Evaluating corridors and linkages

Least cost methods always provide a “best” solution, even when the best is not very good. You need tools that can effectively describe how well your proposed linkage design serves each focal species. These same tools can be used to compare the linkage design to alternative designs that may be proposed to meet cost or political constraints.

Three of the most useful tools are:

  • frequency distribution of habitat quality for each focal species
  • a graph depicting intensity and length of bottlenecks
  • a list of the longest interpatch distances that dispersing animals of each focal species would have to cross.

Why do you need evaluation tools?

As conservation investors try to implement the linkage design you helped produce, they will face some tough choices. For example, they may have an opportunity to buy two huge parcels in the linkage area from conservation-friendly landowners. These two parcels form a continuous swath that overlaps about half of your linkage design, plus an even larger amount of land outside the linkage design. This opportunity represents an alternative linkage design. The investors want to know: How will it compare to the optimal linkage design? Is it almost as good, half as good, or markedly inferior? In another linkage area, a large development company owns most of the land, and wants to develop 3 new cities there. They propose an alternative linkage design that allows them to proceed with their development plans. The developer produces a glossy booklet touting the virtues of their alternative. The County Planning & Zoning Department needs answers to the same questions: How will this alternative compare to the optimal linkage design? Is it almost as good, half as good, or markedly inferior?

There are an endless number of such scenarios. As the analyst, you cannot simply say “We've presented the optimum design. Now please go away.” You want to provide some useful descriptors that allow decision makers to make choices with their eyes open.

No silver bullets

Please avoid the following ways of summarizing utility of alternative linkage designs:

  • A conventional estimate of cost-weighted distance for each species. This is silly. A 90% or 10% difference in cost-weighted distance does not indicate a 90% or 10% change in interpatch movement or gene flow.
  • A percent risk of extinction for each species under each alternative. Population models are useful tools for ranking alternatives, but the numerical “percent extinction risk” should not be trusted.
  • Any other single number that attempts to quantify utility for each focal species.
  • Most especially, any single number that attempts to quantify utility for all species at combined!

There is no silver bullet. You will have to provide several useful descriptors of utility and help summarize them in a way decision-makers can understand.

Useful descriptors

Frequency distributions of habitat suitability

In the illustration, the alternative is clearly inferior. Notice that a frequency distribution is much more informative than the mean habitat suitability. You may also want to provide frequency distributions for other GIS layers, such as land cover types, elevation, and topographic positions. All of these can help decision makers appreciate the differences between alternatives.

Frequency distribution of suitability

Bottlenecks and corridor width

In the illustration, the inset graph shows width of a corridor at each point along the corridor midline (the purple line on the map). By placing your ruler horizontally at any y-axis value, you can see the number and length of bottlenecks that are narrower than any threshold of interest. For instance, in this case, there are no bottlenecks as severe as 250 m. There are six bottlenecks narrower than 400m, the longest of which is 1.2 km long (from 6.8km to 8.0 km along the midline). By producing a similar graph for an alternative, you provide a useful way to compare them.

Graph of corridor width

Interpatch distances

The linkage design below has two strands. For Species A, six potential population patches are fully or completely overlapped by the linkage design. In addition to presenting this map, you can provide a list of interpatch distances that a dispersing animal would have to cross to move between wildland blocks. These distances are represented by the green lines connecting 5 of the patches to the wildland blocks. Notice that the patch in the lower strand is not part of this movement path, and is not included, and that the green line is confined to a single linkage strand. Thus the green line represents our best estimates of interpatch distances that would need to occur after the remaining matrix has been converted to uses incompatible with wildlife movement.

You can produce this same output for any alternative linkage design. The results could be displayed in the format of the following table. The modeled distances are most useful if you compare them to the species' estimated dispersal ability. In the example below, for instance, three interpatch distances in the Alternative exceed the dispersal ability of the species, compared to none in the proposed linkage design.

Map of interpatch distances