POPULATION ECOLOGY

Density and Dispersion

• Density - The number of individuals per unit area

• What are the three patterns of dispersion?

Patterns of Dispersion Within a Population’s Geographic Range

Clumped

Random

Uniform

Estimating Population Size• Mark re-capture method.

• You capture 200 grasshoppers, mark them all and then release them back into the wild. A few days later you capture 100 grasshoppers and 50 of them were marked. How many grasshoppers do you estimate are in this population?

N = # marked x # recaptured number marked & recaptured

200 x 100/50 = 400

Original Population is approx. 400

You capture 50 rabbits, mark all themand release them back into the wild.The following week you capture 30 rabbits and 15 are marked. How many rabbits do you estimate in the population?

50 rabbits marked X 30 recaptured15 marked recaptured

Original Population is approx 100.

Demography• Study of vital statistics that

affect population size

• Life Tables are age-specific summaries of the survival pattern of a population

Life Table for Belding Ground Squirrels (Spermophilus beldini) at Tioga Pass, in the

Sierra Nevada Mountains of California

Age Structure – the relative numberof individuals at each age commonlyrepresented in pyramids. It shows present and future growth trends.

Age Structure Pyramids

Survivorship Curves

• Plot of the numbers of a cohort that are alive at each age.

• Survivorship curves are generally classified into three types:

Survivorship Curves

Life Histories

• Traits that affect time of reproduction and death including:

1. Clutch size

2. Number of reproductive episodes per lifetime

3. Age at first reproduction

• Species that exhibit semelparity, or “big-bang” reproduction

– reproduce

once and die

Figure 52.6

• Species that exhibit iteroparity, or repeated reproduction

– Produce offspring repeatedly over time

•Birds with high probability of dying have larger clutch sizes

• Large clutch sizes mean one reproductive episode per life.

• Small clutch sizes means more than one reproductive episode

• Age at first reproduction is younger in large clutch size reproducers; they invest less energy in their own growth and development.

Models of Population Growth• r = reproductive rate, r = b-d/N

• b = births, d = deaths, N = population size

• multiply both sides of the equation by N, we get: rN = b-d , rN = change in population during a given time interval: N/ t = b - d

Models of Population Growth• r max = the maximum rate of

increase in growth of a species; intrinsic/maximum rate of increase

• The equation for exponential population growth is: dN/ dt = rmaxN

r max = 1

r max = .5

Population Density is the number of

individuals per unit area.

Three factors affect a population’s size:

•number of births

•number of deaths

•number of individuals who immigrate

and emigrate

Exponential Growth occurs whenthe individuals in a population reproduce at a constant rate. At first, the number of individuals in an exponentially growing population increases slowly, but over time the population becomes larger and larger. (J shaped curve)

Logistic Growth occurs when a population’s growth slows or stops following a period of exponential growth. (S shaped curve)

ExponentialGrowth Curve

Logistic GrowthCurve

Carrying Capacity

• The logistic growth equation– Includes K, the carrying capacity,

the largest number of individuals of a population that an environment can support

dN

dt

(K N)

Krmax N

What is occurring here?

?

If N is less than K, then the population growth is positive and population increases

Are all examples of logistic growth?

What sort of limiting factor is seen here?

Density Dependent Selection

A limiting factor causes population growth to decrease.

Density Dependent Factors become limiting only when the per unit area – reaches a certain levelExamples: competition, predation, parasitism, and disease.

Density Independent Factors affect all populations in similar ways, regardless of the populationsize.Examples include unusual weather,natural disasters, seasonal cycles, and certain human activities (such as damming rivers or cutting forests)

What sort of limiting factor is seen here?

Density Independent Selection

K selection (density dependent selection)

•long maturation time•long lifespan•low death rate•few offspring/reproduction•several reproductive episodes•reproduces later in life•large sized offspring•parental care for young

r selection (density independent selection)• short maturation time

• short life-span

• high death rate

• many offspring/reproduction

• one reproductive event

• reproduces early in life

• small sized offspring

• no parental care

Usually more than 1 limiting factor is at work

Describe the population shifts seen below:

Are we at carrying capacity?