Prairie Community Surveys
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Grasshopper Mark & Recapture Data

Biodiversity Assays


Grasshopper Estimated Population Size Data
Month Year
Number Caught & Marked on Day 1
Total Number Caught on Day 2
Number of Recaptured (Marked) on Day 2
Estimated Population Size
September 2008

39

26

3
338
September 2009

19

32

2
304

One of the most wide spread sampling techniques used to estimate wildlife populations is the mark and recapture technique, also known as the Peterson Index.  The population size in a given area is determined as follows: students capture as many grasshoppers as they can and then each grasshopper is painted with nail polish as a tag, counted, and then released into the same area.  After 24 hours, the students repeat the sampling process, capturing both marked and unmarked grasshoppers.  The proportion of marked to unmarked individuals gives an estimate of the size of the entire population.  The following equation, known as the Peterson Index, gives an estimate of the number of individuals in the population:

Total Marked on Day 1 – M
Total Captured on Day 2 – S
Total Marked & Recaptured on Day 2 – R
Estimate of Population Size – N

__R__ = __ S__
M            N

Rearrangement of this equation:

      N =  SM___
          R


Data from 09/14/09

Prairie 10 meters from Pond
Diversity Index = 0.693

Prairie > 10 meters from Pond
Diversity Index = 0.764

ID of Species / #
ID of Species / #
flies / 25
flies / 48
wasps / 3
wasps / 12
brown spiders / 4
mosquitos / 36
brown beetles / 7
brown spiders / 2
green beetles / 7
orange spiders / 1
black beetle / 1
aphids / 27
gnats / 89
weird bugs that Nate caught / 3
lightning bugs / 4
brown beetles / 3
green fly / 1
black beetles / 1
bug with long antenna / 1
gnats / 87
moths / 3
shiny white backed beetle / 1
cricket / 1

Ecologists often use biodiversity measurements to determine the health of an ecosystem.  A declining biodiversity indicates a declining ecosystem and can indicate that the ecosystem is or was undergoing some environmental stress.  There are a great number of ways biodiversity is calculated.  One of the most common and easiest methods is the Simpson’s Diversity Index.  This method calculates not only the number of species represented in a given habitat but also how well each species is represented; therefore, the method takes into account much more than just species richness.  The Simpson’s Diversity value (D) ranges from no diversity at 0.0 to a maximum diversity at 1.0.  These values have no real meaning themselves but merely are used as a means of comparison for different habitats or the same habitat at different times.  A large D value indicates that if you were to go out and collect two organisms from a habitat the odds would be that the two organisms would be different species.  To collect our organisms we set sticky traps out overnight in designated areas and the organisms found on them were counted.

Example Calculation of Simpson’s Diversity Index:
 

            Where
                        s = the number of morphotypes (species)
                        i = a given morphotype
                        ni = the number of individuals in morphotype i
                        N = the total number of individuals collected (for all morphotypes)

            Say:

Community A

Community B

Species 1

1000

Species 1

400

Species 2

100

Species 2

400

Species 3

100

Species 3

400


For Community A:

s = 3
n1 = 1000
n2 = 100
n3 = 100
N = 1200
DA = 1 – [(1000/1200)2 + (100/1200)2 + (100/1200)2]
Da = 0.292

For Community B:

s = 3
n1 = 400
n2 = 400
n3 = 400
N = 1200
DB = 1 - [(400/1200)2 + (400/1200)2 + (400/1200)2]
Db = 0.667