Ranking habitats is only necessary when several examples of the same type of habitat have been identified and there is a need to assign relative levels of conservation significance to them. Those receiving the highest ranking represent the best examples of the habitat in the planning area and probably address significant wildlife habitat at several levels. Often these habitats will have conservation significance at the larger regional scale.

In many cases, ranking will be unnecessary. For example, all poorly or under-represented habitats, habitats of provincially or regionally rare species of conservation concern, and habitats of obvious importance to many wildlife species might automatically be considered highly significant.

9.1 An evaluation of three ranking methods

Three commonly-used comparative evaluation methods that could be used to rank significant wildlife habitats are described and compared below, based on a review of multiple criteria evaluation systems by Smith and Theberge (1987).

  1. Minimum standards

The minimum standards evaluation method is useful when criteria are measured on different scales and when different criteria are not comparable, as is the case in evaluating numerous natural areas for several ecological criteria. This method ranks candidate sites based on whether they meet an acceptable minimum standard for at least a few criterion. Therefore, if the minimum standard for species diversity is 20 percent more recorded species than the average for all candidate sites, then all of the candidate sites in Table 9.1 might be considered “significant”, as indicated by the “✔”. The minimum standards evaluation method does not overlook sites that are outstanding in one criterion as compared to another evaluation method called additive weighting, where the summed score for candidate sites that have “average” scores for all criteria may be higher.

Table 9-1. Minimum standards evaluation method example.
Candidate site Diversity Rarity Productivity
Site A ✔ ✔ ✔
Site B ✔    
Site C ✔ ✔  

The minimum standards evaluation method is more ecologically and mathematically valid than the other two evaluation methods discussed in this section. For many reasons, the aggregation of criteria measurements into one index (as with the additive weighting and ranking evaluation methods) obscures the complexity of the evaluation process. Aggregating criteria ignores relationships among ecological criteria. Also, not all criteria are applicable to every kind of natural area. For example, rarity may be more applicable to smaller areas while diversity may be more applicable to larger areas.

Evaluators will also find that the minimum standards method is the simplest to explain to non-specialists in government, industry, and the public. Using the minimum standards evaluation method will enhance understanding of why a natural area has been determined significant. For example, a particular woodland may be significant because it meets the minimum size criterion and it is believed that larger woodlands support area-sensitive bird species, are less sensitive to invasion by exotic species, and more likely to have associated woodland ecosystem functions and processes intact.

  1. Additive weighting

Using the additive weighting evaluation method, candidate sites are scored for several criteria. The criteria may also be weighted in some manner to reflect their relative importance. Scores for each criterion are first multiplied by the weighting for that criterion and then summed for all the criteria to obtain an overall index for each candidate site. This index is used to determine the comparative value of two or more candidate sites.

Scores must therefore be numerical and comparable among criteria. Criteria must be measured using an interval or ratio scale and in comparable units so that a drop in one criterion can be offset by an increase in another. For example, the two hypothetical candidate sites in Table 9.2 have equivalent ecological value.

Table 9-2. Additive weighting evaluation method example.
Candidate site Rarity value Productivity value Recreation value Sum
Site A 0 3 0 3
Site B 0 3 0 3
Site C 1 1 1 3

Although this evaluation system is simple, it makes a number of false assumptions. For example, it assumes that criteria are independent of each other when in fact ecological data are highly correlated (e.g., size is positively correlated with diversity [Domon and Bergeron 1987]). The criteria must also be weighted using some reasonable basis for determining the relative importance of each criterion. Weights are often subjective and vary widely. The additive weighting evaluation method may result in the identification of average sites as significant while sites that are outstanding in one criterion are classified as not significant.

  1. Ranking

The ranking evaluation method is similar to additive weighting except that each candidate site is ranked for each criterion. For example, three candidate sites may be ranked 1, 2, 3 for rarity and 2, 1, 3 for diversity. The criteria are also ranked (e.g., rarity = 1 and diversity = 2, see Table 9.3). Each candidate site’s rank for each criterion is then multiplied by the criterion’s rank, all these values are summed for each site, and the sums are used to rank the sites.

Table 9-3. Ranking evaluation method example.
Candidate site Rarity × 1 Diversity × 2 Sum
Site A 1 × 1 = 1 2 × 2 = 4 5
Site B 2 × 1 = 2 1 × 2 = 2 4
Site C 3 × 1 = 3 3 × 2 = 6 9

The ranking evaluation method assumes that each candidate site can be ranked for each criterion (a difficulty if there are many candidate sites), the criteria can be ranked (based on some reasonable basis for relative importance), and the criteria are independent. Moreover, the sums or total scores obtained are the result of mathematically non- permissible numerical operations on ordinal numbers (i.e., the evaluator subjectively ranks each candidate site for each criterion, the criteria are also ranked, and then the two ranks are multiplied).

With this evaluation system, there may be considerable uncertainty in field measurements, variation among people in assigning scores and in the weights given to different criteria, as well as fuzziness in the definitions of the criteria. This is an important consideration for evaluators who want to have a high degree of confidence in the derived scores or ranks in order to defend them and base official plan designations on their accuracy. A Conservation Advisory Committee can help establish criteria and ranking.

9.2 Recommended method for ranking similar habitats

Planning authorities are advised to use the minimum standards evaluation method whenever possible because it provides one of the simplest and most ecologically sound approaches to ranking significant wildlife habitats. Numerous examples of evaluation criteria that can be used with it are listed in the tables in Appendix Q and in Table 8-1. However, only a few key criteria will need to be used to evaluate most candidate sites. These are listed for selected habitats, in Appendix Q. The majority of them are recommended for initial ranking of similar habitats because they can be deduced from available information (maps, aerial photographs, site reports, expert opinion). The effective use of other criteria frequently requires extensive knowledge of each site and/or field investigation.

9.2.1 Importance of representation of habitats

When designing a Natural Heritage System, the most important criterion is “current representation of habitat within the planning area.” If identification of wildlife habitats is conducted in a thorough manner, the application of this criterion to the evaluation of these sites will ensure that the full range of wildlife habitats existing within the planning area is included within the Natural Heritage System.

This criterion has other advantages. It applies to most habitats within the four significant wildlife habitat categories and it is easy to use. Usually, evaluators only need to know the number of examples of a specific habitat in order to determine its conservation significance (all under-represented habitats would be considered very important and worthy of some form of protection, regardless of their ranking according to other criteria). Furthermore, field investigations are less likely to be required when this criterion is used.

9.2.2 Establishing minimum standards for representation

It is suggested that whenever habitats appear to be under-represented according to the established minimum number of examples required for adequate representation of the habitat within the planning area, all existing examples should be ranked highly. For many of them, there will be no need to apply additional evaluation criteria.

To ensure adequate representation of habitats within the planning area, two or three examples of a specific habitat, depending on the habitat type, are suggested as minimum standards for the criterion of current representation. This does not mean that more of these habitats cannot be protected but, as a very minimum, the number identified as a standard should be protected. Generally, habitats for species of conservation concern, species sensitive to human activities and disturbances, and rare vegetation communities should automatically be considered highly significant if they are found at three or fewer locations. If there are more than three examples, then other criteria, in addition to ‘current representation’ should be used to rank them.

For most habitats of more common or less sensitive species (e.g., white-tailed deer) the value of two examples is presented as a reasonable minimum standard for current representation. It must be stressed that this is a minimum standard; i.e. if there are one or two deer yards, they would be significant based on representation; additional deer yards may be significant based on other criteria. Protection of only one habitat example may not provide enough long-term protection for many species. This is particularly true for species of small habitats, isolated habitats, and habitats located near or in developed or settled parts of the planning area. A possible exception to this approach concerns very extensive habitats. For example, a single, large site may be resilient enough to provide significant habitat for a variety of wildlife species for many years.

9.2.3 Minimum standards of other selected evaluation criteria

The choice of minimum standards for evaluation of some criteria is subjective and can be difficult. For example, how many species should be present on a site in order for it to be recognised as significant because of diversity? What should be the minimum size of a site, to be considered significant? How rare would a habitat or species have to be before it was considered significant? The answers to these questions and others will vary across the province depending on the quality and amount of habitat remaining in different planning areas, as well as the knowledge and aims of the evaluators.

The minimum standards in Appendix Q are presented as guidelines. Planning authorities electing to use values other than those provided are urged to develop minimum standards that do not unnecessarily preclude potentially significant sites from consideration. For example, if an objective of the Natural Heritage System were to protect habitat for area-sensitive birds, a minimum standard of 10 ha of forest interior would eliminate smaller sites from consideration, a potentially serious problem in many parts of southern Ontario where forest cover is limited and heavily fragmented. A minimum standard of 4 ha might result in several sites being considered significant or at least being further assessed using other criteria. The use of more generous or inclusive minimum standards represents a more cautious approach to Natural Heritage System planning and design. Since little is known about the specific habitat requirements of many species and because unforeseen future events can destroy or seriously degrade habitats, it seems reasonable to protect more wildlife habitat whenever possible.

Highest conservation significance might be assigned to habitats meeting the greatest number of minimum standards for the evaluation criteria, although any habitat meeting the minimum standards for only one criterion should be considered sufficiently significant to merit some form of protection. No candidate habitats should be considered significant unless they meet a certain minimum standard.

9.2.4 Avoiding numerical values for some minimum standards

There are several reasons why, for some criteria, the use of absolute numerical values for minimum standards has been avoided (e.g., size or numbers of animals occupying a habitat). It is very difficult to develop specific and yet comprehensive minimum standards that can be applied to different landscapes across the province with varying amounts and quality of habitat. What may be considered significant in one area may not be in another. It is hard to assign minimum standards to certain criteria (e.g., level of disturbance, degree of threat, location of habitat). Often these minimum standards are either unknown or poorly understood. For example, spatial area is considered an important criterion when assessing the conservation value of forest stands to area-sensitive bird species. Although most biologists believe that larger, contiguous forests have greater value to these species than smaller patches, they are still learning about the minimum areas required to support local populations and to maintain long-term population viability. A suggested minimum area of 50 ha (a commonly-cited value) could be criticised by some people as being too large or too small. More important, if a forest stand has to be at least 50 ha to be considered significant as habitat for area-sensitive bird species, planning areas with only smaller stands remaining could decide that there is no significant forest habitat for these species in their jurisdiction. However, these smaller patches of forest may have value to some of these birds, as well as local wildlife.

For these reasons, suggested minimum standards for some criteria (e.g., size, diversity) are based on comparisons made between similar habitats. If five sites are to be ranked for diversity, rough estimates of plant and animal diversity for each site can be calculated based on reports about the sites and/or informed opinions from knowledgeable people. A mean diversity value for all five sites can also be easily determined. As a minimum standard for diversity, the diversity of a single site would have to exceed the mean diversity value for all five sites by at least 20 percent.

9.2.5 Explanation of the tables in Appendix Q

The tables in Appendix Q list important evaluation criteria for seasonal concentration habitats, rare vegetation communities or specialised wildlife habitats, and habitats of species of conservation concern that have been discussed in this guide. By definition, many species of conservation concern are rare, declining, or have a large proportion of their global population in Ontario and urgently need some protection. The criteria in Table Q-3 were selected because they are closely tied to the definition of these species and minimum standards for them are more easily derived than for some criteria (e.g., abundance, location of habitat, degree of threat or decline of habitat, ability of habitat to meet species’ requirements).

As mentioned earlier, other criteria can be used and the suggested minimum standards are only guidelines. In addition, not all criteria listed in these tables for each habitat need to be used, especially when there are only a few habitats to be ranked. Having several criteria to choose from for each habitat can prove helpful where information for some criteria is unavailable, out-dated, or incomplete. For example, if planning authority staff do not have accurate information about the size of a deer yard or the number of deer it supports, they might rely more on some of the other criteria (current representation, provision of other significant wildlife habitats, provision of suitable habitat or habitat diversity). These criteria are more easily determined from readily available information such as maps, aerial photographs, and from local experts.

Finally, whenever the minimum standard for current representation is met, planning authorities are advised to use at least three evaluation criteria. The highest ranked habitats would meet the minimum standards of the largest number of criteria. Ideally, significant habitats should meet the minimum standards of at least two criteria to reduce the potential for conflict should some people disagree with one of the criteria. There does not need to be a minimum number of habitats that are protected. The number protected should be determined by the number that meet the minimum criteria.