Modifying a habitat map for special circumstances

Sometimes a habitat suitability model needs to be modified to better reflect what we know about a species or study area. We give three scenarios:

  • A recent unmapped housing development in the analysis area may be still be mapped as natural vegetation in a land cover layer, resulting in an a poor habitat suitability map
  • Species that take several generations to move across a patchy landscape ('corridor dwellers') may depend more heavily on the use of habitat patches as stepping stones through the landscape
  • Suitable habitat for some species must be within close proximity to a habitat factor critical for its survival, such as escape terrain or perennial waters

Modifying a habitat map to account for unmapped influences

Habitat factors such as land cover are often developed using remotely-sensed data that is several years old. When used to create a habitat suitability model, this can result in a model depicting newly developed land as optimal habitat, or recently restored land as unsuitable.

To create more realistic habitat and corridor models, we recommend modifying habitat suitability models to account for previously unmapped influences. One simple way to do this in GIS is to simply digitize the unmapped influence, then reassign all pixels falling within the digitized feature a new score which better reflects your understanding of the new habitat suitability.

We suggest caution if tweaking a habitat map to account for possible new developments. When this modified habitat map is used for corridor analyses, you will still get a corridor, and the corridor may run right through the pixels you just re-mapped as developed. A naïve comparison of the 2 maps would suggest “The development won't affect the corridor.” But even if a change does not affect the location of the corridor, it may affect its quality. To assess the impact on corridor quality, use the metrics outlined in Evaluating corridors.

Modifying a habitat map for patchy landscapes

The typical corridor model is for a species that can move from one wildland block to another in a single movement event of a few days or weeks. Some species—corridor dwellers—take more than one generation to move between wildland blocks. CorridorDesigner lets you build more realistic corridors for such species.

Passage dwellers vs. corridor dwellers

Most corridor models assume that an individual animal can move between wildland blocks in a single movement event of a few hours to a few weeks. These animals can be called passage species, in contrast to corridor dwellers, which require more than one generation to move their genes between wildland blocks. The distinction is based on an interaction between the species and the landscape. Thus a species could be a passage species if the wildland blocks are within dispersal distance, but a corridor dweller where wildland blocks are farther apart. Corridor dwellers must find suitable breeding opportunities within the linkage.

Accomodating corridor dwellers

One way to accommodate corridor dwellers is to assign the highest suitability value to patches of potential breeding habitat. This tends to produce a corridor that links those patches in steppingstone fashion. By dispersing from patch to patch, one interpatch movement per generation, these animals can gradually recolonize a linkage and wildland block after a local extinction event, or move their genes between wildland blocks.

In our experience, this procedure makes sense only when modeling a species with a few habitat patches imbedded in a matrix dominated by poor habitat. Do not use this procedure if most of the matrix is breeding habitat. In such a case, the procedure creates a highly linear corridor that often fails to include the highest-quality habitat. Similarly, unless you know the threshold between breeding and non-breeding habitat precisely, don't use this procedure when a large fraction of the matrix is near the estimated threshold. In that situation, a tiny error in the threshold can drastically affect modeled patches and the modeled corridor.

Modifying habitat map to account for critical habitat factors

Using a standard habitat suitability model for a species that is dependent on proximity to a critical resource can greatly overpredict the amount of suitable habitat in an analysis area. Neither geometric mean nor arithmetic mean habitat models adequately account for a situation where a species is absolutely dependent on close proximity to one specific resource. For example:

  • Bighorn sheep must be close (‹300m) to steep slopes, which they use as escape terrain.
  • Many amphibians must be close to perennial waters, which they may use for food, cover, and thermoregulation.
  • Many rattlesnakes and lizards must be close to rocky outcrops, which they use for cover.

One way to create a more realistic model is to reclassify a habitat suitability model to better reflect declining suitability with increasing distance from a particular critical factor. Using GIS, the basic steps are:

  • Create a new raster or feature class layer of just the critical factor by performing a GIS query.
  • Use distance bands to reclassify an existing habitat suitability model (HSM) based on proximity to the critical factor. For an aquatic species dependent on perennial waters, this could be stated as:
    • From 0-30 m from perennial streams, the new HSM is 100% of the existing HSM
    • From 30-60 m from perennial streams, the new HSM is 80% of the existing HSM
    • From 60-100 m from perennial streams, the new HSM is 60% of the existing HSM
    • Anything ›100 m from perennial streams is not habitat for the species; the new HSM is 0% of the existing HSM.