Transcript Population Ecology

Population Ecology
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Population Dynamics
• Theoretically, if reproduction and mortality
rates in a non-mobile population are equal
and constant, the number of individuals in the
population would remain constant
• Natural population are not static
– Constantly subject to change and motion because
of many variable factors both in the environment
and within the organisms themselves
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Population Ecology
• Study of distribution, density, numbers of
individuals and structure(gender, age), rates of
birth and mortality, factors that affect growth
• Density – number of individuals per unit area
(ex. Per acre or hectare) or unit volume (ex. In
a column of water)
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Population Ecology
• Composition – number of individuals, gender
and age
– Changes result of different factors
• Reproduction, invasion, emigration, migration,
mortality, and cyclic fluctuations of considerably
greater length and magnitude involving several years or
more
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Obtaining Population Information
• Direct data on most population numbers is
difficult or impossible to obtain
– Rarely able to count the entire population
• Count all the individuals in a prescribed area
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Techniques of Obtaining Populations
1) Simple counts
1) # seals/island, #burrows/area, # wildebeest/herd
2) Can use aerial photographs to obtain population
estimates
1) Seals and sea lions
2) Wintering waterfowl and marine birds
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Techniques of Obtaining Populations
2) Mark-recapture technique
1) Capture and mark individuals
1) Trapping, marking, ID tags, radio transmitters
2) Recapture at a later point in time
1) Provide estimate of population size for a given area
3) Calculation = (total number marked)(total
number recaptured)/(number of recapture that
were marked)
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Techniques of Obtaining Populations
• Mark-recapture technique
– Example
• Initial capture of 50 individuals
• Second capture of 100 individuals, 10 of the 100 were
marked from the first capture
• Estimated population size = 50*100/10 = 500
individuals
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Techniques of Obtaining Populations
3) Census techniques
1) Transect methods
1) Walk or drive a line (transect) and count the number
of individuals at specific locations, evenly distributed
along the line
2) Used for pheasant counts
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Distribution
• Distribution – way species are organized in an
area
• Can be due to abiotic factors (rocks, water, the
environment) or biotic (species interactions,
plants, food sources)
• Can look at an individual species or the
assemblages of species – description at the
community level
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Distribution
• Type 1 – Uniform or
regular or nearly
uniform
• Possible explanations
– Territorial species
– Dispersed resources
• Telephone poles used as
perching sites for birds
– Behavioral interactions
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Distribution
• Type 2 – clumped
distribution
• Possible explanations
– Patchy distribution of
resources
– Organisms live in groups
or close together
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Distribution
• Type 3 – Random
distribution
• Possible explanations
– Random distribution of
resources
– Absence of strong
attractions or repulsions
among individuals of a
population
• Very uncommon
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Demographics
• Characteristics of a population that affect
growth
• Two characteristics that are important
– Age structure
– Sex ratio
• In human population of characteristics are
considered
– Race, education, marital status, religious beliefs,
etc.
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Age Structure
• Methods – follow
a group of
individuals from
birth to death over
time
– Construct a life
table for the group
0-9
Number of
Survivors
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10-19
10
1
20-29
8
2
30-39
7
1
40-49
5
2
50-59
3
2
60-69
2
1
70-79
2
0
80-89
1
1
90-99
100+
0
0
1
0
Age Class
Number of Deaths
0
Mortality
rate
0.000 = 0/11
0.090 = 1/11
0.200 = 2/10
0.125 = 1/8
0.286 = 2/7
0.400 = 2/5
0.330
0.000
0.500
1.000
1.00
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Age Structure
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Sex Ratio
• Rate at which a
population may grow
can be dependent on
the sex ratio
• Sex Ratios by age (males per
1000 females
– Fewer females –
slower rate of
population growth
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Calculate Rates for Populations
• 3 Rates that are looked at
– Survivorship – number of individuals that reach
the next year of life
– Birth – number of individuals born within a
designated time frame
– Mortality – number of individuals that die each
year
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Survivorship
• Number of
survivors/age group
• Probability of newborn
individuals of a group
surviving to particular
ages
• Yields 3 different curves
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Survivorship
• Type 1 – high
survivorship for most
age groups except older
individuals
• Examples – humans,
large mammals,
organisms that produce
few offspring but
provide extensive
parental care
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Survivorship
• Type 2 – constant
survivorship rate for
most age groups
• Examples – some
species of birds, lizards,
annual plants,
invertebrates and
rodents
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Survivorship
• Type 3 – low survivorship
early but individuals that
do make it live longer
• Examples – many species
of fish and marine
invertebrates, perennial
plants, trees, species that
produce many young and
no parental care
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Survivorship Curves
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Birth Rates
• Also called reproduction rate
• Number of individuals born within a certain
period of time
• Population increase primarily dependent upon
reproduction
• In order to avoid extinction a species must
produce new individuals in numbers sufficient
to replace those that die
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Reproduction Potential
• Maximum number of individuals that a
population could produce
• Number of new individuals that could be
produces is greater than the number that is
actually produced
– Actual number takes into account survival rate
– Actual number could be close to potential
• Single young produced once a year by certain large
mammals
– Actual number could be small fraction of potential
• Fish that lay several million eggs
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Factors of Reproductive Rates
Clutch size – number of young produced per
reproductive event
Small
Large
• Animals with long life span
• 1 or occasionally 2 young a
year
• Large herbivorous mammals
– elephants, zebras, cows
• Semi-aquatic mammals –
seals, walrus
• Marine mammals – whales,
dolphins
• Animals with short life span
• Large number a year
• Small mammals – mice,
voles
• Fish – salmon, sturgeon,
trout
• Reptiles – snakes, turtles
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Factors of Reproductive Rate
• Number of reproductive episodes per year
– Small clutches – usually once per year or every
couple years
• Long gestation period
• Long life span
– Middle to Large clutches – multiple times per year
• Short gestation period
• Short life span
• Record example – captive vole produced 17 litters
within 1 year
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Factors of Reproductive Rates
• Number of reproductive episodes per lifetime
Semelparity
• Reproduce one time in life
• Plants – annuals
• “Big bang” reproduction
– Reproductive event usually
large and fatal
• Examples
– Pacific salmon – lay eggs and
die
– Many insects, squid, octopus,
arachnids
Iteroparity
• Reproduce many times in
life
• Plants – perennials
• Examples
– Humans
– Vertebrates – birds, reptiles,
virtually all mammals, and
most fish
– Invertebrates – most
molluscs, many insects
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Factors of Reproductive Rate
• Age of reproductive maturity – how old the
animal must be to reproduce
• Some species have delayed maturity
– Condor – can’t breed until they are about 5 yrs old
– Many large mammals – 1-2 years
– Voles – breed at 3 to 6 weeks
• Some are born pregnant
– Species of mite
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Factors of Reproductive Rate
• Density
– High density – may cause decrease in fertility,
resulting in shortening of the breeding season and
reduction in number of young per litter
– Low density – becomes harder to find mates,
inbreeding
• Age
– Effects breeding abilities
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Measure/Model Population Growth
• Strait counts are hard to achieve
• Use mathematical model to predict
population size in the future
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Model Population Growth
Immigrate
Birth
Population
size
Emigration
Death
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Measure/Model Population Growth
• N = population size – total number of
individuals in a specific area at a given time
• B = number of births
• b = birth rate
– N = 1000
– B = 34
– b = B/N = 34/1000 = 0.034
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Measure/Model Population Growth
• D = number of deaths
• d = death rate
– N = 1000
– D = 16
– d = D/N = 16/1000 = 0.016
• T = time
• r = rate of increase
– r = b-d
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Measure of Population Growth
• Change in population size would be the
number of births minus the number of deaths
in a specific period of time
• ΔN/Δt = B-D
• This requires us to count number born and
number that die in specific period of time
• Easier to use rates
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Measure of Population Growth
• The simplest case – no limitations on growth
within the environment
• Two things occur
– Population displays its intrinsic rate of increase
– Population experiences exponential growth
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Intrinsic Rate of Population Increase
• Rate of growth of a population when
population is growing under ideal conditions
and without limits
– As fast as it possibly can
• Difference between birth rate and death rate
is maximized
• Characteristic of population and not of the
environment
– Usually can’t be achieved in most environments
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Intrinsic Rate of Population Increase
•
•
•
•
Higher intrinsic rate – grow faster
Lower rate of increase – slower growth
Intrinsic rate – rmax
Influenced by different factors
– Age of reproduction maturity
– Number of young produced
– How well the young survive
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Measurements – Unlimited growth
• Formula
• Produces J shaped
curve (called J curve)
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Population size
– Nt = N0(er)t
– Nt = number of
individuals at present
time
– N0 = number of initial
organisms
Unlimited exponential
growth
100
80
60
40
20
0
0
2
4
Time
6
8
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Limits on Population Growth
• Exponential growth cannot go on forever
– Population will eventually run into limits in their
environment
• Environment has finite amount of resources
• Each environment has a carrying capacity
– A specific number of individuals the environment
can support
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Carrying capacity
• Environment has finite amount of available
resources
• Population has to share the available resource
• As population increases – more individuals
have to share limited resources
– Each individual gets an increasingly smaller share
• Carrying capacity
– Maximum stable population size that a particular
environment can support over a long period of
time
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Carrying Capacity
• Symbolized – K
• Property of the environment
• Vary over space and time
– Affected by abundance of limiting resources
• If number of individuals exceeds carrying
capacity – environment will be destroyed to
the point where it can no longer support that
number of individuals
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Carrying Capacity
• As population approaches carrying capacity
– Individuals experience either a higher death rate
or a lower fecundity
– Rate of population growth declines towards zero
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Logistic Growth
• Logistic growth
– Mathematical description that takes into
consideration carrying capacity
– Employs two parameters
• rmax
•K
– Curve is S-shaped
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Logistic Growth
• Initially the population grows exponentially at
a rate which is determined by rmax
• As population size approaches carrying
capacity – population growth rate slows
– As population gets larger – rate gets slower
• Ultimately the rate of growth reaches zero at
the carrying capacity
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Logistic Growth
𝑁𝑡 =
𝑁0 −𝐾
𝑁0 + 𝐾−𝑁0 ∗𝑒 −𝑟𝑡
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Logistic Growth
• Logistic model – density dependent
– Rate at which population changes with density of
organisms that are currently in the population
• Population do not typically display idealized
logistic growth seen with the model
• Deviation – delayed feedback
– Overshoot
– Vary up and down around the carrying capacity
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Logistic Model
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K or r Selected Populations
K - selected
• Equilibrium populations
• Species good at maintaining
population sizes at carrying
capacity
r-selected
• Opportunistic populations
• Species good at growing
rapidly in disturbed
environments
– Significantly less capable of
maintaining its population at
carrying capacity
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K or r selected Populations
• Few populations are either purely r or K
selected
r-selected
K-selected
Organism size
Small
Large
Energy used to make
each individual
Low
High
# offspring produced
Many
Few
Timing of maturation
Early
Late (with much
parental care)
Life expectancy
Short
Long
Lifetime reproductive
events
One
More than one
Survivorship curve
Type III
Type I or II
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r-selected curves
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K-selected curves
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Population Regulation
• Density-dependent factors
– Based on competition within the species
– Help determine carrying capacity
– Factors
•
•
•
•
•
•
•
Food
Mates
Increased rates of be coming prey and parasitism,
Stress and behavioral problems
Nesting habitat
Waste buildup
Water
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Population Regulation
• Density-independent factors
– Occur to same extent regardless of population size
– Factors
• Weather and climate
– Drought, typhoon, hurricane, excessive rain or snow
• Geological disturbances
– Earthquake, tidal wave, volcanic eruption
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Cycles
• Regular fluctuation in density
• Fluctuates on annual cycle
– High just after reproduction
– Low just before
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Cycles
• Long term cycles – periods of 3- 10 years
• Possible causes
– Hormonal changes
– Change in food quality
– Lag in response to predator population density
– An adaptation to reduce predation
• Lemming (4 yr)
• Cicadas (13-17 yr)
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Irregular Cycles
• Changes
without regular
patterns
• Can be due to
strong weather
or climate
events
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