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NATURAL HERITAGE

THE NATURAL COMMUNITIES OF VIRGINIA
CLASSIFICATION OF ECOLOGICAL COMMUNITY GROUPS


Second Approximation (Version 2.6)
Information current as of July, 2013



Statistics

The table below shows an example of compositional summary statistics for a small group of plots within a larger dataset. These statistics are calculated using custom macros written by DCR-DNH ecologist Philip Coulling and are routinely used to evaluate the adequacy of groups recognized in cluster analysis and ultimately to assist in naming and describing the community types. Initially, total mean cover and total frequency across all plots in a dataset are determined for every taxon. For each taxon in each group under consideration, the following summary statistics were then calculated:

  • Frequency - the number of samples in a group in which a species occurs.
  • Mean Cover - mean percent cover calculated from midpoint values of cover class ranges.
  • Relative Cover - the arithmetic difference between mean cover (for a given group of samples) and total mean cover (for the entire data set) (= Mean Cover - Total Mean Cover). Expressed by plus or minus symbols, this value provides a relative approximation of how much more, or less, abundant a particular species is in a community type compared to the overall data set.
  • Constancy - the proportion of samples in a group in which a species occurs, expressed as a percentage (= Frequency / Number of samples in group x 100). Because they are scaled to 100, constancy values can be compared across community types with unequal numbers of plots.
  • Fidelity - the degree to which a species is restricted to a group, expressed as the proportion of total frequency that frequency in a given group constitutes (= Frequency / Total Frequency x 100).
  • Indicator Value (IV) (= Constancy x Fidelity / 100). A synthetic value indicating species that are both frequent within and relatively restricted to a group of plots.
  • Indicator Value Adjusted by Cover, Scaled (Scaled Adj IV) (= Indicator Value x Mean Cover / 9). By dividing IV by 9, the maximum possible cover value, this statistic synthesizes information about frequency, diagnostic value, and mean abundance. A species entirely restricted to a particularly community type, occurring in every sample of that type, and attaining maximum mean cover will have a Scaled Adjusted IV of 100 for that type. Empirically, taxa with Scaled Adjusted IVs = 15 are almost always those most characteristic of a type, although the exact range of values in any given type or data set may vary considerably.
  • Indicator Value Adjusted by Cover, Unscaled (Unscaled Adj IV) (= Indicator Value x 2relative cover). An alternative, unscaled synthetic measure of adjusted IV, using relative cover as the modifier of IV. Since cover classes form a logarithmic, rather than linear scale of values, Unscaled Adjusted IV is a statistically more legitimate means of incorporating information on cover, and has the advantage of not favoring only dominant species and better identifying species that are considerably more abundant within a given type than in the data set as a whole. This statistic is sensitive, however, to vegetation types containing few samples and to species with low overall frequency
Additionally, the following statistics are generated for each group:
  • Mean Species Richness - the average number of species present per plot (S); only species rooted inside plot boundaries are included in this calculation.

  • Homoteneity - the mean constancy of the S most constant species, expressed as a fraction. This value represents the constancy of the average species in a community type; higher values for homoteneity indicate greater uniformity in species composition among plots. Although homoteneity is not independent of group size, often increasing as the number of group members decreases, it can be used to evaluate whether community types have been defined at an appropriate level.

Community Type Mean Species Richness = 14
Community Type Homoteneity = 0.738
 
Taxon
Mean Cover
Relative Cover
Constancy
IV
Scaled Adj. IV
Unscaled Adj. IV
Acer rubrum
2
----
67
2
0
0
Amelanchier arborea var. arborea
2
0
33
2
0
2
Baptisia tinctoria
2
+
33
33
7
67
Carex tonsa
2
+
33
5
1
10
Carya alba
2
--
33
1
0
0
Carya glabra
2
--
33
1
0
0
Chimaphila maculata
2
0
33
1
0
1
Cypripedium acaule
1
0
33
11
1
11
Danthonia spicata
4
++
33
3
1
11
Dichanthelium depauperatum
2
+
33
17
4
33
Dichanthelium dichotomum
2
0
33
1
0
1
Gaylussacia baccata
7
++
100
11
8
43
Kalmia latifolia
7
+
100
8
6
15
Nyssa sylvatica
1
-----
33
1
0
0
Pinus pungens
7
++++
100
100
78
1600
Pinus virginiana
4
+
67
7
3
15
Polypodium virginianum
2
0
33
8
2
8
Prunus serotina var. serotina
2
0
33
1
0
1
Quercus marilandica var. marilandica
1
-
33
4
0
2
Quercus montana
7
0
100
5
4
5
Quercus rubra var. rubra
4
-
100
6
3
3
Quercus velutina
2
--
67
3
1
1
Sassafras albidum
2
-
67
3
1
2
Sericocarpus asteroides
2
+
33
33
7
67
Smilax glauca
2
0
67
5
1
5
Solidago bicolor
2
+
33
8
2
17
Tephrosia virginiana
2
+
33
33
7
67
Vaccinium pallidum
6
+
100
6
4
12
Vitis aestivalis var. aestivalis
2
0
33
5
1
5