To get from a habitat suitability map to a corridor map, we follow three steps:
We have been operating under the assumption that habitat suitability and habitat permeability are synonyms. In Overview of habitat modeling, we admitted that we don't know this for sure. The big leap of faith is assuming that habitat suitability is the same as habitat permeability. At this point, we simply define resistance or travel cost as the inverse of suitability or permeability, such that
Resistance (cost of travel through a pixel) = Maximum suitability minus pixel suitability.
In this tutorial, we have defined habitat suitability and permeability on a scale of 0-100. Thus,
Resistance (travel cost) = 100-pixel suitability.
Resistance or travel cost reflects the ecological cost of travel through a pixel. In general resistance increases with the risk of dying in the pixel and the energetic cost of travel through the pixel. Resistance decreases with how much food, water, and cover an individual might find in the pixel, or the probability of finding a mate and breeding in an area that includes the pixel. It is not necessarily related to the speed of travel through the pixel. In fact, animals may tend to move quickly through costly pixels, to minimize risks and more quickly get to better habitat.
The starting and ending points of a corridor can greatly impact the location of the modeled corridor. A terminus is a part of a wildland block that forms one end of a modeled corridor. You can define it as a point (or pixel), a linear edge (e.g., the wildland boundary), or a patch (population patch or breeding patch). Often there is more than one potential terminus in each wildland block.
We discuss this step further in Defining end points for corridors.
Cost distance of each pixel is the lowest possible cumulative resistance from that pixel to terminuses in each habitat block. A map of cost-distance always produces continuous swaths of permeable pixels; these swaths have been used as the basis for all published corridor designs. The most difficult issue is selecting a corridor width ('slice') wide enough to facilitate movement, but narrow enough to minimize monetary costs of conservation.
We discuss this step further in Cost distance and single-species corridors.
We use the term “corridor” for a swath of land that is best expected to serve movement needs of an individual species after the remaining matrix has been converted to other uses. We use “preliminary linkage” to refer to the union of all single species corridors. We use “linkage design” to refer to this map after it has been modified to (a) serve focal species for which you could not build a corridor model, (b) remove redundant strands, and (c) meet other conservation goals. The linkage design includes not only this map, but also recommendations for mitigating barriers and managing lands in and adjacent to the mapped area.