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Methodology – Guide to the Use of Field Survey Summary Tables
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Relative mean field density (all fields)
for a species (RD) is the mean field density value for a species divided by
the sum of mean field density values for all species, expressed as a percentage.
Relative abundance
for a species = RF + RU + RD. The total of the relative abundance values for all species
equals 300. This measure was used to rank the species in the field survey summary tables. This calculation
assumed that the frequency, field uniformity and field density measures were equally important in estimating the
abundance of a species. Relative abundance has no units attached to it, but the value for one species can be
compared with the value of another species. For example, if green foxtail has a value of 36 and wild buckwheat 18,
then green foxtail is twice as abundant as wild buckwheat. Relative abundance does not necessarily relate to the
competitive ability of the species. A species may have a high relative abundance value but not be very competitive.
The geographic distribution of each species is presented in maps produced with ArcMap GIS 9.3 from
Environmental Systems Research Institute, Inc.. Inverse Distance Weighting was used to estimate frequency of the
species by interpolating presence/absence data. The interpolation included all sites within used a fixed radius of
0.05° (approximately 5.5 km) extended if necessary to include a minimum of ten sites. The sites were given a
weight of inverse of the square root of the distance to the interpolated point. The data presented is smoothed using
nearest neighbour focal statistics, based on the means within a four cell radius.
Guide to the Use of the Field Survey Summary Tables
For the purposes of illustration, the meaning of the variables is explained for the species dandelion in the provincial
summary table (Table 6). A
frequency
of 36.8% shows that dandelion occurred at least once in 210 of the 571
fields surveyed. This variable estimates the geographic extent of the weed.
Field uniformity shows the proportion of quadrats (20 per field) in which the species occurred. In the example, the
value for
all field uniformity
means that dandelion occurred in 14.9% of the quadrats surveyed. This variable can
be used as an estimate of the area occupied by a weed. The
occurrence field uniformity
value means that
dandelion was present in 40.6% of the quadrats when considering only the 210 occurrence fields.
Three density variables are included in the tables. Dandelion had an
occurrence field density
of 8.2 plants per
square metre and an
all field density
of 3.0 plants per metre square. The density for fields in which the species
occurred is always equal to or higher than the density for all the fields in the summary. The
maximum density
shows that at least one field had a density of 98.6 dandelion plants per square metre.
The final column shows the abundance of each species surveyed relative to each other. Values in this column add
up to 300.
Relative abundance
is derived from the values of the frequency, field uniformity (all fields) and density
(all fields) variables. The relative abundance variable is used for ranking species such as dandelion and wild
buckwheat. Because all field uniformity and all field density values of dandelion are larger than that for wild
buckwheat, dandelion is ranked higher, even though wild buckwheat has a slightly higher frequency. The relative
abundance value of 53.3 for dandelion is higher than the value of 30.1 for wild buckwheat
.