Transcript POPULATION ECOLOGY

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 them
and 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 recaptured
15 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 number
of individuals at each age commonly
represented 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: DN/ Dt = 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 when
the 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)
Exponential
Growth Curve
Carrying Capacity
Logistic Growth
Curve
• The logistic growth equation
– Includes K, the carrying
capacity, the largest number
of individuals of a population
that an environment can
support
dN
dt
 rmax N
(K  N)
K
If N is less than K, then the
population growth is
positive and ?population
increases
What is occurring here?
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 level
Examples: competition,
predation, parasitism, and
disease.
Density Independent Factors
affect all populations in similar
ways, regardless of the population
size.
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)
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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?