Population ecology: Essentials of ecology chapter 5
a) Appreciate the difficulty of counting individuals; note its importance
b) Understand the nature and importance of life tables and fecundity schedules
c) Appreciate the range of life cycles and patterns of birth and death exhibited by different
organisms
 Definition of a Population:
a group of organisms of the same species occupying a particular space at a particular
time.
 What are the main characteristics of a population?
a) the density of the population- the number of individuals per unit of area or per unit
volume.
b) age structure of the population -the relative proportions of individuals of each age
c) the sex ratio of the population
d) the size of the population
 the four parameters that affect the size of the population: births, deaths,
immigration and emigration.
Understanding nature, predicting and controlling it
What is an individual?
 Unitary organisms
 Modular organisms
 Growth by the repeated production of modules
 In many cases they form branches, are rooted, attached, not motile
 Modular organisms: distinguishing individuals can be difficult
 Distinguishing between the genetic individual (genet) and the module
 a module starts life as a multi-cellular outgrowth of another module
 the genet is the individual that starts life as a single-celled zygote and is considered dead
when all component modules are dead.
How many organisms are there in a population?
 Measuring absolute density
 total counts
 sampling methods
Two currently used sampling techniques are:
 quadrat sampling
 capture-recapture method
Quadrat size and shape
 A quadrat is a four-sided figure, circular, hexagonal or irregular in shape.
Two main questions:
 What size of quadrat should one use?
 What shape of quadrat is best?
Problems involved with shape
 Edge effect- minimal in a circular quadrat and maximal in a rectangular one.
 The ratio of length of edge to the area inside a quadrat changes as:
What shape of quadrat is best?
Why are long thin quadrats better?
 Habitats are heterogeneous
 Areas are never uniform, and organisms are patchily distributed within the overall
sampling zone
 Long quadrats cross more patches.
 The reliability of population estimates (size and density) depend on
 estimating the population in each quadrat exactly
 knowing the area of each quadrat
 making sure the quadrats counted are a good representative of the whole area.
 Modular organisms: what should be counted?
When is a tree a tree?
 Do we include saplings and seedlings?
 Do we include live seeds in the soil?
 Do we include trees with only one or two shoots with only a few leaves? all other shoots
are dead.
Capture-recapture method
 Capture, marking, release and recapture is an important technique for mobile animals,
because it allows the estimation of density, birth and death rates of the population.
 All capture-recapture methods make three critical assumptions that can be violated:
 marked and unmarked individuals are captured randomly
 marked individuals are subject to the same mortality rates as unmarked individuals
 marks are not lost or overlooked.
 the capture-recapture technique has been used on butterflies, beetles, snails, and many
vertebrates e.g. fish, manatees, waterfowl etc.
Measuring relative density
 traps
 catch per unit effort
 percentage cover
 Life cycles and reproduction
 Annuals and perennials
 Iteroparous species: breeding seasonally once a year e.g.
 Iteroparous species: breed continuously e.g.
 Semelparous species: single reproductive episode followed by death e.g.
Life tables and longevity
 What is a life table?
 It is an age-specific summary of the mortality rates operating on a cohort of individuals.
What is a cohort?
 A cohort may include the entire population, or all the males in the population, or all
individuals in a population that started life at the same time.
Questions addressed when examining a life table:
 is mortality high among juveniles?
 do older organisms have a higher mortality rate than younger organisms?
 what is the life expectancy of an individual in a population?
 how can we estimate the rate at which the population increases or decreases?
 how many female offspring will be produced by a cohort or generation?
Types of life tables
Static life tables
 Provide a snapshot of a population’s age structure
 Assumes that equal number of offspring are born each year
 not good for species with short life-spans; biased towards stage common at the moment
examples
Cohort life tables
 Follows a group of individuals throughout their life (problem: Intermingling cohorts for
long-lived species with overlapping generations)
 These difficulties are eased if the organism is sessile.
 Excellent for species with short life-span, e.g.
Defining symbols used in a life table
 x = age interval (years, days, months, or hours depending on species)
 nx = number of survivors at start of age interval x
 lx = proportion of organisms surviving at start of age interval x
 dx = number dying during the age interval x to x+1
 qx = mortality rate during the age interval x to x+1
 ex = life expectation, the average time left to an individual at the beginning of the interval
x
How are the columns related?
 nx+1 = nx - dx
(number of survivors)
 qx = dx/nx when numbers of individuals are used.
 lx = nx/no
(proportion of organisms surviving)
 Lx = nx+nx+1/2 (the number of individuals alive)
 ex = SLx/nx or SLx/lx (life expectation)
 Ro = Slxmx
(the reproductive rate)
Symbols used in a life table
 Lx = the number of individuals alive in each age interval
 Fx = the total number of eggs deposited during each stage
 mx = the mean number of eggs produced per surviving individual.
 Ro = the reproductive rate = the mean number of offspring produced per original
individual by the end of the cohort.
How many were alive in the 6th year?
 n5-6 = 734
 How many were alive in the 7th year?
 n6-7 = 688
Number dying = 734-688 = 46
What is the survivorship in the 6th year?
 lx = nx/no (proportion of organisms surviving)
 734/1000 = 0.734
What is the mortality rate qx? qx = dx/nx
 46/734 = 0.063

Three steps to calculating life expectancy
 Calculate Lx = the average number of individuals alive in each age interval
 Lx = nx+nx+1 / 2
 Calculate Tx
 Tx = SLx
 Calculate ex = life expectation, the average time left to an individual at the
beginning of the interval x
 ex = SLx/nx or Tx/nx
Reproductive rate/population growth
 To calculate population growth,
 a) the fecundity should be known (mx)
 Fecundity is the
 number of female offspring produced by each breeding female.
 the total number of offspring produced
 mx = age-specific fecundity
 b) lx = survivorship (number of females surviving in each age class
Two formulas used to calculate the population’s net reproductive rate
 Ro = Slxmx
(sum of the number of fertilized eggs produced per original individual during each of
the stages )
 Ro = SFx/nx
(Fx = total number of eggs, seeds, or young deposited
 during one generation divided by the original number of individuals)
 Survivorship curves
 As a result of time-specific (static) and age- specific (cohort) demographic techniques,
three general types of survivorship curves are recognized.
Type I curve
 Example, elephants
Type II curve
 Example, lizards and many birds.
Type III curve
Many birds, small
mammals, lizards,
turtles
Number of
survivors
(log scale)
QuickTime™ and a
decompressor
are needed to see this picture.
Many
invertebrates
Many mammals