Digests for Debate: Coarse vs. fine filters for connectivity planning

From Yellowstone to Yukon to individual fish-passage culverts, efforts to increase habitat connectivity are moving forward at a range of scales. This push for connectivity conservation has generated increasing need for methods that can identify priority areas for connectivity, and given fresh life to a long-running debate regarding the use of coarse-filter and fine-filter approaches to conservation planning.

The issue, in a nutshell, is that we simply don’t know how each landscape feature (e.g., roads, lakes, agricultural fields) hinders or promotes movement for each species. For that reason, connectivity planners have to choose Cartoon_mountain_pass_symbolizing_path_of_least_resistanceproxies to represent all the species in a landscape. There are currently two leading contenders for this proxy role: focal species and landscape naturalness.

The focal species approach identifies corridor networks based on one or several species in a landscape, chosen because we actually know something about how they move and why. The focal species approach is a fine-filter conservation strategy; it will probably do a good job accommodating the movement needs of the focal species and others that move in similar ways. The more numerous and diverse the focal species used, the fewer cracks there will be in the filter. The drawback is that a full focal species approach can be time consuming and expensive, especially at large landscape scales, where many focal species are needed to represent diverse habitats. It’s also difficult to know how many focal species is enough.

Thus, the naturalness of the landscape itself has recently become a popular alternative to focal species. This approach assumes that species will find it more difficult to move through areas heavily modified by human activities, and identifies corridor networks connecting areas with low levels of modification. The naturalness approach is a coarse-filter conservation strategy; it will probably accommodate the movement needs of the many species that prefer to move through areas in good natural condition, but its coarse general assumptions may result in many species (particularly specialists) falling through the cracks.

Over the past several years, my colleagues and I from the Washington Wildlife Habitat Connectivity Working Group completed both types of connectivity analyses for Washington State, USA, and surrounding areas. Our recent paper in Landscape Ecology describes how the two approaches in fact identify fairly similar areas, and that the landscape integrity approach (our version of the naturalness approach) performed about as well as 3-4 randomly selected focal species at capturing the connectivity network identified by our full focal species approach (16 and 11 species, respectively, for our statewide and Columbia Plateau analyses). However, the landscape integrity network agreed better with the corridor networks of large-bodied species with good movement abilities. Furthermore, about half of the focal species used at each scale captured a large proportion of the connectivity network produced using the full suite of focal species.

We interpreted these results to suggest that naturalness-based connectivity approaches offer a good analytical bang for your buck, and that there are decreasing returns to adding focal species beyond a relatively modest number. However, a full focal species approach (or a naturalness approach supplemented with a few focal species) may better accommodate the movement needs of a landscape’s full complement of species, particularly small-bodied species with poor movement abilities. An even more complete assessment will come when we can compare both sets of results to the actual movement patterns of all the species in our landscape. Unfortunately, these data simply don’t exist for large or even small landscapes, and are unlikely to become available anytime soon. In the meantime, additional studies comparing these and other connectivity analysis methods will go a long way toward informing efforts to reconnect fragmented landscapes.

Resources

Krosby M., I. Breckheimer, B. Cosentino, B. Gaines, S. Hall, K. Halupka, R. Long, B. McRae, J. Pierce, J. Schuett-Hames, and P. Singleton. 2015. Focal species and landscape “naturalness” corridor models offer complementary approaches for connectivity conservation planning. Landscape Ecology. doi:10.1007/s10980-015-0235-z.

Theobald D. M., S. E. Reed, K. Fields, M. Soulé. 2012. Connecting natural landscapes using a landscape permeability model to prioritize conservation activities in the United States. Conservation Letters 5:123–133.

WHCWG (Washington Wildlife Habitat Connectivity Working Group). 2010. Washington Connected Landscapes Project: statewide Analysis. Washington Departments of Fish and Wildlife, and Transportation, Olympia, Washington. Available at http://waconnected.org/statewide-analysis/

WHCWG (Washington Wildlife Habitat Connectivity Working Group). 2012. Washington Connected Landscapes Project: analysis of the Columbia Plateau Ecoregion. Washington Departments of Fish and Wildlife, and Transportation. Olympia. Available at http://waconnected.org/columbia- plateau-ecoregion/

2017-03-19T23:54:21+00:00 September 8th, 2015|

About the Author:

Meade Krosby

Meade Krosby is a research scientist with the Climate Impacts Group at the University of Washington, where she works with land and wildlife managers and policy makers to incorporate climate change into conservation planning. Connectivity analysis and planning under climate change is a central focus of her work; she is the climate lead for the Washington Wildlife Habitat Connectivity Working Group, and is currently spearheading an effort to integrate climate change and connectivity into the decision-making of land and wildlife managers in the Washington-British Columbia transboundary region.