population ecology. population – a group of organisms of the same species occupying a particular...
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Population Ecology
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Population – a group of organisms of the same species occupying a particular space at a particular time.
Populations are units of study.
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Population Attributes
• Density – size of a population in relation to a definite unit of space
• Affected by:– Natality – the reproductive output (birth rate) of
a population– Mortality – the death rate of organisms in a
population– Immigration – number of organisms moving
into the area occupied by the population– Emigration – number of organisms moving out
of the area occupied by the population
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Population Density
Four primary population parameters:
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Density Examples
Two fundamental attributes that affect our choice of techniques for population estimation are the size and mobility of the organism with respect to humans.
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Range of Population Density
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Two Types of Density Estimates
• Absolute Density – a known density such as #/m2
• Relative Density – we know when one area has more individuals than another
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Measuring Absolute Density
• Total Count – count the number of organisms living in an area– Human census, number of oak trees in a
wooded lot, number of singing birds in an area– Total counts generally are not used very often
• Sampling Methods – use a sample to estimate population size– Either use the quadrat or capture-recapture
method
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Quadrat Method• A Quadrat is a sampling area of any shape
randomly deployed. Each individual within the quadrat is counted and those numbers are used to extrapolate population size.– Example: a 100 square centimeter metal
rectangle is randomly thrown four times and all of the beetles of a particular species within the square are counted each time: 19, 21, 17, and 19. This translates to 19 beetles per 100 cm2 or 1900 per m2.
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Transects as Quadrats
• Each transect was 110 meters long and 2m wide (220 m2 or 0.022/ha). All trees taller than 25 cm counted.
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Capture-recapture Method
• Important tool for estimating density, birth rate, and death rate for mobile animals.
• Method:– Collect a sample of individuals, mark them,
and then release them– After a period, collect more individuals from
the wild and count the number that have marks– We assume that a sample, if random, will
contain the same proportion of marked individuals as the population does
– Estimate population density
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Assumptions For All Capture-Recapture Studies
• Marking technique does not increase mortality of marked animals
• Marked individuals are allowed to mix with population
• Marking technique does not affect catch probability
• Marks are not lost or overlooked• No significant immigration or emigration• No significant mortality or natality
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Marked animals in second sample
Total caught in second sample
Marked animals in first sample
Total population size
=
520 N
16=
Peterson Method or Lincoln Index
N = 64N = (20)(16)5
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Indices of Relative Density
• Assume that samples represent some relatively constant but unknown relationship to total population size.– # cars in the Piggly Wiggly parking lot
• Provides an index of abundance– Is population increasing, decreasing, or
staying the same– Are there more animals in one location than
another?– Can not quantify differences between sites
Twice the number of tracks does not = twice as many animals
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Some Indices Used
• Traps• Number of Fecal
Pellets• Vocalization
Frequency• Pelt Records• Catch per Unit Fishing
Effort
• Number of Artifacts• Questionnaires• Cover• Feeding Capacity• Roadside Counts
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Natality (birth rate)• Fecundity – physiological notion that
refers to an organism’s potential reproductive potential – Usually inversely proportional to the amount of
parental care given to young
• Fertility – Ecological concept that is based on the number of viable offspring produced during a specific period– Realized fertility – actual fertility rate
One birth per 15 years per human female in the child-bearing ages
– Potential fertility – potential fertility rate One birth per 10 to 11 months per human
female in the child-bearing ages
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Natality Continued• Absolute or crude natality – number of
new individuals per unit time– 50 protozoa increase to 150 by division in one
hour, then crude natality = 100
• Specific natality – number of new members divided by the population size– 100 new protozoa in one hour from original 50 – 100/50 = 2 protozoa per original per hour
• 400 births in one year in a town of 10,000– Absolute natality = 400– Specific natality = 400/10,000 = 0.04 = 4%
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Mortality
• Opposite of mortality is survival• Longevity focuses on the age of death of
individuals in a population– Potential longevity – maximum lifespan by an
individual of a particular species Set by the organisms physiology (dies of old age) Sometimes described as the average longevity of
individuals living in optimal conditions– Realized longevity – actual life span of an organism
Measured as an average for all animals living under real environmental conditions
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Determining Mortality• Mark several individuals and measure how many survive
from time t to t+1.• If abundance of successive age groups is known, then you
can estimate mortality between successive age groups.• Can use catch curves for fish:
147292
Survival between age 2 and 3=
147/292=0.50
Or develop regression equation
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Immigration and Emigration
• Seldom measured
• Assumed to be equal or insignificant (island pop’s)
• However, dispersal may be a critical parameter in population changes
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Life Tables - Mortality
• Mortality is one of the four key parameters that drive population changes.
• We can use a life table to answer particular questions about population mortality rates.– What life stage has the highest mortality?– Do older organisms die more frequently than
young organisms• A cohort life table is an age-specific summary of
the mortality rates operating on a cohort of individuals.
• Cohort – a collective group of individuals– Fish year class, all mice born in March, tadpoles from a
single frog, freshman year class
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X = age
nx = number alive at time t
lx = proportion of organisms surviving from the start of the life table to age x (ex: l1 = n1/n0, 0.217 = 25/115; l2=n2/n0, 0.165=19/115)
dx = number dying during the age interval x to x + 1 (ex:d0=n0-n1, 90 = 115-25; d1=n1-n2, 6=25-19)
qx = per capita rate of mortality during the age interval x to x + 1 (ex: q0=d0/n0, 0.78 =90/115; q1=d1/n1)
Cohort Life Table:
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Per Capita Rates• Per capita is a presentation of data as a
proportion of the population.
• Suppose a disease kills 400 ducks:– If total duck population = 250,000 then the per
capita mortality = 400/250,000 = 0.16%.– If total duck population = 2,500 then the per
capita mortality = 400/2,500 = 16%.
• Per capita gives us an idea of how the entire population is affected.
• Allows us to standardize a population
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Suvivorship Curve
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Types of Survivorship Curves
• Type 1 – Mortality rates high late in life span
• Type 2 – Mortality rates fairly constant with age
• Type 3 – Mortality rates highest early in life span
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Survivorship Curves
• Can be a reflection of the amount of parental care
• Can be affected by population density
Managed for hunting
Unmanaged area
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bx = natality; (lx)(bx) = reproductive output for that age class
R0 = (lx)(bx) = net reproductive rate
Population net reproductive rate
Fertility Schedule
0.6% increase each generation
Adding Reproduction
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Net Reproductive Rate
Under stable conditions R0 is usually around 1.
R0 < 1 population is declining, R0 = 1 population is stable, R0 > population is increasing.
Can greatly affect the population growth rate, and natural selection works towards an adaptive reproductive schedule
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Population Age Structure
R0 > 1 R0 = 1 R0 < 1
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Population Age Pyramids
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Dominant Year Class
• Fish reproduction strongly affected by year-to-year fluctuations
• Population Age Pyramids will be different
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Relationships
Environmental factors
Mortality Rates
Natality Rates
Age Composition
Rate of increase or decrease of the population
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Growth With Discrete Generations• Species with a single annual breeding season
and a life span of one year (ex. annual plants).• Population growth can then be described by the
following equation:
• Where– Nt = population size of females at generation t
– Nt+1 = population size of females at generation t + 1
– R0 = net reproductive rate, or number of female offspring produced per female generation
• Population growth is very dependent on R0
Nt+1 = R0Nt
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Multiplication Rate (R0) Constant
• If R0 > 1, the population increases geometrically without limit. If R0 < 1 then the population decreases to extinction.
• The greater R0 is the faster the population increases: Geometric Growth
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Multiplication Rate (R0) Dependent on Population Size
• Carrying Capacity – the maximum population size that a particular environment is able to maintain for a given period.– At population sizes greater than the carrying capacity,
the population decreases– At population sizes less than the carrying capacity, the
population increases– At population sizes = the carrying capacity, the
population is stable
• Equilibrium Point – the population density that = the carrying capacity.
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Y = mX + b
Y = b – m(X)
Intercept
Net Reproductive rate (R0) as a function of population density:
N = 100, then R0 = 1.0 population stable
N > 100, then R0 < 1.0 population decreases
N < 100, then R0 > 1.0 population increases
Remember, at R0 = 1.0 birth rates = death rates
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Growth With Overlapping Generations
• Previous examples were for species that live for a year, reproduce then die.
• For populations that have a continuous breeding season, or prolonged reproductive period, we can describe population growth more easily with differential equations.
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Multiplication Rate Constant
• In a given population, suppose the probability of reproducing (b) is equal to the probability of dying (d).– Instantaneous rate of
population growth = r = b – d
– Then dN/dt = (b – d)N = rN
– Population grows geometrically
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Multiplication Rate Dependent on Population Size
dNdt = rN
K - N
K
Where:
N = population size
t = time
r = intrinsic capacity for increase
K = maximal value of N (‘carrying capacity’)
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K
r Pop. Size (K-N)/K Growth Rate
1.0 1 99/100 0.99
1.0 50 50/100 25.00
1.0 75 25/100 18.75
1.0 95 5/100 4.75
1.0 99 1/100 0.99
1.0 100 0/100 0.00
dNdt = rN
K - N
K
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Logistic Growth
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Theoretical Growth Forms
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Population Fluctuations
• Irruption – a boom in numbers followed by a bust
• Unpredictable, but usually happen when weather, food, and shelter are all ideal
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Population Cycles
• Population increase and decrease follow a multiple year cycle
• Usually predator prey cycles, but not always
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Modeling Population Cycles
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Population Level Pulsing
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Density Independent / Dependent Factors
• Physical factors such as unpredictable weather, water currents, chemical limiting conditions, or pollution can affect the population no matter the size
• Biotic factors such as competition, parasites, and predation often work as a density dependent factor (more important at higher densities)
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Metapopulations
• Subpopulations occupying discrete patches of suitable habitat separated by unsuitable habitat (except dispersal corridors)
• Source vs. sink
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r vs K selection
• r – intrinsic rate of increase– r-selected species have evolved to put a lot of
energy into reproduction and growth
• K – carrying capacity– K –selected species have evolved to put a lot
of energy into maintenance
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r vs K selection