The goals of establishing conservation corridors are frequently to allow animals to shift ranges with climate change, maintain gene flow, or avoid inbreeding. To achieve these goals, the corridors need to facilitate dispersal movements or long mating movements outside the home range.
We often estimate resistance to long-distance movement as the linear inverse of habitat suitability within the home range, because habitat suitability in the home range has already been studied or because it is easier and cheaper to study than landscape use during dispersal. Thus we often scale habitat suitability on a scale of 0 to 100, and then estimate resistance as 100 minus suitability.
In a recent study on desert bighorn sheep (part of my dissertation under Paul Beier), we found that landscape resistance to long-distance exploratory movements was indeed negatively related to habitat suitability. But, contrary to the common assumption, the slope was not minus one – indeed we found a highly nonlinear concave relationship.
As habitat suitability decreased from 1 to 0.8 to 0.6 to 0.4, resistance stayed near zero. As suitability decreased toward zero, resistance increased dramatically (Keeley 2015). Thus during long-distance movements bighorn sheep avoid only the most unsuitable habitat conditions, and readily use moderately unsuitable conditions.
I found several studies supporting this nonlinear pattern, and no studies supporting a linear relationship. Landscape resistance to exploratory movements of red-cockaded woodpeckers was a strongly negative exponential function of habitat suitability (Trainor et al. 2013). Landscape genetic patterns of brown bears (Mateo-Sánchez et al. 2015) and kinkajous (Keeley 2015) were consistent with a negative exponential relationship between resistance and habitat suitability. Hispid cotton rats (Bowne et al. 1999), lions (Elliot et al. 2014), Iberian lynx (Gastón et al. in review), and Siberian flying squirrels (Selonen et al. 2006) also showed little aversion to habitat conditions that were moderately unsuitable in the home range.
Less-direct support comes from studies of mountain goats (Shirk et al. 2010), American marten (Wasserman et al. 2010), and American pikas (Castillo et al. 2014). Each species was constrained by fewer landscape variables during dispersal and mating movements than during daily movements.
If, as we believe, most mobile animals readily traverse habitat of moderate and low suitability for dispersal, planning for wildlife corridors would be more flexible than if resistance is assumed to be a linear function of habitat suitability in the home range. In this circumstance, corridor designers would no longer focus on the location of the least-cost path, but rather on identifying the major barriers to movement (ex. roads, fences, canals, rail lines) and the major mortality factors in connective zones (road kill, human conflict). They would then devise strategies to mitigate those barriers and mortality factors.
We also believe that there is probably a threshold size for unsuitable habitat patches, such that animals will rarely or never cross patches larger than that threshold. If this belief is correct, determining that threshold distance should be an important focus of corridor research.
Keeley, A.T.K. 2015. Comparing Estimates of Landscape Resistance to Animal Movement. Ph.D. thesis. Northern Arizona University.
Bowne, D.R., J. D. Peles, and G. W. Barrett. 1999. Effects of landscape spatial structure on movement patterns of the hispid cotton rat (Sigmodon hispidus). Landscape Ecology 14: 53-65.
Castillo, J.A., C. W. Epps, A. R. Davis, and S. A. Cushman. 2014. Landscape effects on gene flow for a climate‐sensitive montane species, the American pika. Molecular Ecology 23: 843-856.
Elliot, N.B., S. A. Cushman, D. W. Macdonald, and A. J. Loveridge. 2014. The devil is in the dispersers: predictions of landscape connectivity change with demography. Journal of Applied Ecology 51: 1169-1178.
Gastón, A., S. Blázquez-Cabrera, G. Garrote, M. C. Mateo-Sánchez, P. Beier, M. A. Simón, and S. Saura. (in review) Response to agriculture by a woodland species depends on cover type and behavioural state: insights from resident and dispersing Iberian lynx. Journal of Applied Ecology.
Mateo-Sánchez, M.C., N. Balkenhol, S. Cushman, T. Pérez, A. Domínguez, and S. Saura. 2015. A comparative framework to infer landscape effects on population genetic structure: are habitat suitability models effective in explaining gene flow? Landscape Ecology 30: 1-16.
Selonen, V. and I. K. Hanski. 2006. Habitat exploration and use in dispersing juvenile flying squirrels. Journal of Animal Ecology 75: 1440-1449.
Shirk, A.J., D. O. Wallin, S. A. Cushman, C. G. Rice, and K. I. Warheit. 2010. Inferring landscape effects on gene flow: a new model selection framework. Molecular Ecology 19: 3603-3619.
Trainor, A.M., J. R. Walters, W. F. Morris, J. Sexton, and A. Moody. 2013. Empirical estimation of dispersal resistance surfaces: a case study with red-cockaded woodpeckers. Landscape Ecology 28: 755-767.
Wasserman, T.N., S. A. Cushman, M. K. Schwartz, and D. O. Wallin. 2010. Spatial scaling and multi-model inference in landscape genetics: Martes americana in northern Idaho. Landscape Ecology 25: 1601-1612.