2.3.2 Abundance of organisms.
Methods for Estimating Population Size
1. Quadrats
2. Capture/Mark/Release/Recapture
(Lincoln Index)
• Knowing population size is important
in making environmental decisions
that would affect the population.
• Making a decision on an estimate that
is too high  extinction.
• Making a decision on an estimate that
is too low  unnecessarily hurt people
that depend on the animals for food &
income.
• When estimating population size it is
important to collect RANDOM
SAMPLES.
• A sample is a part of a population,
part of an area or part of some other
whole thing, chosen to illustrate what
the whole population, area or other
thing is like.
• In a random sample every individual in
a population has an equal chance of
being selected.
Using Quadrats
1. Mark out area to be sampled.
2. Place quadrates ( 1 m2, 10 m2)
randomly within the area.
3. Count how many individuals are
inside each of the quadrates.
4. Calculate the mean number of
individuals per quadrate.
5. Pop. Size = mean X total area
area of each quadrat
RANDOM
QUDRATS
SYSTEMATIC
QUDRATS
Quadrat sampling is suitable for
plants that do not move around and
are easy to find.
Quadrat method can be used to
determine:
 POPULATION DENSITY = number of
individuals of each species per area.
 PERCENTAGE FREQUENCY = percent
of each species found within an
area.
 PERCENTAGE COVER = percent of
plant covering a given area.
Capture/Mark/
Release/Recapture
Lincoln index
1. Capture as many individuals as possible in
the area occupied by the animal
population, using netting, trapping or
careful searching.
2. Mark each individual, without making
them more visible to predators and
without harming them.
3. Release all the marked individuals and
allow them to settle back into their
habitat.
4. Recapture as many individuals as
possible and count how many are marked
and how many are unmarked.
10 marked
14 unmarked
Capture and Marking
5. Calculate the estimated population size
by using the Lincoln Index:
population size = N1 X N2
N3
N1 = number caught and marked initially
N2 = total number caught in 2nd sample
N3 = number of marked individuals
recaptured
Most suitable for animals that move around
and are difficult to find.
Assumptions:
1. The population of organisms must be closed,
with no immigration or emigration.
2. The time between samples must be very small
compared to the life span of the organism
being sampled.
3. The marked organisms must mix completely
with the rest of the population during the time
between the two samples.
4. Organisms are not hurt or disadvantaged by
being caught and marked and therefore all
organisms have na equal opportunity of being
recaptured
Change in the relative abundance of a
species over an area or a distance is
referred to as an ECOLOGIAL GRADIENT
Also known as Zonation.
Changes in the distribution of animals with
elevation on a typical mountain in Kenya. Another
example of Zonation