The previous previous describe how to construct one single-species corridor. The union of single-species corridors constitutes a preliminary linkage design. This is morphed into a final linkage design after you tweak the design to accommodate focal species for which no corridor was modeled, buffer against edge effects, and remove areas from the design that likely increase financial costs of acquisition or management without substantially improving ecological utility.
The previous procedures produce a corridor model for a single species. You will repeat these procedures for several focal species. Now you are ready to create a comprehensive linkage design. There is more than one way to do this, but we recommend that this process include the following steps:
After creating corridor models for all focal species, we recommend joining all single-species corridors into a preliminary linkage design. This union of corridors is the most obvious way to fulfill our goal of “no species left behind.”
By overlaying the linkage design on a map of modeled habitat patches for each focal species, you will usually find that most breeding patches for each species were already captured by the union of corridors. Sometimes one or two species-specific habitat patches can be added if the addition would reduce the need for individuals to move distances longer than the estimated dispersal capability of the focal species. Because dispersal distances are only known for few species, and probably biased low due to difficulties associated with collecting dispersal data, we recommend using the longest known dispersal distance of the species as an estimate of dispersal capability. If there are no data for a species, data for a closely related species can be used, or another species of similar body size, mobility, and natural history.
Least-cost procedures will always produce a least cost corridor or path—even if the best is entirely inadequate for the focal species. This overlay procedure is a good way to assess how well the linkage design serves each species. Evaluating corridors and linkages describes additional descriptors you can use to assess how well a linkage design or corridor serves each species.
In Who to connect: selecting focal species, we mentioned that it may not be possible or appropriate to develop a corridor model for some focal species. In our efforts, we address needs of these species in one of the following ways:
If there are enough data to model potential habitat, map potential population and breeding patches. Overlay this patch map on the preliminary linkage design. Ask a species expert if the preliminary linkage design captured enough of these habitat patches to conserve the species. If not, add any additional patches near the preliminary linkage design that would improve the species' prognosis.
Overlay a map of known occurrences of the species on the preliminary linkage design. Ask a species expert if it would be helpful to expand the linkage design to include more of these occurrences, and if so, whether the occurrences should be included as disjunct steppingstones or by widening part of the linkage design.
In the arid southwest, adding perennial streams to the linkage design is a simple way to meet the needs of fishes and other riparian obligates. Buffering each stream 100m from the edge of the riparian zone is useful to reduce pollutants, sedimentation, and other edge effects. While many studies document edge effects on streams only out to about 50m, there are already many irreversible human alterations within 50m of most streams. Attempting to have 100m buffers in other areas may compensate for these impacts. Finally, the uplands adjacent to the stream are important movement areas for many terrestrial species. A 100m buffer helps reduce human disturbance and edge effects in this upland zone.
At this point, your preliminary linkage will likely have multiple strands, each serving the needs of one or more focal species. You may have some deeply looped strands. The thin strands winding through the linkage design in the map below capture the only continuous strands of perennial waters between the two wildland blocks. A linkage design like this is clearly not analogous to the linear corridors—such as hallways in our office buildings, or interstate highways—we construct to facilitate human movement. This is not surprising. We want the linkage to fit the species needs. We do not try to make the species fit the corridor.
Next, you will evaluate the preliminary linkage design as modified so far, and widen one or more strands to provide the following benefits of wide linkage strands:
Although edge effects and home range widths of focal species are relevant to linkage width, we recommend asking not “how narrow a linkage strand might possibly be useful to focal species?” but rather “what is the narrowest width that is not likely to be regretted after the adjacent area is converted to human uses?”
Conservation dollars are limited, so you do not want to propose a linkage design that includes large areas that do not substantially improve connectivity for a species. In our linkage designs, we examined the various slices of single species corridors to identify the “balloon” areas that emerged when we selected a slice that met our target minimum width (e.g., areas in the eastern ends of Slice 3 and Slice 4 in the linkage below).
To determine if the balloon areas are important, we overlay the selected slice on a map of population patches, breeding patches, and habitat quality. We look for opportunities to delete areas so long as the deletion will not significantly increase the travel cost for that species, or any other focal species.
This entire section describes procedures that are only weakly quantitative. Many procedures, such as the “no-regret” standard for width, are quite subjective. Does it really matter? Is it worth it?
Modeling is a tool to help you examine your landscape using all of your brain-power and the best available scientific knowledge. Modeling is not a substitute for such hard thinking!
In practice, all this tweaking has rarely caused big changes in our linkage designs. But we still go through the checklist. Mulling over the linkage design in this way is better than rushing a plan out the door and later wishing we had addressed these issues.
In a few cases, these considerations have improved the linkage design, and we sleep better at night for that. In many cases, it helps us more fully discuss the likely benefits (or lack thereof) for particular species and other conservation goals.