Element Occurrence Ranking

Ecology Contents

About CDC Ecology Ranking Methology Element Ranking (pdf 258 KB) Element Occurrence Ranks Element Occurrence Ranking Factors Element Occurrence Specifications Spatial Patterns

Introduction

Because of the relatively great complexity of ecological communities, due in part to the interaction of species and successional change, it is difficult to consider the influence of all rank factors concurrently. Thus, each factor is assigned a separate “A”, “B”, “C” or “D” rating, sequenced and weighted according to priority, and combined in an algorithm to calculate an EO rank value. This value is then translated according to an EO rank scale to determine the EO rank. The process for developing an EO rank for a community is described in detail below.

Specifications for determining “A”, “B”, “C”, and “D” ratings for condition, size, and landscape context factors should be entered into corresponding EO Rank Factor Specifications fields in the Element Occurrence Specifications. In addition, the prioritization sequence and weightings used in calculating an EO rank should be documented in the EO rank specifications. There are general guidelines for both the sequencing and weighting of rank factors to be used for calculating EO ranks. For most Elements, the general guidelines for the rank factor prioritization sequence are determined according to community pattern type (described below), while the general guidelines for weighting rank factors apply to any community, regardless of pattern type. In most cases, community EO rank specifications can simply incorporate these general guidelines. However, for Elements for which these general guidelines do not apply, the rationale for the specified alternative prioritization sequence and/or weightings should be included in the EO rank specifications.

While the procedure for ranking community EOs may seem complicated, the actual application of factor weightings and calculation of the final EO rank can be automated. EO ranks should be reviewed by an ecologist; in rare cases, adjustment of the rank may be necessary and the reasons for doing so documented.

Prioritizing EO Rank Factors

The first step in the process of determining a rank for a community EO is prioritizing the rank factors on the basis of the relative importance of each factor for that Element. The factor that is most important is considered the primary rank factor, the factor with less importance is the secondary rank factor, and the remaining factor, having the least importance, is the tertiary rank factor.

EO rank specifications developed for a particular Element should indicate the prioritization sequence of the rank factors. The pattern type of the community may generally serve as a guide to determining the prioritization sequence of rank factors that is appropriate for the Element. A prioritization sequence indicated in the rank specifications that differs from these general guidelines should include justification for the modified sequence. In ranking ecological communities for which Element-specific EO rank specifications have yet to be developed, the general guidelines may be used to prioritize the rank factors. The following table summarizes the general guidelines for priority sequencing on the basis of community pattern type (described in a-d below).

a) Matrix Community Pattern Type
Size and landscape context are generally identified as the primary and secondary factors for a matrix community type. A matrix community, by definition, occupies a very large area with high connectivity to other community types; thus, size and landscape context are typically more important than condition, which could be quite variable (and in some cases, difficult to measure).

b) Large Patch Community Pattern Type
Condition and size are generally identified as the primary and secondary factors for a large patch community type; however, this sequence is quite flexible. Because this community type conceptually occupies the “middle ground” between matrix and small patch types, some large patch communities may be more similar to matrix types, while others more closely resemble small patch types, or linear types. In such cases, the general guidelines for rank factor prioritization for the community type most similar could be utilized for the large patch type.

c) Small Patch Community Pattern Type
Condition and landscape context are generally identified as the primary and secondary factors for a small patch community type. Small patch types vary much less in size, often contain more specialized species, and, because of their small size, are sensitive to factors affecting landscape context.

d) Linear Community Pattern Type
Landscape context and condition are generally identified as the primary and secondary factors for a linear community type. Linear types, having a large amount of edge and typically dependent on currents or flow regimes, are extremely sensitive to factors affecting landscape context. In addition, linear types often support very specialized species.

Weighting EO Rank Factors

The second step in the process of determining a rank for a community EO is assigning weightings to each of the rank factors. Weightings are determined on the basis of the relative influence of the factors on the viability of the EO. Thus, the weighting of a particular rank factor is directly related to its position in the prioritization sequence. According to the general guidelines for weighting EO rank factors, the values are: primary factor weighting = 4, secondary factor weighting = 3, and tertiary factor weighting = 2. While these general weightings apply in most cases, for some Elements a different weighting scheme may be more suitable. For example, each of the rank factors may have a relatively equal effect on the viability of an occurrence of some large patch community types such that all three rank factors should be weighted equally (e.g., 1-1-1).

The general weightings for rank factors are provided in the EO rank specifications for most communities. However, in cases where an alternative weighting scheme is indicated in the specifications, justification for the modified weightings should be included.

Calculating an EO Rank Value

The third step in the process of determining a rank for a community EO utilizes the ratings for each of the sequenced rank factors along with the assigned weightings to calculate an EO rank value. In order to perform the calculation, numeric equivalents must first be assigned for “A”, “B”, “C”, and “D” rank factor ratings as follows:

A rating = 4
B rating = 3
C rating = 2
D rating = 1

The following simple formula may then be used for the calculation:

[(P * x) + (S * y) + (T * z)] ÷ (x + y + z) = EO Rank Value

where

P = numeric equivalent for primary rank factor rating
S = numeric equivalent for secondary rank factor rating
T = numeric equivalent for tertiary rank factor rating

and

x = weighting assigned to primary rank factor
y = weighting assigned to secondary rank factor
z = weighting assigned to tertiary rank factor

Determining a Final EO Rank

The final step in the process of determining a rank for a community EO translates the calculated EO rank value derived in the previous step into an “A”, “B”, “C”, or “D” rank according to the EO rank scale. The rank scale provides a range of numeric values associated with each EO rank, as shown in the table below.

EO Rank	Numeric Values
A	>3.25 and 4.00
B	>2.50 and 3.25
C	>1.75 and 2.50
D	>1.00 and 1.75

A simplified alternative to the process of performing a calculation to determine an EO rank is the use of a matrix. The matrix displays all possible combinations of the three ratings initially assigned to the primary, secondary, and tertiary rank factors for a particular occurrence, along with the resulting calculated EO rank value and the final EO rank. As stated above, however, the calculation of a community EO rank will be an automated process, so this matrix will likely be of limited use, if any, in the future. The table below illustrates the EO rank matrix based on the general guidelines for weighting rank factors (i.e., primary factor = 4, secondary factor = 3, tertiary factor = 2); note that the use of rank factor weightings that differ from the general guidelines would result in a different matrix.

Source: Draft Element Occurrence Data Standard