Understanding the role of behavioral mechanisms and impacts of habitat fragmentation on structuring small declining populations

Understanding and mitigating the impacts of habitat fragmentation due to anthropogenic causes (e.g. logging) on wildlife populations consists of three major components: (1) understanding both historical and current population dynamics, structure and behavior of the species of interest; (2) identifying the role and extent that various human activities and changes in land-use patterns play in causing the resulting population structure; (3) understanding how best to mitigate negative anthropogenic impacts on a given landscape. As pointed out by van Oort, McLellan & Serrouya (2011), in many instances the assumption is made that where humans have fragmented once contiguous habitats, wildlife will inevitably establish a new metapopulation structure for the species. If land and wildlife managers and conservation practitioners begin with the premise that a metapopulation structure is in place before confirming this with data, and if these managers proceed to attempt to mitigate our impacts to putatively ensure and enhance habitat connectivity, landscape permeability and genetic and demographic connectivity, our conservation efforts and funds may be misguided. As a hypothetical example, species X is perceived to have been historically distributed fairly evenly across an intact landscape as a relatively panmictic population. At some point in time a road network is constructed within the once contiguous habitat. As time proceeds, the population begins to decline and the management and conservation community decides that these anthropogenic structures (i.e. roads) have severed habitat connectivity for species X forcing the species into a new metapopulation structure. As a result, the decision is made to spend funds to reduce habitat fragmentation through best management practices that reduce road densities and/or via some mitigating structure(s) (e.g. road crossing structures) to enhance both genetic and demographic connectivity (i.e. rescue effect; Simberloff, 1988; Hanski & Gilpin, 1991; Hanski et al., 1995). In this hypothetical example, behavior studies are even conducted demonstrating that species X would utilize patches of minimum size Y and crossing structures with design parameters A, B and C. Once the identified best management practices are used to reduce fragmentation levels and correctly designed crossing structures constructed, species X still continues in decline ultimately going locally extinct. In this hypothetical example, what was clearly missing was an empirically based, baseline understanding of population structure and behavior before fragmentation of the landscape. In the case of mountain caribou Rangifer tarandus caribou, van Oort et al. (2011) claim that spatial severing of habitat by human activities did not lead to the creation of a metapopulation structure in which natal dispersal (or even breeding dispersal) links subpopulations, because the weak ‘innate’ ability to disperse continues to prevent metapopulation structuring and associated rescue effects of these small, isolated subpopulations. Did caribou in this study area demonstrate this lack of ‘innate ability’ to disperse historically for long periods of time or is the inability and/or unwillingness to disperse a function of current social dynamics? In other words, caribou may likely not disperse in the current situation in British Columbia due to population sizes now falling below some critical threshold level of size/ density that would drive both natal and breeding dispersal (Dobson, 1982; Travis, Murrell & Dytham, 1999). This idea is touched on by van Oort and colleagues in their discussion of population eruption and expansion by other caribou populations (e.g. theMulchatna caribou herd). In the woodland caribou case study it appears that human activities (logging and roads) have fragmented the habitat, but that the lack of dispersal and connectivity among populations may not be due to habitat fragmentation and barriers per se and instead the proximate cause of the lack of connectivity is the low-density populations’ unwillingness to disperse. Disentangling between social/behavior-caused and habitat fragmentation-caused lack of dispersal has huge implications for how best to recover populations or mitigate the impacts of anthropogenic-caused habitat fragmentation. For example, if the lack of demographic connectivity is simply a spatial habitat fragmentation issue, then mitigation measures such as