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Malta Rocky Beaches
Biostatistics
In our study, Biostatistics was identified as one the most relevant topics linked to fieldwork by both teachers and students. This page hopes to guide students through calculations required as part of the Biology Advanced-Level MATSEC syllabus.
A video-clip was added to each description so as to cater for different cognitive styles.
Species Density
Species Density is a measure of the individuals of a species present. This is found by simply counting the individuals of each species present.
Species Density = 3
Species Density =1
Species Frequency
Species Frequency is a measure of probability of finding a given species with any throw of a quadrat in a given area. This is found by noting how many times a species occurs in say, 10 throws.
The species is present in 3 of 10 quadrats. Therefore, 3/10= 30%; This would show the probability of finding this species in a randomly thrown quadrat.
Species Cover
Species Cover is measure of the proportion of ground occupied as an estimate of the total area
Area covered is approximately 50%. A grid can be used when necessary to get more reliable results
1.Quadrat Studies
2. Estimating Population Size
Lincoln Index
Also known as Mark-release method. This method involves capturing animals, marking them, setting them free again and recapturing them at a later stage.
Marking can be done by aluminum tags on opercula of fish, rings on bird legs, small mammals by dyes, arthropods by paint. NO ANIMALS ARE TO BE HARMED DURING SUCH A PROCESS.
Estimated total population=
(number of organisms in first sample) x (number of organisms in second sample)
(number of marked organisms recaptured)
(5) x (8)
(2)
= 20 organisms
Assumptions
1. Random mixing: organisms mix randomly within a population. Now we know that this Is not always the case as some live in colonies for example.
2. Time: sufficient time must be allowed between the capture and recapture to allow for random mixing. The less mobile the organism the longer the time
3. Restricted movement: only applicable to populations whose movement is restricted geographically
4. Even dispersion: organisms disperse evenly within the geographical area of the population
5. Negligible changes in population size: immigration, emigration, births and deaths are considered to be negligible
6. Marking does not hinder movement or make them conspicuous to predation
Simpson Index
Simpson's Index takes two factors into account; 'Species Richness' and 'Species Evenness'.
Species Richness- The number of species in a sample. However, this is not very useful on its own as it ives the same weighting to species which have a few individuals to others which are in abundance.
Evenness- This is a measure of the relative abundance of each species in an area.
3. Estimating Population Diversity
Number of Individuals
Sample 1
Sample 2
20
30
25
5
11
59
75
75
Species 1
Species 2
Species 3
Total
Sample 2 is less diverse than sample 1 because the area is dominated by one species, whereas more evennessis found in Sample 1.
As species richness and evenness increase, so does diversity.
Drawback- Simpson's Index gives more importance to abundant species as the addition of a rare species to a sample causes only small changes to the value of D
Simpson's Index (D)=
N (N-1)
n(n-1)
Sample 1 = 75(75-1) = 5550
20(20-1) + 30 (30-1) + 25 (25-1) 1850
D= 3
N= Total number of individuals in an area
n= Number of individuals belonging to a particular spcies
N= Total number of individuals in an area
n= Number of individuals belonging to a particular spcies
Simpson's Index (D)=
N (N-1)
n(n-1)
Sample 2 = 75(75-1) = 5550
5 (5-1) + 11 (11-1) + 59 (59-1) 3552
D= 1.56
4. Comparing Data
Chi-Squared Test
Students were asked to investigate the foraging habits of wasps at Zonqor Point. This was done by noting the number of times two plants were visited.
Null hypothesis: There is no significant difference in
the number of visits to each plant.
When to use Chi-Squared
-Data is categorical or nominal
-The expected frequency is not below 5 in more than 20% of the cells
Sample Observed Frequency Expected Frequency Difference (O-E)
Plant 1 32 27 32-27=5 0.925
Plant 2 22 27 22-27=-5 0.925
Chi-squared=0.925+0.925= 1.85
Degrees of freedom= (number of classes of data)-1 =2-1 =1
Xcrit coreesponding to 1 degree of freedom and 5% level of significance = 3.84
Since the value of chi squared obtained is 1.85, it is less than Xcrit =3.84, the null hypothesis is accepted.
t-Test
The T-Test is used to check whether the mean of two groups are statistically different from each other.
There are two formulae depending on whether the samples are independent or matched.
(i) Independent Samples
(ii) Matched Samples
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