Big Picture Methodology

A natural heritage system includes core natural areas, other significant natural areas and linking habitat corridors. This often includes potential areas for restoring and enhancing natural areas and corridors.

The "Big Picture" natural heritage system for Carolinian Canada was developed to combine the best available data on natural features, plants and animals with a landscape-level ecological approach and state-of-the-art information management. A multi-agency Technical Committee was struck in 1999 to provide expert review throughout the analysis.

The Big Picture method generally followed the following principles.

Photo: NASA  

1) Core natural area selection should preferentially identify intact natural areas with larger habitat blocks, and regions with high overall percentages of natural vegetation cover.

2) Core areas should include viable occurrences of globally rare species and vegetation community types, and concentrations of rare species and vegetation.

3) Minimum core natural area size should exceed 200 hectares where possible, with smaller high-quality areas in areas with lower amounts of natural vegetation cover.

4) Minimum corridor widths should be at least 200 metres plus any adjacent areas of natural cover. As a result, almost all of the natural heritage system is currently in natural vegetation cover with the exception of some portions of 200 metre-wide corridors. These latter areas may represent opportunities for future naturalization. On the ground, it may be important for ecological reasons to establish functioning corridors more than 200 metres wide. In other situations, it may not be feasible ecologically or socially.

5) Where possible, more than 30% of the non-urban area of each ecological district should be identified as part of the natural heritage system.

What are Carolinian Core Natural Areas?

To identify Carolinian core natural areas, digital data sets were assembled to show the locations of natural features including life science Areas of Natural and Scientific Interest (ANSIs), evaluated wetlands, Carolinian Canada sites, older growth woodlands, forest cover (click to see map of forest concentrations), streams, rivers and element occurrences (rare species and vegetation types; click to see map of these). These data were converted to a grid of 25 metre by 25 metre units for the entire Carolinian zone (individual pixels).  

Each feature type was assigned a point value based on its relative ecological "value". For example, each pixel within a documented older growth forest received 15 points, whereas all pixels in the path of rivers and streams received three points. The following variables were assigned scores for each of the 25m x 25m map unit in the Carolinian zone.

Land Cover Types from LANDSAT Satellite imagery (e.g. wetland, forest, agriculture, urban) Life Science Areas of Natural and Scientific Interest (ANSIs) Concentrations of Rare Species & Rare Vegetation Communities (Element Occurrences) click to see map of these
Slope heterogeneity Wetlands Forest Interior Outlines
Aspect heterogeneity Carolinian Canada Sites Forest Interior "Internal" Buffers
Drainage (30m buffer around rivers, streams and lakes) Older Growth Woodlands Forest Cover Concentration within 2 and 10km (click to see map of forest concentrations)

Scores assigned for each of these variables and additional information on each variable can be viewed here. Scores for these variables were then summed to create a total score for each 25x25m pixel. Click here to see a map of the higher scores across the Carolinian region.

To identify core areas, this map of scores for each 25x25m map unit needed a minimum value that would result in a system of core areas of reasonable extent and configuration. Two values were considered, 11 and 12. Use of a cut-off of 11 would have resulted in larger core areas, but would include features with low ecological integrity (e.g., drainage ditches). A cut-off value of 12 points resulted in representation of over 30% of the ecoregion in cores, with fewer patches of low integrity.

Clusters of adjacent 25 metre by 25 metre map units with values 12 or greater were grouped. Any group larger than 200 ha was automatically considered a "core area". Core natural areas over 200 ha were concentrated mainly in areas with higher percentage natural vegetation cover. 

Sizable regions of the study area had no core areas larger than 200 ha. In such areas, the OMNR-recommended minimum size for a "significant woodland" was used, based on percentage forest cover by township, 40 hectares where cover is 15-30%, and 4 hectares where cover is less than 15%. As a result, in some townships, core natural areas were as small as 4 ha. 

Other Significant Natural Areas were identified as natural areas with 12 or more points and larger than the minimum area for significant woodland in each township (i.e. 40 hectares where cover is 15-30%, and 4 hectares where cover is less than 15%). These are another important component of the natural heritage system.

What are Potential Habitat Corridors?

The best potential habitat corridors to connect the system of Carolinian core natural areas were also identified using the best available ecological inventory data, a landscape-level ecological approach and state-of-the-art information technology. This approach used an algorithm to find the best ecological corridor for the "least cost".

To determine the best potential connections between the core natural areas, additional data on each 25mx25m portion of the Carolinian zone was added including variables which would encourage facilitate connection paths between the core areas ("Linkages") and some which discourage ("Barriers") connections between the core areas. Recreational Class Provincial Parks, for example, would make good potential additions to the connections network, but connections across divided highways would be unlikely. The variables used are listed below:

"Barriers" "Linkages"
  • Urban areas
  • Extraction sites
  • Highways
  • Abandoned pits and quarries
  • Utility Corridors
  • Abandoned railways
  • Agreement forests
  • Recreational Provincial Parks
  • Regenerating fields

Each value for each of these variables was assigned a score. The "barrier" scores were negative and the "linkages" variable scores were positive. For example industrial zoning was assigned a score of -10 while greenspace zoning was assigned a score of +5. Click here to see the scores assigned to each variable. An overall composite score was determined by adding together the scores for these variables for each 25m x 25m map unit in the region. The composite scores formed a "resistance" layer to determine the "least-cost" (i.e. greatest ecological value with fewest practical obstacles) connections between core natural areas.

These least-cost corridor alignments generally follow existing areas of natural vegetation cover, especially riparian areas, because of the positive score values. Where no such alignments along positive values could occur, the cores were connected along the "highest value per unit length" route (i.e., in areas without natural vegetation cover), and a minimum width of 200 metres was assigned to that highest-probability corridor alignment. These areas are the only significant part of the mapped natural heritage system not currently in natural cover.

The general goal was to connect all core natural areas within 20 kilometres of each other. The minimum score of the least-cost connection between two core natural areas furthest apart was calculated, and this figure was used as the maximum cost to form a path between any two core natural areas. This ensured that all core natural areas were linked to at least one other core area, and often to many more where there was a high density of core areas and natural values. These connections were widened to incorporate adjacent natural areas having a score greater than 12.

All connections were reviewed visually to supplement the digital analysis which could not make judgements about including or excluding adjacent natural features, ensure that recreational provincial parks were included in the natural heritage system wherever practical and ensure that each connection follows an ecologically and practically optimal route at least 200 metres in width.

Not all connections achieved the targeted minimum width of 200 m. Connections that narrowed to less than 200 m were highlighted as potential restoration areas.

Want more background on the methodology used to develop the Big Picture? Click here!

NHIC and MNR expanded the Big Picture to other parts of Ontario in 2002. Click here to get more information on the Big Picture 2002.

The Nature Conservancy of Canada has used the Big Picture as a basis for Conservation Blueprints to guide protection of significant lands


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