the death rate.

r(rate of growth)=birth rate – death rater(rate of growth)=birth rate – death rate

PREDICTING POPULATION PREDICTING POPULATION GROWTH, con’tGROWTH, con’t

Exponential growth curve:Exponential growth curve: population growth plotted population growth plotted against time.against time.

As a population gets As a population gets larger, it also grows at a larger, it also grows at a faster rate. faster rate.

This is the maximum This is the maximum population growth under population growth under ideal circumstances. ideal circumstances.

Includes plenty of room Includes plenty of room for each member, for each member, unlimited resources (food, unlimited resources (food, water) and no hindrances water) and no hindrances (predators). (predators). FACT: No population exhibits this type of growth for long.

PREDICTING POPULATION GROWTH, PREDICTING POPULATION GROWTH, con’tcon’t

Logistic model:Logistic model: This model accounts for the declining This model accounts for the declining resources available to populations as they grow.resources available to populations as they grow.

It assumes the birth and death rates are not constant.It assumes the birth and death rates are not constant. As the population grows, births decline and death As the population grows, births decline and death

rises. rises. Eventually birth=death so the population stops Eventually birth=death so the population stops

growing.growing.

Carrying capacity (K):Carrying capacity (K): The number of organisms of The number of organisms of one species that an environment can support one species that an environment can support indefinitely.indefinitely.

PREDICTING POPULATION GROWTH, PREDICTING POPULATION GROWTH, con’tcon’t

Two modes of population growth.

The Exponential curve (also known as a J-curve) occurs when there is no limit to population size.

The Logistic curve (also known as an S-curve) shows the effect of a limiting factor (in this case the carrying capacity of the environment).