Separation Distance

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).

A separation distance is the amount of intervening area that determines whether Source Features of an Element should be grouped as part of the same (complex) Element Occurrence (EO), or should be considered as discrete Element Occurrences.

Separation distances will be provided in the EO specifications for the Element. For species, distances are provided for intervening areas of unsuitable habitat, and for suitable habitat that is not known to be occupied. For communities, distances are provided for intervening areas of different natural/semi-natural communities, and cultural vegetation.

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

In the absence of EO specifications providing separation distances, minimum values have been recommended.

Species: Separation by Unsuitable and Suitable Habitats

When applicable, two separation distances should be specified for species Elements: one across unsuitable habitat, and another across apparently suitable habitat that is not known to be occupied (regardless of whether surveyed). The use of these distances in defining EOs is designed to reflect hypothesized differences in gene flow across suitable vs. unsuitable habitats. However, for some species Elements, there will likely be no significant differences in gene flow across the different habitats. In these cases, only one separation distance need be specified. To promote consistency in the application of separation distances, they should be measured along the shortest route of expected travel of the Element between the edges of the known or minimally estimated occupied habitat, although this may not be a straight line (see Section 7, EO Spatial Representation).

For all species Elements, the distance of unsuitable habitat needed to separate EOs is always less than or equal to the distance of apparently suitable but unoccupied habitat needed to separate EOs. Because the unsuitable habitat cannot support the Element, a specified distance of this habitat can be more prohibitive to dispersal and residence by the Element than the same distance of apparently suitable habitat. Thus, separation by unsuitable habitat is presumed to be more definitive. Further survey work is unlikely to result in the discovery that the separation was inaccurate. It is also unlikely that unsuitable habitat will become occupied over time, and therefore, the separation between two EOs will presumably remain.

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Communities: Separation by Different Community Types

For community Elements, habitat suitability or unsuitability is not applicable. Instead, 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.

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Separation Across Mixed Areas

Frequently, the area located between populations or patches may consist of a mixture of apparently suitable and unsuitable habitat, or a mixture of other natural or semi natural community types and/or cultural vegetation. When applying EO specifications, if no mixed habitat guidance is provided, the separation distances to be applied should bec onceptually based on the relative amounts of apparently suitable and unsuitable habitat.

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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 species Elements, minimum separation distances are generally 1 km or greater for both unsuitable habitat, and for apparently suitable habitat that is not known to be occupied. For 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. Table 4.1 summarizes the recommended minimum separation distances for species and community EOs.


Type of Separation Species EOs Community EOs
barrier qualitatively defined qualitatively defined
unsuitable habitat ≥ 1 km N/A
apparently suitable habitat not known to be occupied ≥ 1 km N/A
cultural vegetation N/A ≥ 0.5 km
different natural or semi-natural communities N/A ≥ 1 km


Although some Elements may occur as truly separate populations at scales of separation less than 1 km, the practical value (for conservation planning and action) of delineating finer-scale EOs is often questionable. Nevertheless, a few Elements may require separation distances that are less than the established minimum; in such cases, these distances should be justified in the EO specifications.

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Factors Determining Separation Distances

Several factors that may be considered when determining separation distances to be written in the EO specifications for a given Element:

Dispersal Distance

For species Elements, dispersal distance is the distance that individuals or propagules (e.g., pollen, seeds, spores, larvae) travel from an existing location to a new location. Success of dispersal depends on whether suitable habitat for establishment is reached within that distance. Typical dispersal distance for an Element is rarely known and may be extremely variable. However, since dispersal allows genetic connectivity between otherwise apparently distinct populations, separation distances between EOs should be greater than the distance of routine dispersal events.

For many Elements, a small percentage of individuals or propagules may disperse great distances. While potentially significant for establishing new populations and for reducing genetic differentiation of populations, these rare, long-distance dispersal events should not be factored into separation distances. For migratory species, dispersal distance is not a useful concept for determining separation between populations since these Elements may typically disperse over enormous distances. Considering dispersal distances in determining separation distances for such Elements may lead to impracticably large EOs.

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Home Range

In the absence of information about dispersal distance for animals, home range size may be a useful surrogate for that knowledge based on a presumed relationship between the two. For some animals, home range is the average area occupied, utilized, and/or defended by an individual, either during its lifetime or for a given breeding season. The true extent of home range is often not well known, and may vary from year to year, and between different habitats. Generally, separation distances should be at least three times the average home range for the Element (i.e., based on the length of the largest axis). In cases where the area of a home range is not known but information is available on movement (excluding dispersal and migration), use three times the distance of that movement. This distance would ensure that EOs that are, in fact, distinct remain separate despite fluctuating home range boundaries through defining adequate space between them to allow for such fluctuations.

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Spatial Patterns of Occurrence

The relative degree of spatial patchiness of an EO is an important factor when determining separation distances for EO specifications. Spatial patterns can be measured by the size of the EO, separation between EOs, and/or the surrounding context of the EO (e.g., the degree of unsuitability of the surrounding landscape).

For matrix communities, it may be difficult to develop separation distance guidelines due to their extensive and complex spatial patterns. Large readily recognizable stands that qualify as distinct EOs according to the separation distance guidelines may, nonetheless, be connected by smaller less apparent stands located within the prescribed separation distance. When such cases are found in natural or semi natural landscapes, the smaller and larger stands may be grouped into one principal EO, with sub EOs used to define the individual stands. However, in more altered landscapes, the intervening small stands are less likely to create a meaningful connection between the large stands; thus, large stands would be maintained as separate principal EOs.

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Temporal Patterns of Occurrence

Changes in spatial patterns over time, including many successional phenomena, may also be considered when writing EO specifications. In general, separation distance guidelines will depend on the rate of change.

If spatial changes occur relatively frequently (e.g., within a practical time frame of 25 years), then separation distance guidelines should be adjusted to incorporate the relatively dynamic temporal/spatial nature of an occurrence. In other words, because a principal EO with dynamic characteristics represents all potential varying locations of that population or community over a given time period, it encompasses an area larger than what is actually occupied at the time of survey. Thus, greater separation distances should be specified to ensure that a shifting population or patch is not recorded as multiple separate occurrences over time.

On the other hand, if spatial changes occur relatively infrequently (e.g., the population or community remains at a particular location for longer than 25 years), then for all practical purposes, separation distance guidelines should reflect the relatively stable nature of the occurrence. In other words, temporal factors should be considered largely irrelevant, and separation distance guidelines should be based on current factors only.

Temporal patterns of occurrence may be an important consideration for many species (e.g., birds that are dependent on grassland communities; plants characterized by seed banking that may only be apparent for discontinuous periods of time). Temporal patterns of occurrence may also be an important consideration for very dynamic communities (e.g., meadow and marsh communities that move up and down streams in relation to beaver dams). In each of these cases, occurrences may not appear to persist locally if considered at one time only, but do persist in the larger landscape over a longer time frame.

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Comparability with Similar Functional Groups

Similarity in components of species biology or community processes (e.g., a - d above) between Elements may be an important consideration in developing EO specifications. This functional similarity is often found in groups that are related through taxonomy, shared ecological factors, or some combination of the two (e.g., “alliance” for communities, “genus” for species, ecological groups within an alliance). However, groups may be functionally related without having any taxonomic relation (e.g., conifer and mixed matrix communities occurring in the same pattern in a boreal ecoregion, riffle dwelling mussel species occurring in similar patterns of abundance). Functionally similar Elements should have comparable separation distances; it would normally not make sense to specify separation distances for functionally similar Elements that differ by an order of magnitude.

These factors to be considered in determining separation distances may be dependent on other components (e.g., landscape may affect dispersal distance, population density may influence home range size, and sex may determine average movement distance). Although multiple factors may influence the decision on separation distances specified, the most significant factor(s) should be provided as justification in the EO specifications.

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Separation Distance Analysis & Application


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See also:

Separation Barriers