Who to connect: selecting focal species

Above all, remember that your goal is to conserve or restore a functioning wildland network that maintains ecological processes and provides for the movement of all native species between wildland blocks. Your goal is not to use a particular GIS tool. Work with biologists who know the analysis area to select 10 or more focal species that collectively will serve as an umbrella for all native species and ecological processes.

Examples of focal species include

  • area-sensitive
  • habitat specialists
  • dispersal limited
  • sensitive to barriers
  • otherwise ecologically important

While large carnivores are excellent focal species for linkage designs, a linkage should never be designed solely to serve large carnivores. For species for which a corridor model cannot be created, we give recommendations in From corridors to linkages.

Select a wide range of focal species

We encourage the selection of focal species likely to collectively serve as an umbrella for all native species and ecological processes. In linkage designs we created in California and Arizona, we often had 10-20 focal species, including reptiles, fish, amphibians, plants, and invertebrates.

Important types of focal species include:

  • Area-sensitive species: the first to disappear or become ecologically trivial when corridors are lost.
  • Habitat specialists: species that most need continuous swaths of a specific vegetation type or topographic element in the planning area.
  • Dispersal limited: species with short or habitat-restricted dispersal movements.
  • Barrier-sensitive species: the species hardest to get across the road, canal, fence or other barrier in the area.
  • Metapopulations: species requiring dispersal between wildlands for metapopulation persistence; species requiring connectivity to avoid genetic divergence of a now-continuous population.
  • Ecologically important species: species that represent important ecological processes; currently important species that would become ecologically trivial if connectivity were lost.

Do not design a linkage solely for large carnivores

Relying solely on large carnivores to design a linkage will likely harm more than help a linkage design. Because large carnivores like bears and wolves live at low density and are among the first to be harmed by loss of connectivity, they are excellent focal species for linkage design. They also make popular flagships to increase stakeholder support for a linkage. Large carnivores were the only focal species in almost half of the linkage designs published to date.

But we argue against designing a linkage solely for large carnivores—or any single species. Many species besides large carnivores need linkages to maintain genetic diversity and metapopulation stability. Furthermore most large carnivores are habitat generalists that can move through marginal and degraded habitats, and a corridor designed for them does not serve most habitat specialists with limited mobility. Worst of all, successful implementation of a single-species corridor for large carnivores could have a “negative umbrella effect” if land use planners and conservation investors become less receptive to subsequent proposals for less charismatic species. The umbrella effect of large carnivores best serves biodiversity if these species are part of a linkage designed for a broad array of native species.

Work with biologists to determine focal species

Biologists familiar with the study area should be invited to identify focal species. Even the foremost ecologist in the linkage area cannot provide a comprehensive list of all focal species. As an analyst or planner, you would hate to publish your plan and then discover that you had failed to include an important focal species. To avoid this, contact biologists working for agencies, NGOs, academic institutions, consulting companies, and major landowners in the area to develop a comprehensive list of focal species.

If stakeholders are concerned that a linkage may increase the spread of invasive species into wildlands, then one or more invasive species could be included in the suite of focal species. Any expected invasion via the linkage should be compared to invasion expected from edges and matrix land regardless of the conserved linkage.

Should “adequate data” be a criterion to qualify as focal species?

As an analyst, you will groan when someone proposes a focal species about which little is known. How can you possibly design a linkage to serve such a species? Shouldn't we just forget about it? In general, our answer is “no.”

Recall that our motto is “no species left behind.” We can't just say, “Sorry, butterfly, we don't know how you move, so you are out of luck.” We may not be able to model movement, but as conservationists, we must do what we can. We return to this issue below.

There is one circumstance in which it can be OK to exclude a poorly-known species. This can occur when there is another, better-understood focal species that plausibly captures the needs of this focal species. For instance, if a species was proposed because it is suspected to prefer steep slopes, but little else is known about the species, you can identify another focal species whose close affinity to steep slopes is more susceptible to modeling.

Is it appropriate to have a focal species that occurs only in the matrix?

Even though species may not occur in one or both wildland blocks, they may still be important to the functioning of an ecosystem linkage. Managing for species endemic to the linkage can help us ensure that we are managing the linkage as a semblance of a fully-functioning ecosystem, rather than a narrow gauntlet that lets focal species pass between wildland blocks.

One example of this scenario that we have had to face was with the plant Rainbow manzanita in California. Rainbow manzanita does not occur in either the Santa Ana or Palomar Mountains protected wildland blocks to be connected, but is widely distributed in the matrix between them. The plant's geographic range is nearly 20 miles long, and contained almost entirely in the matrix between wildland blocks. Since part of our underlying goal is to conserve evolutionary processes, including the crucial processes of evolution, range shifts, and response to climate change, Rainbow manzanita was a most appropriate focal species.

We now routinely include such species, and refer to them as “species the corridor needs” (to ensure its ecological integrity) in contrast to “species that need the corridor” (to get from one wildland block to the other). This leaves the question of what exactly we do with such species, because we cannot handle them in the same framework as species that occur in both wildland blocks.

What to do with species for which you cannot build a model?

Just because we cannot build a corridor model for some species, we do not just remove these focal species from consideration!

In the last two sections we mentioned two types of focal species that don't fit well in our standard framework of designing corridors between wildland blocks. The first group consists of species for which we cannot model movement as function of GIS layers. For example, this would include animals that can fly (birds, many insects) and plants or insects whose propagules are wind-dispersed. We may be able to model suitable patches of habitat, but we don't know how they move from patch to patch.

The second group consists of species that occur in the matrix, but not in the wildland blocks. Even if we can model their movement, they differ from all the other focal species in that they are not moving between wildland blocks—we have no logical start and end points for the corridor. More precisely, we have an impractically large number of patches within the matrix, any of which could be considered start and end points. You will doubtless find other types of species that are legitimate focal species, but for which a modeled corridor would not pass the “laugh test.” We describe methods to accommodate species that don't fit well in our framework of designing corridors in From corridors to linkages