Population ecology is the study of populations in relation to their environment. Key concepts include population size, density, growth rates, and limiting factors. Population growth can be exponential in unlimited environments but becomes logistic as resources are depleted. The logistic model describes sigmoid growth with a carrying capacity K, where population growth rate is highest at N=K/2. Natality, mortality, immigration, and emigration influence population size. Estimating population density can involve direct counts, sampling methods like quadrats, or capture-recapture of marked individuals.

1. Population EcologyPopulation Ecology
Cyra Mae R. Soreda

2. Population EcologyPopulation Ecology
Population ecology is the study of
populations in relation to the
environment. It includes environmental
influences on population density and
distribution, age structure, and variations
in population size.

3. Characteristics of PopulationCharacteristics of Population
Population size
Population density
Dispersion patterns
Demographics
Survivorship curves
Population growth

4. Population sizePopulation size
“In population genetics and
population ecology, population size
(usually denoted N) is the number of
individual organisms in a population”.
Factors that Govern Population Size
1.Crude Birth Rate (CBR)
2.Crude Death Rate (CDR)
3.Immigration
4.Emigration

5. Factors That Increase Population Size
1.Natality is recruitment to a population
through reproduction.
2.Immigration from external populations e.g.
Bird migration.
Factor Reducing Population Size
1.Mortality which is the death rate from any
source e.g. predation.
2.Emigration, where individuals leave the
population for another habitat.

7. NatalityNatality
The production of new
individuals by birth, hatching,
germination or fission
2 aspects of reproduction must
be distinguished:
 Fecundity
 fertility

8. NatalityNatality
Fecundity-physiological notion that refers to
an organism’s potential reproductive capacity
Fertility-ecological concept based on the no.
of viable offspring produced during a period
time
Realized fertility and potential fecundity-we
must be able to distinguish between them

9. NatalityNatality
E.g, realized fertility rate for a
human pop may be only 1 birth per
15 years per female in the child-
bearing ages
While the potential fecundity rate
for humans is 1 birth per 10 to 11
months per female in the
childbearing ages

10. MortalityMortality
Biologists-interested not only in why
organisms die but also why they die
at a given age
Longevity-the age of death of
individuals in a population
2 types:
◦ Potential longevity
◦ Realized longevity

11. MortalityMortality
Potential longevity
◦ The maximum life span of an individual of
a particular sp is a limit set by the
physiology of the organism, such that it
simply dies of old age
◦ The average longevity of individuals living
under optimum conditions
◦ However, organisms rarely live under
optimum conditions-most die from
disease, or eaten by predators or succumb
to a number of natural hazards

12. MortalityMortality
Realized longevity
◦ The actual life span of an organism
◦ Can be measured in the field, while
potential longevity only in labs or
zoos

13. examplesexamples
European robin has an average
life expectation of 1 year in the
wild, whereas it can live at least
11 year in captivity

14. Have more births than deaths?
◦ Population increases
Have more deaths than births?
◦ Population decreases
Have equal amounts of births and deaths?
◦ Population remains constant
What happens to the populationWhat happens to the population
when we….when we….

16. ImmigrationImmigration
“im”= in
Migrate= to move from one place to
another
Immigration is the individual movement
into an area
Animals in search of mates and food in
new areas

18. EmigrationEmigration
“E” means ‘out’
Migrate means to move from one
place to another
Emigrate means individuals moving out of
one place and into another
Young wolves and bears leaving as they
mature
Shortage of food

20. NATALITYNATALITY
The birthrate, which is the ratio of total live
births to total population in a particular area
over a specified period of time
MORTALITYMORTALITY
The death rate, which is also the ratio of the
total number of deaths to the total population.
IMMIGRATIONIMMIGRATION
The number of organisms moving into area
occupied by the population is called
immigration.
EMIGRATIONEMIGRATION
The number of organisms moving out of the
area occupied by the population is called
emigration.

21. How to estimate populationHow to estimate population
density?density?
Techniques differ between organisms
such that the technique to estimate deer
cannot be applied to bacteria or protozoa
or vice versa
There are 2 fundamental attributes that
affect and ecologists choice of technique
for population estimation

22. 2 attributes
Size
-small animals/plants are usually more abundant
than large animals/plants
Mobility
-based on movements of these organisms

23. Why the need to estimateWhy the need to estimate
population density?population density?
Estimates of population are made for two
reasons:
◦ How to quantify nature – ecologist role
◦ Estimates are allows for comparisons between
different populations in terms of space and time
measure

24. 2 BROAD APPROACHES TO ESTIMATE POP DENSITY
Absolute density
No of individual per area/ per volume
Important for conservation and management
Relative density
Comparative no of organisms
Two areas of equal sizes, which area has more organism
e.g, between area x and y
Area x has more organism than area y

25. ABSOLUTE DENSITYABSOLUTE DENSITY
Making total counts and by using
sampling methods
Total counts - direct counting of
populations
- human pop census,
- trees in a given area,
- breeding colonies can be photographed
then later counted
- in general total counts are possible for
few animals

26. Measurements of Absolute densityMeasurements of Absolute density
 Sampling methods
◦ to count only a small proportion
of the population - sample
 Using the sample to estimate
the total population
 2 general sampling techniques:
1) Use of quadrats
2) Capture-recapture method

27. Use of Quadrats
Count all individuals on several quadrats of known size,
then extrapolate the average count to the whole area
Quadrat- a sampling area of any shape (may be a
rectangle, triangle or circle)
3 requirements:
• the pop in the quadrat must be determined exactly
• area of the quadrant must be known
• quadrant/s must be representative of the area
• achieved by random sampling

28. Quadrant sampling in plantQuadrant sampling in plant
populationpopulation
Conduct a transect in the upland
hardwood forest
3 transect line, 110 meters long, count all
trees taller than 25cm within 1meter of
each line
By utilizing the quadrant method sampling
for old trees and seedlings, we can
determine if populations were likely to
change over time

29. Capture Recapture Method
Capture, marking, release, and recapture-important for mobile animals
Why?-it allows not only an estimate of density but also estimates of birth rate
and death rate for the population being studied
Capture animal, mark (tag) them and then release them
Peterson method:
Involves 2 sampling periods
Capture, mark and release at time 1
Capture and check for marked animals at time 2
Time intervals between the 2 samples must be short because this method
assumes a closed population with no recruitment of new individuals into the
Population between time 1 and 2 and no losses of marked individuals

30. Formula for capture-recaptureFormula for capture-recapture
methodmethod
Marked animals in 2nd
sample = Marked animals in 1st
sample
Total caught in 2nd
sample Total population size

31. e.g of capture recapture methode.g of capture recapture method
Dahl marked trout in small
Norwegian lakes to estimate the size
of the population that was subject
to fishing. He marked and released
109 trout, and in 2nd
sample a few
days later caught 177 trout, of which
57 were marked. From the data,
what is the estimate population size?

32. e.g of capture recapture methode.g of capture recapture method
By using the formula
57 = 109
177 Total pop size
Total pop size = (109 X 177)
57
= 338 trout

33. RELATIVE DENSITYRELATIVE DENSITY
Traps – no caught per day per trap –
animals caught will depend on their density,
activity and range of movement, skill in
placing traps – rough idea of abundance –
night flying insects, pitfall traps for beetles,
suction traps for aerial insects
Fecal pellets – rabbits, deer, field mice –
provides an index of pop size
Vocalization frequency – bird calls per 10
mins, can be used for frogs, cicadas,
crickets
Pelt records – trapper records dates back
300 years – of lynx

34. Relative densityRelative density
Catch per unit effort – index of fish abundance –
no of fish per cast net or no of fish per 1 hour
trawling
Number of artifacts – thing left behind – pupal
cases of emerging insects
Questionnaires – to sportsmen (eg fish)and
trappers
Cover - % ground surface covered – in botany,
invertebrate studies of the rocky intertidal zone
Feeding capacity – bait taken – for rats and mice –
index of density
Roadside counts – birds observed while driving
standard distances

35. Population dispersionPopulation dispersion
patternspatterns
3 types
random
uniformclumped

37. Population dispersion patterns
Random-when the position of each
individuals in a pop is independent of
the others
Uniform-it results as a form of some
negative interactions
Common among animal pop where
individuals defend an area for their own
exclusive use (territoriality) or in plant
pop where severe competition exist for
belowground resources, i.e water or
nutrients

38. Population dispersion patternsPopulation dispersion patterns
Clumped-where individuals
occur in groups
Reason-suitable habitat or
resources may be distributed as
patches on a larger landscape

39. POPULATIONPOPULATION
GROWTHGROWTH

40. Population growthPopulation growth
Refers to how the number of
individuals in a population increases or
decreases with time (N, t)
Reflects the difference between rates
of birth and death
 in pop, if new births occur
 in pop, if death occurs

41. 2 types of pop growth
Exponential population growth
dN = rmaxN
dt
Logistic population growth
dN = rmaxN (K-N)
dt K
Population
Growth
Mathematically
Defined

42. N=K/2

43. Exponential GrowthExponential Growth
Continuous population growth in an unlimited
environment can be modeled exponentially.
dN / dt = rmax N
Appropriate for populations with overlapping
generations.
◦ As population size (N) increases, rate of population
increase (dN/dt) gets larger.

44. Exponential GrowthExponential Growth
For an exponentially growing population, size
at any time can be calculated as:
Nt = Noert
Nt = number individuals at time t.
N0 = initial number of individuals.
e = base of natural logarithms.
r (= rmax ) = per capita rate of increase.
t = number of time intervals.

45. PracticePractice
If the human population size in 1993 was
5.4 billion, what was the projected
population size in the year 2000?
r=0.0139
No = population size in 1993 = 5.4 billion
t = 7 years (year 2000 - 1993)
r = 0.0139

46. Nt = No ert
Nt = (540,000,000) e(0.0139)(7)
Nt /540,000,000 = e 0.0973
Dust off your high school math skills. To get rid
of the exponent, simply take the (ln) of both
sides of the equation. Remember, when we
take the natural log of a quotient we end up
taking the ln of one value and subtracting it
from the ln of the other value (see below).

47. ln (Nt /540,000,000) = ln (e 0.0973)
[here we're taking the natural log of the
quotient]
= ln(Nt) - ln(540,000,000) = 0.0973
[rewrite it as natural log of one value
minus natural log of the other value]
Nt = 595,000,000 or 5.95 billion

49. Logistic Population GrowthLogistic Population Growth
As resources are depleted, population growth
rate slows and eventually stops: logistic population
growth.
◦ Sigmoid (S-shaped) population growth curve.
◦ Carrying capacity (K) is the number of individuals of a
population the environment can support.
 Finite amount of resources can only support a finite number of
individuals.

50. Logistic Population GrowthLogistic Population Growth
dN/dt = rmaxN(1-N/K)
rmax = Maximum per capita rate of increase under
ideal conditions.
When N nears K, the right side of the equation
nears zero.
◦ As population size increases, logistic growth rate
becomes a small fraction of growth rate.
 Highest when N=K/2.
 N/K = Environmental resistance.

51. ProblemProblem
Suppose a population of butterflies is
growing according to the logistic
equation. If the carrying capacity is
500 butterflies and r = 0.1 individuals/
(individual*month), what is the
maximum possible growth rate for
the population?

52. To solve this, you must first determine
N, population size. From the plot of dN/dt
vs. N, we know that the maximum
possible growth rate for a population
growing according to the logistic model
occurs when N = K/2, here N = 250
butterflies. Plugging this into the logistic
equation:
DN/dt = rN [1- (N/K)]
= 0.1(250)[1-(250/500)]
= 12.5 individuals / month

55. Fig. 11.9Fig. 11.9

56. Limits to Population GrowthLimits to Population Growth
Environment limits population growth by altering
birth and death rates.
 Density-dependent factors
 Disease, Parasites, Resource Competition
Populations do not show continuous geometric increase
When density increases other organisms reduces the fertility and
longevity of the individuals in the population
This reduces the rate of increase of the pop until eventually the pop
ceases to grow
The growth curve is defined as the sigmoid curve, S – shaped
K = carrying capacity (upper asymptote or maximum value) – the
maximum number of individuals that environment can support
 Density-independent factors
Natural disasters
Climate

57. r- and k-speciesr- and k-species
Characteristics of r- species
 high biotic potential
 Rapid development
 Early reproduction
 Single period reproduction per individual
 Short lifecycle
 Small body size
 Regulated by the density-
independent factor

58. Characteristics of k- species
 low biotic potential
 slow development
 delayed reproduction
 multiple period reproduction per individual
 long lifecycle
 large body size
 Regulated by the density-dependent
factor

59. Life history strategiesLife history strategies
K and r selection (MacArthur and Wilson 1967)
r-selected species
•r refers to the per capita rate of increase
•Selection favoring rapid growth
•Should be favored in new or disturbed
environments
•Less competition
K-selected species
•K refers to carrying capacity
•More prominent in species that are
typically at their carrying capacity
•Favors more efficient use of resources
•Live with competition

60. THANKYOU FOR LISTENING.THANKYOU FOR LISTENING.