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Montana Field Guide

Ecological Systems in Montana

Conservation Status Ranking Factors


Management of Montana’s biological diversity rests on our ability to understand how the individual components of that diversity –species, natural communities, ecosystems and landscapes— are distributed across the state, and to make meaningful assessments of their condition.  Over the years,the Montana Natural Heritage Program, in partnership with the other Heritage Programs forming part of the Natureserve network, has refined methodologies for ranking the conservation status of individual species.  But species, of course, exist as part of biological communities, and the integrity of those communities is often the factor determining species distribution and condition.  Consequently, land managers need to be able to link species to mid-scale ecological units that can be easily identified, evaluated, and managed.  In response to this need, the Natureserve network has put forth the concept of ecological systems, which represent "recurring groups of biological communities that are found in similar physical environments and are influenced by similar dynamic ecological processes."  Ecological systems offer a classification unit that is easily mappable and identifiable in the field, and that can be crosswalked to other classification systems in use by land management agencies.

Montana’s new Land Cover GIS theme, part of the Montana Spatial Data Infrastructure, uses ecological systems as its core map unit.  Descriptions of these ecological systems, available from the MTNHP website, describe the distribution, extent, dynamics, biological components, and management and restoration concerns particular to each system.  Now we are taking the next step, proposing conservation status ranking factors that will allow managers to identify ecological systems of particular concern within their jurisdiction.  As part of this process, we have assigned provisional conservation status ranks to all the mapped ecological systems in Montana.  This document outlines those factors, and lists the proposed status ranks for Montana’s ecological systems.  Our intention is to make this document available for review until January of 2011, at which time we will evaluate the assigned ranks for individual ecological systems based on feedback from partners and users.

It should be noted at the outset that these conservation status factors, originally developed for species, have not been applied at the ecological system level in the past, but only at the species and ecological community level.  Ecological communities are assemblages of co-occurring species and growth forms with the potential to interact with each other in defined habitats at certain times (McPeek and Miller 1996).  Although they do not lend themselves to large-scale mapping, their boundaries are easily defined, and the threats that they face are easily quantified.  By contrast, the composition of one occurrence of an ecological system may be quite different than the composition of another.  For example, there are over thirty ecological communities associated with the Rocky Mountain Alpine-Montane Wet Meadow ecological system, some of which are at risk within the state.  As a general rule, the conservation status factors proposed in this document consider the ecological system in general rather than specific terms. We are assessing most of the wet meadow occurrences in the state rather than focusing on the ones that might harbor at-risk communities (although the prevalence of these communities in a given system is considered as part of the Intrinsic Vulnerability factor).  Therefore, ecological system ranks should be considered as only provisional guidance for management.  Although it is likely that at-risk ecological systems will contain similarly vulnerable communities and species, it does not follow that systems that are secure statewide will always include secure communities and species in any given occurrence.  Therefore, the status of ecological systems should carry less weight –and should not substitute for a consideration of- the status of communities and species within a management unit.

We welcome public comment on the rankings.  Please send email to us at  Broad participation in this process will help ensure that the conservation status rankings are broadly accepted and that appropriate management strategies can be crafted in response.

Conservation Status Factors

Heritage Network member programs have adopted a suite of factors to use in assessing the conservation status of plants, animals, and ecological communities.  These factors can be applied to evaluate conservation status of a species or community at the global (i.e. entire range), national, or subnational (i.e. state or province) scale, or even within the geographic boundaries of a management unit (e.g. a National Forest, a BLM Field Office, etc).  In proposing conservation status ranking factors for ecological systems in Montana, we have incorporated that methodology as much as possible.  For each factor listed below, we give a definition, methods for calculation, and thresholds for ranking.

1. Range Extent

Describes the estimated current range of the ecological system in Montana.  This is based on the sum of area calculations of all Omernik Level IV ecological systems in which the system is believed to occur.

Z Zero (no occurrences believed extant)
A <100 km2
B 100-250 km2
C 250-1,000 km2
D 1,000-5,000 km2
E 5,000-20,000 km2
F 20,000-200,000 km2
G 200,000-2,500,000 km2
H > 2,500,000 km2
U Unknown
Null Rank factor not assessed

2. Area of Occupancy

A measurement of the current land area covered by the ecological system, regardless of its integrity or viability (i.e. a 50 km2 forest will occupy the same area if half the trees have severe insect damage as it would have prior to the outbreak).

Z Zero (no occurrences believed extant)
A <1 km2
B 1-4 km2
C 4-10 km2
D 10-20 km2
E 20-100 km2
F 100-500 km2
G 500-2,000 km2
H 2,000-20,000 km2
I >20,000 km2
U Unknown
Null Rank factor not assessed

3. Number of Occurrences

This is an estimate or inference of the number of occurrences of the system within the area of interest.  Although the viability of known occurrences is not included in this estimate, very small instances of systems that by their nature should be large (i.e., non-viable occurrences) are not counted.  For example, if there is a single pixel of Aspen-Mixed Conifer Forest on a land cover map in the midst of a Montane Douglas Fir Forest, this would not count as an occurrence.

Z 0 (zero)
A 1-5
B 6-20
C 21-80
D 81-300
E >300
U Unknown
Null Rank factor not assessed

4. Percent of area with excellent or good integrity

Represents the percent of the area occupied by the system that is considered to have excellent or good integrity, defined as a 95% probability of persistence over the next 20-100 years (accounting for successional factors), with only minor to moderate alterations in composition, structure and/or ecological processes.

When individual occurrences have been ranked (e.g. wetlands), ranks of "A" or "B" indicate excellent to good viability.  These ranks provide an assessment of estimated viability, or probability of persistence (based on condition, size, and landscape context) of occurrences of a given system.

A No system occurrences believed to have excellent or good integrity
B Very small percentage (<5%) with excellent or good integrity
C Small percentage (5-10%) with excellent or good integrity
D Moderate percentage (11-20%) with excellent or good integrity
E >Good percentage (21-40%) with excellent or good integrity
F >Excellent (>40%) with excellent or good integrity
U Unknown what number of occurrences with excellent or good integrity
Null Rank factor not assessed

5. Long-term trend

This metric reflects the degree of change, whether known, estimated, inferred or suspected in the extent, area of occupancy, number of occurrences, and or condition of the ecological system over the last 200 years in Montana.  Change can be human in origin (agricultural conversion, wetland draining) or natural (wildfires, drought).

A Decline of more than 90%, with less than 10% of the ecological system’s original size, range extent, area occupied, or high-integrity occurrences remaining).
B Decline of 80-90%
C Decline of 70-80%
D Decline of 50-70%
E >Decline of 30-50%
F >Decline of 10-30%
G >Relatively stable (<10% changed)
H >Increase of 10- 25%
I >Increase of >25%
U Unknown.  Insufficient information to identify long-term trends in size, range, area occupied, or number of high-integrity occurrences)
Null Rank factor not assessed

6. Short-term trend

This metric reflects the degree of change, whether known, estimated, inferred or projected in the extent, area of occupancy, number of occurrences, and/or condition of the ecosystem’s occurrence over the last 10-100 years in Montana.  Change can be human in origin (agricultural conversion, wetland draining) or natural (wildfires, drought).  The short-term trend may be recent, current, or may be projected into the near future.  It need not be known with certainty, and it may be smooth, irregular, or sporadic.  (For example, forest fire frequency and intensity may increase, decrease or stay the same over the next 10 years.  However, the degree of change brought about by forest fire can be projected based on recent fire history).  In assigning a score, do not consider increases in number of occurrences resulting from fragmentation of once-large occurrences into multiple small ones.  Instead, consider this to be evidence of a decreasing area of occupancy.

A Decline of >90%
B Decline of 80-90%
C Decline of 70-80%
D Decline of 50-70%
E >Decline of 30-50%
F >Decline of 10-30%
G >Relatively Stable (unchanged or remaining within a 10% upward or downward fluctuation)
H >Increase of 10 to 25% in occurrences, extent, condition etc.
I >Increase of >25%
U Unknown.  Insufficient information to identify long-term trends in size, range, area occupied, or number of high-integrity occurrences)
Null Rank factor not assessed

7. Threats (Scope and Severity)

Assesses the impact of extrinsic threats, whether anthropogenic, natural, or mixed, using the classification of threats proposed by Salasky et al (2008).  The impacts may be direct (e.g. conversion of native prairie to agriculture) or indirect (spread of introduced invasive species).  Effects of natural threats like fire or insect damage may be particularly important when the ecological system is concentrated in a particular area or when it has only a few occurrences.  However, features of the ecological system that make it especially susceptible to threats (e.g. a narrow environmental tolerance) should be considered under the “Intrinsic vulnerability” rank factor.  Threats should be considered as present and future.  Past (and realized) threats should be assessed under rank factor 4.  For example, population growth in the Flathead, Bitterroot and Gallatin Valleys would be evaluated when looking at the short-term trends affecting riparian woodlands and shrublands.  By contrast, the impact of climate change on alpine ecosystems would be considered a threat.

Threats are assessed in two separate categories:

  • Scope (how much of the area of occupancy may be affected?)
  • Severity (how badly and how irreversibly will the ecological system be affected?)

When an ecological system faces more than one threat, all threats are ranked using the criteria below.  The threats with the greatest severity and largest scope are used to calculate an impact rating.


Pervasive > 71-100% of the total area of occupancy of the ecological system may be affected
Moderate 31-70% % of total area of occupancy of the ecological system may be affected
Restricted 11-30%% of total area of occupancy of the ecological system may be affected
Small < 1-10% of the total area of occupancy of the ecological system may be affected


Pervasive Threat is likely to destroy or eliminate the ecological system, or reduce its extent by 71-100%
Large Threat is likely to seriously degrade the ecological system, or reduce its extent by 31-70%
Restricted Threat is likely to moderately degrade the system, or reduce its extent by 11-30%
Slight The threat is likely to only slightly degrade the system, or reduce its extent by 1-10%

The overall IMPACT of the theat is calculated using this table: More guidance can be found in Master et al. (2009)

SEVERITY   Pervasive Large Restricted Small
Extreme Very High High Medium Low
Serious High High Medium Low
Moderate Medium Medium Low Low
Slight Low Low Low Low

8. Intrinsic Vulnerability (optional)

An assessment of the intrinsic characteristics of the ecological system that might make it vulnerable (or resistant to) natural or anthropogenic stressors or large-scale catastrophes.  For example, a system will be intrinsically vulnerable if its dominant species or vegetation structure take a long time to recover following disturbance, or if it depends on some stochastic factor like flash flooding events for regeneration.  Intrinsic vulnerability is similar to, but can be distinguished from, the environmental specificity found in the next section.  In some cases, a system may exhibit intrinsic vulnerability because of that specificity.  For example, fens typically require a stable ground water source and prefer calcareous substrates (specificity).  If climate change is likely to alter ground water levels across the extent where fens occur, they could be said to have intrinsic vulnerability as well.

In assessing this factor, focus on the ecological system as a whole, rather than on disjunct or peripheral occurrences that might be particularly vulnerable because of their isolation.  In all cases, note that intrinsic vulnerability factors exist independently of human disturbances, although they may render the system more susceptible to the impacts of those disturbances.

It is appropriate to consider the state and global ranking (if assigned) of ecological communities associated with this system when ranking this factor.  For example, if the majority of the communities have an S4 or S5 rank, it can be inferred that the system as a whole is not highly vulnerable.

A Highly Vulnerable. System occurrences are highly susceptible to changes in composition and structure that would require more than 100 years to reverse, or that could not be reversed through natural processes.
B Moderately Vulnerable. System occurrences may be susceptible to changes in composition and structure but tend to recover through natural processes within 10 to 100 years.
C Not Intrinsically Vulnerable. System occurrences are resilient or resistant to irreversible changes in composition and structure and quickly recover (within 10 years).
U Unknown
Null Rank factor not assessed


These conservation status factors follow the conservation status assessment factors developed by Master et al. (2002).  Those factors have recently been revised (Master et al. 2009) and those revisions are reflected here.  Individual calculations for ecological systems in the Online Field Guide were done using the Natureserve Conservation Status Assessments Rank Calculator Version 2.0, available at:

Literature Cited

  • Master, L. L., L. E. Morse, A. S. Weakley, G. A. Hammerson, and D. Faber-Langendoen. 2002. Heritage Conservation Status Assessment Factors. NatureServe, Arlington, Virginia, U.S.A.
  • Master, L.L., D. Faber-Langendoen, R. Bittman, G. A. Hammerson, B. Heidel, J. Nichols, L. Ramsay, and A. Tomaino. 2009. NatureServe Conservation Status Assessments: Factors for Assessing Extinction Risk. NatureServe, Arlington, VA
  • McPeek, M. A., and T. E. Miller. 1996. Evolutionary biology and community ecology. Ecology 77:1319-1320
  • Salafsky, N., D. Salzer, A. J. Stattersfield, C. Hilton-Taylor, R. Neugarten, S. H. M. Butchart, B. Collen, N. Cox, L. L. Master, S. O’Connor, and D. Wilkie. 2008. A standard lexicon for biodiversity conservation: unified classifications of threats and actions. Conservation Biology 22:897-911.