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Purpose: Even for organisms that are large and sessile, like trees, estimating population abundance is challenging. Study sites are often too vast to tabulate all individuals within a population and therefore require
unbiased sampling of a subset of that "population". How many samples are needed to accurately describe population abundance and where should these samples be taken in order to be representative and unbiased? Defining the area and individuals to be subsampled may in itself be challenging where organisms overlap each other and individuals are hard to distinguish or where vegetation is too dense to easily establish plots. In addition, abundance can be expressed in different ways (which species has more individuals, which occupy the most area, which are most likely to be encountered?). This indoor laboratory exercise will introduce you to methods useful in addressing these problems.
Instructions:
Sample the study area on the desert map provided. See the Map Legend page for explanation of the map. Plant species are designated by letter.
All sampling must be random in order to obtain an unbiased sample set.
Biased samples are those that are not representative of the entire population. Use the random number table to avoid biased sampling (e.g. to avoid unconsciously sampling easiest to reach on the map). You will use the following three techniques to measure abundance (express density as individuals per 100 m 2 ):
1) Quadrat sampling: For two common species sampled, estimate
density, dominance, and frequency by sampling ten 8m X 8m quadrats.
Procedure - select two 2-digit random numbers (from the table at the back of the lab handout) as coordinates along the left side and along the bottom of the map (blindly select a starting point on the random number table and then use successive numbers following that point for the remainder of the exercise). Use these coordinates to determine the position of your first quadrat (the transparency square provided).

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Count the number of individuals whose centers fall within the quadrat for each of the two species you have chosen. Record the sizes expressed as area of these individuals (the easiest way is to use the size legend at the bottom right corner of the map). Repeat the above procedure for nine more randomly selected quadrats. Record area and number as soon below in the appropriate data table located at the end of this lab.
Density = number of individuals for a species * 100
area sampled
(we mulitiple by 100 so that density in this lab can
be express as individuals per 100 m 2 )
Dominance = total area covered by a species area sampled
Frequency = number of quadrats in which the species occurs
Total number of quadrats
For example:
Quadrat #
#1
Species _g___
Area covered
3.14, 4.91, 4.91
Species __t__
No. Area covered
3 6.28
No.
1
#2 4.91, 6.28 2 - 0

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2) Line-intercept analysis: For the same two common species, estimate
density by sampling 10 transects.
Procedure - Use the scale along the bottom of the map as a
"baseline" from which your transect lines will extend perpendicularly. Select one random number (from the table) as the point from which your first transect line will extend.
Draw a transect line vertically across the map. Record the sizes (maximum plant width) of all individuals that are touched by the left edge of the line (note that the size key on the map is in area, not width so data must be converted).
Repeat the above procedure for nine more randomly selected transects. Record widths in the appropriate data table located at the end of this lab.
Density =[ ∑ (1/plant width) ] * (unit area / total transect length)
Where unit area is the size of the area on the basis of which density is to be expressed (in our case, per 100 m as measures of width and transect length.
2 ) and must be in the same units
Area (m)
0.01
0.03
Width (m)
0.1
0.2
Area (m)
1.77
3.14
Width (m)
1.5
2.0
0.12
0.28
0.5
0.78
0.4
0.6
0.8
1.0
4.91
7.07
9.62
12.59
2.5
3.0
3.5
4.0
(Dominance and frequency can also be calculated by different equations using this method)

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3) Point-quarter technique: For the same two common species, estimate density by sampling seven points (making 28 quarters).
Procedure - select two random numbers as coordinates along the left side and along the bottom of the map. Use these coordinates to determine the position of your first point. Lay your quarter transparency so that the intersection of the two perpendicular lines overlay the random point
(orient the lines parallel to the map edges).
Construct a “ruler” along the edge of a sheet of paper using the scale along one edge of the map (each tick mark is equivalent to one meter in the actual desert). Record the distant (in map units) from the center of the nearest individual (for each of the two species) to the intersection point
(leave cell blank if no individuals exist within that quarter from the point to the map edge). Do this in each of 4 quarters formed by the perpendicular lines.
Repeat the above procedure for six more randomly selected points. Record the distances in the appropriate data table located at the end of this lab.
Density = unit area
(mean point-to-individual distance)
2
Where unit area is the size of the area on the basis of which density is to be expressed (in our case, per 100 m 2 )
(Dominance and frequency can also be calculated by different equations using this method)

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1 092725 012157 827052 297980 625608 964134
2 104460 007903 484595 868313 274221 367181
3 676071 388003 266711 323324 044463 762803
4 881878 862385 203886 261061 096674 811548
5 534500 336348 086585 241740 581286 008435
6 094276 615776 242112 985859 075388 082003
***+------+------+------+------+------+------+
7 333848 513630 474798 841425 331001 542740
8 847886 629263 596457 589243 576797 800957
9 942495 695172 523982 264961 771016 118797
10 450553 679145 324036 715835 963418 533048
11 024670 615375 717260 171144 340939 208712
12 932959 205554 113225 704406 263818 633643
***+------+------+------+------+------+------+
13 039831 202271 212602 089507 469224 639594
14 988302 547676 746372 209684 217507 290574
15 175854 538509 553171 532143 360606 597321
16 495315 444204 428810 387020 053680 418895
17 753091 146845 416561 310332 393625 548843
18 953724 288220 149970 386262 386069 597807
***+------+------+------+------+------+------+
19 173274 702895 911389 217714 646684 378324
20 911011 662819 646737 239435 685692 543131
21 392306 802141 254886 782821 289220 130020
22 882896 081149 367042 007910 643061 512454
23 772248 022364 524302 141025 973919 650811
24 430182 217550 407747 587048 155578 688888
***+------+------+------+------+------+------+
25 279938 605231 938268 988723 539349 215567
26 740111 274197 062044 375939 197343 376793
27 157048 106084 723570 494784 370919 520523
28 412312 454163 328878 920713 285022 359910
29 465304 485797 554382 089535 312344 905379
30 391433 216755 698507 029714 692654 577724
***+------+------+------+------+------+------+
Lab adapted from Cox, GW. 1996. Laboratory Manual of General Ecology. Wm. C. Brown
Publishers.

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#3
#4
#5
#6
#7
#8
#9
#10
Group member names:
Raw data table for quadrat sampling technique.
Quadrat #
Species ____
Area covered
#1
Species ____
No. Area covered
#2
No.

#1
#2
#3
#4
#5
#6
#7
Raw data table for line-intercept technique.
Intercept #
Species ____
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
Species ____
Raw data table forpoint-quarter technique.
Species ______ Species ______
Point #
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Summary table 1: Comparison of quadrat sampling parameters
Species ____ Species ____
Density
Dominance
Frequency
Summary table 2: Comparison of sampling techniques estimating
density in individuals/square meter
Quadrat
Species ____ Species ____
Line- intercept
Point- quarter
Summary table 3: Comparison of sampling techniques estimating
density in individuals/square meter from all groups.
Species ____ Species ____
Group:
1
Quadrat Line- intercept
Point- quarter
Quadrat Line- intercept
Point- quarter
2
3
4
6 mean
Standard deviation coeff. of variation coefficient of variation = 100 * stand. dev. / mean

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Question set:
1.
Density, abundance and frequency are all measures of abundance. For the following questions an ecologist might interested in, would one type of measure be more appropriate than the others? Why or why not?
An ecologist who is interested in which of two desert plant species is using more resources over the study area?
An ecologist who is interested in which of two desert plant species is more likely to be consumed by an herbivore that tends to consume an entire patch of that species if the herbivore happens to encounter that patch.
2. How could one species have a higher density than a second species, but the second species have a higher calculated frequency than the first
(assume both density and frequency were measured accurately)?

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3. Density is measured as individuals per unit area. However, a line has no area, so how can the line-intercept method estimate density?
4. If the density of a species you plan to estimate is very low, but individuals of this species can be seen when far away, which of the three techniques would be best to use (i.e. most efficient given limited time)? Why? If the average size of individuals for a species is very small, which of the three techniques would be least efficient given limited time? Why?

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5. Based on the data from all groups (Table 3), which species tended to least variable among estimates of density? Why might this be?
6. Do you think that the same total area of desert floor was measured by all three techniques? How would this affect variation among replicate estimates (i.e. among the estimates made by each group)?

8 m X 8 m 8 m X 8 m
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8 m X 8 m 8 m X 8 m
8 m X 8 m 8 m X 8 m