determining population size

Question 2

What are the principles of ecology are based on? Where is the data obtained?

Answer 2

qualitative and quantitative data

data obtained from studies carried out on animals, plants and the
abiotic environment.

Question 3

What are the three types of distribution in a population? Explain each one

Answer 3

Regular distribution
- normal and even distribution throughout
the area. The animals mark out a small area around themselves, defend this area territorially

Random distribution
- distribution pattern where the location of
an individual is not affected by the locations of other individuals. No territoriality is evident.

Cluster distribution
- distribution where individuals group together
in clusters across a defined area. This is a trend with social animals such as a pride of lions or herd of elephants.

Question 4

What are the requirements to use a direct method?

Answer 4

• organisms are large enough to count accurately
• not move around too much

Question 5

What are the direct technique method(s)?

Answer 5

A Census

Question 6

When are censuses used?

Answer 6

• the animals are large
• they inhabit a fixed area
• they can be counted quickly and easily
• the organisms are relatively sessile

Question 7

List 3 advantages and 3 disadvantages of using a census

Answer 7

+ accurate and reliable, original data is collected.
+ data for sub-populations/ categories may be available e.g. gender, age etc
+ detailed cross-tabulations of the population categories may be possible.

• it is an expensive process because every parameter of the population is checked and recorded.
• it is time-consuming, requires manpower to collect original data.
• it does not take into account births and deaths which occur in already surveyed place
• lack of preciseness of the definition of statistical units
• personal bias of the investigators.

Question 8

What are the requirements to use an indirect method?

Answer 8

• when needing to estimate the population size.

Question 9

What are the indirect technique methods?

Answer 9

• Quadrat methods
• Mark and recapture

Question 10

What is the quadrat method used to determine? How is it performed?

Answer 10

determine the species density and the
species frequency of more than one species at a time.

it involves counting the individuals found in a number of squares that are randomly spaced over the total area.

Question 11

Define ‘Species Density’

Answer 11

number of individuals of a species found in a given area

Question 12

Define ‘Species Frequency’

Answer 12

number of individuals of a species found in each
quadrat. It is the distribution of the species,

(If the species is found in every tenth quadrat, then the frequency is 10% of the total area)

Question 13

Estimated population formula

Answer 13

numbers in sample size x total size of area
——————————————
size of quadrat

Question 14

Avg number of all quadrats formula

Answer 14

1+2 +6+8+ 4+3+2+1+2+4
——————————
Number of quadrats
= 33/ 10
= 3

Question 15

List 4 factors that will affect the reliability of the quadrat method

Answer 15

• must be selected at random (the quadrat samples are representative of the study area as a whole).
• Each individual inside each quadrant must be accurately counted.
• must cover at least 10% of the total area.
• size of the quadrats is known.
• total area must be known.

Question 16

List 4 Advantages and 4 disadvantages of quadrat sampling

Answer 16

+ sampling is easy to use, inexpensive
+ suitable for studying plants, slow-moving animals and faster-moving animals with a small range.
+ requires the researcher to perform the work in the field and, without care.
+ measures abundance and needed cheap equipment.

• not useful for studying very fast-moving animals which don’t stay within the quadrat boundaries
• biasness in favour of slow moving taxa
• Collect only taxa that are present in the sampling time and not buried deep in sediment.
• it is a low estimate of taxonomic richness and assemblage composition
• It is also a low detectability of among-site differences in assemblage composition
• Some animals may experience harm if the scientist collects the population within the quadrat rather than studying it in the field.

Question 17

How would one apply/use quadrat sampling?

Answer 17

• broadly used in plant studies
• applied in ecology especially in biodiversity
• used to find the taxonomic richness of slow moving taxa in a certain area
• used to evaluate the impact of weathering, trampling or erosion in an ecosystem or at a cliff or rock face.

Question 18

Define a ‘Transect’

Answer 18

A line across a habitat or part of a habitat; string or rope placed in a line on the ground.

Question 19

What is the purpose of a transect?

Answer 19

It is used to investigate a gradual change in a habitat rather than to simply estimate the number of organisms within it.

Question 20

What is a disadvantage of using a transect and why?

Answer 20

Any species not touching the transect line would not be recorded

A line transect only provides information about the number and distribution of species along an environmental gradient- fixed line

Question 21

Define ‘Mark and Recapture Method (M&R)’

Answer 21

Method used to establish the estimated population size of animals that do not remain in the same area permanently or animals that are not always visible.

Question 22

What are the 4 steps of the M&R method

Answer 22

1. We catch, mark and release the animals.
2. The animals are allowed time to mix with the rest of the population.
3. After a time, we capture a second sample and count the individuals where marked and unmarked animals will be in the second sample.
4. We can use the following formula to calculate the estimated population
   size

Question 23

What is the formula for estimated population size?

Answer 23

N = CxM/ R

N= Estimated population size
C= Total in 2nd catch
M= Total in 1st catch
R= marked individuals in 2nd catch

Question 24

What are safe marking/ tagging methods to use?

Answer 24

• paint that is non-toxic and waterproof.
• rubber rings on the legs of birds
• plastic tags through the ears of buck
• branding an emblem on the animal’s rump
• shaving hair in a distinctive pattern
• aluminium discs on the operculum of fish.

Question 25

List advantages and disadvantages of the M&R method

Answer 25

+ allows an estimate of a population to be made when a direct count is impossible
+ Accuracy does not depend on an assessment of the amount of habitat

• Marked animals may be more easily spotted by a predator.
• Births, deaths, immigration and emigration may still occur during the period between the first capture and the recapture.
• Some animals may avoid the trap after being trapped the first time.
• The animal usually needs to be captured to be marked, which may injure it, or alter its behaviour pattern.
• Accuracy does depend on capturing a large proportion of the population.
• Capturing and marking animals can be cost prohibitive and affect animal behaviours, which can bias population estimates.
• The mark used may harm the animal (for example a dot of a particular paint
  may turn out to be toxic to the animal)

Question 26

List 3 assumptions of the M&R method

Answer 26

• population is physically and demographically closed (i.e., no immigration
  or emigration, no birth or mortality so that the population is constant over the
  time of sampling).
• Marks or tags are neither lost nor missed
• Marked individuals returned to the population and mixed randomly with
  unmarked individuals.
• All individuals within a sample have an equal probability of capture.
• Individual behaviour or vulnerability does not change after being marked. The marking technique must not harm the individual or affect its survival by predation.
• The individuals have a lifespan which is long enough for you to capture, mark, release and recapture.
• The proportion of marked individuals recaptured in the second sample represents the proportion of marked individuals in the population as a whole.