Element Occurrence Specifications

Ecology Contents

About CDC Ecology Ranking Methology Element Occurrence Ranking Element Occurrence Ranks Element Occurrence Ranking Factors Element Occurrence Specifications Spatial Patterns

Introduction

EO specifications are used to delineate and differentiate EOs. In other words, EO specifications define precisely what evidence constitutes a valid EO (i.e., the minimum size, quality, or persistence required), and what distances or factors separate one principal EO from another. This will affect the number of EOs tracked. Low thresholds for minimum size or quality in the EO specifications (i.e., lax criteria) may result in a proliferation of EOs having little practical conservation value, and high development and maintenance costs for biologists and data managers. Conversely, high thresholds (i.e., stringent criteria) may result in a failure to designate EOs for significant occurrences of an Element.

Although the number of principal EOs is often used as one of many factors in determining Element conservation priorities (i.e., Element ranks), this number should be used judiciously. For some Elements, the number of EOs may be, in part, a result of fragmentation of historically more extensive occurrences. Whether a given Element in such a fragmented landscape is represented as a single large principal EO having multiple sub-EOs or multiple small principal EOs is of little importance in ranking the Element; both means of recordation should reflect the reduced viability of the Element at that location. In such situations, consistent delineation of the EOs is important, and should be based on separation distances that are useful for delineating viable units that are practical for conservation action.

For communities, EO specifications information may be organized according to the spatial patterns and ecological dynamics typical of groups of Elements. These groups can be described as matrix, For communities, EO specifications information may be organized according to the spatial patterns and ecological dynamics typical of groups of Elements. These groups can be described as matrix, large patch, small patch, and linear. Ecological factors and ranking considerations may be similar for Elements within a group that share the same spatial patterns and dynamics; thus, EO specifications for Elements within a particular group may also be similar.

Minimum Criteria for EOs

For communities, minimum criteria for EOs are implicit in the classification of the Element. A brief description of the Element (e.g., composition, structure) that includes information on characteristics that distinguish it from similar communities should be provided in a global Element summary field. Any area that is large enough to be classified as a particular community Element has, in essence, met the minimum criteria for an occurrence of that community type. Practically, however, minimum sizes may be helpful and should be provided in the EO specifications.

Recommended minimum sizes for the different community pattern types are: 2 hectares for matrix; 0.4 hectare for large patch; 0.05 hectare for small patch; and 30 meters in length for linear. Stands/areas below the recommended minimum size become difficult to judge in terms of community type characteristics, and, if isolated, become heavily influenced by edge effects. For conservation purposes, generally only larger sized occurrences of each community type are tracked and the threshold for minimum size is seldom approached.

Separating EOs

Principal EOs are typically separated from other principal EOs, either by barriers or breaks, or by specified distances across intervening areas. For ecological communities, separation distances will be measured across intervening areas of different natural or semi-natural communities, or cultural vegetation.

Barriers

In addition to minimum criteria for identifying an EO, known barriers for Elements, either naturally occurring or manmade, should also be described in the EO specifications. For community EOs, barriers may be obstacles that limit the expansion or alter the function of communities. In effect, these barriers separate populations of most of the component species within the community, thus obstructing or severely limiting gene flow. Barriers may be common for many aquatic and wetland communities, but are typically less common for many upland terrestrial communities.

Separation Distances

In addition to barriers that totally, or almost completely, prevent movement and/or dispersal, distances of intervening area that restrict movement may also separate EOs. These distances are used to delineate the population units between which gene flow is significantly reduced. For comparison, IUCN (1996) characterizes reduced gene flow between units as “typically one successful migrant individual or gamete per year or less”.

For most species, data from gene flow studies does not exist; thus, decisions on separation distances should be made on the basis of best information available. Also, consideration of gene flow is not applicable to Elements that disperse widely (e.g., birds, wind-dispersed plants or insects), Elements having very long generation times (e.g., giant tortoises, plants characterized by long-term seed banking or dormancy, persisting clones), or Elements that are dependent on rare but recurrent phenomena for dispersal (e.g., floods, major
storms).

The intent of assigning values for separation distances is to achieve consistency in the manner in which EOs are defined and mapped. The degree of restriction to movement and/or to dispersal of the Element resulting from the intervening area determines the distance(s) required to separate one EO from another. Thus, areas that are highly restrictive to the Element’s movement or dispersal require smaller distances for separating EOs than areas less prohibitive to movement or dispersal. Several factors may be used to set separation distance(s) for EOs:

• dispersal distance
• home range
• spatial patterns of occurrence
• temporal patterns of occurrence
• scomparability with similar functional group

The factors used to determine separation distances for EOs should be cited as justification in the EO specifications.

Separation by Different Community Types

Ecological community EOs may be separated by expanses of different natural or semi-natural community types, or cultural vegetation. Intervening natural and semi-natural areas will likely inhibit the expansion or function of community EOs to a lesser degree than intervening cultural vegetation. In a like manner, intervening natural and semi-natural areas with similar kinds of habitat characteristics will inhibit expansion or function of a community less than those with very different kinds of characteristics. For example, bogs separated by intervening areas of upland jack pine on bedrock are more definitively identified as distinct EOs than bogs separated by areas of black spruce swamp.

Recommended Minimum Separation Distances

Minimum values for separation distances have been recommended to ensure that EOs are not separated by unreasonably small distances, which would lead to the identification of unnecessarily fragmented populations as potential targets for conservation planning or action. For ecological communities, the minimum separation distance delineated for intervening areas of different natural or semi-natural communities is 1 km or greater, and a distance of at least 0.5 km for interjacent areas of cultural vegetation.

Element Occurrence Rank Specifications

In order to ensure that EOs having high conservation value are ranked as priorities, EO rank specifications must provide stringent guidelines for assigning consistent and appropriate “A” through “D” ranks to EOs. Specifications for each of the EO ranks should provide, as appropriate for the Element, criteria related to one or more of the three identified ranking factors: size, condition, and landscape context. Criteria for “D”-ranked EOs must be consistent with the minimum criteria in the EO specifications for that Element. Time frames for the persistence of viable EOs should be considered when writing EO rank specifications.

Criteria in EO rank specifications should be based on the scientific information available, including historical precedents, knowledge of current occurrences, results of experimental restoration projects, and current constraints on restoration. Justification for the criteria used in setting the “A” rank threshold should be documented. In addition, when available, justification for the “C”/”D” threshold (i.e., the distinction between EOs with “fair” viability and “poor” viability) should also be provided. Justification for the “D” rank threshold (differentiating EOs from non-EOs) should be recorded in the EO specifications for that Element.

For Elements whose biology and ecology are poorly understood, EO rank specifications may be developed with consideration of taxonomically and functionally similar Elements. When available, well-developed rank specifications for similar, better-known Elements may serve as models for such poorly known Elements.

Because Elements within a particular functional group may have similar components of species biology or community processes, EO rank specifications for the Elements within that group may differ only minimally. In these cases, it may be practical to initially conceptualize a set of criteria that would be broadly applicable to the entire functional group, and then modify them as appropriate for each individual Element in the group. This approach may be particularly useful for groups of community associations (e.g., an alliance), and to a lesser extent for species (e.g., cave amphipods).

Element Occurrence Specifications Report     

The following format is used for Element Occurrence Specifications for Communities:

Sample Element Occurrence Specification Reports (PDF 59 KB)

Adobe Acrobat Reader is required to view the sample reports.  

Source: Draft Element Occurrence Data Standard