Space-use responses to habitat fragmentation and connectivity in the root vole Microtus oeconomus

Ecology, June, 1998 by Harry P. Andreassen, Karine Hertzberg, Rolf A. Ims

INTRODUCTION

Although habitat fragmentation is widely regarded as a major threat against the viability of wildlife populations (Rolstad 1991, Fahrig and Merriam 1994, Wiens 1995), there is little empirical knowledge about the proximate mechanisms underlying population responses to fragmentation (Wiens et al. 1993, Diffendorfer et al. 1995, Ims 1995). Clearly, the effects of fragmentation and the processes involved will depend on spatial scale or extent of the fragmentation (sensu Wiens 1989). Large-scale habitat fragmentation affecting metapopulation dynamics (Hanski and Gilpin 1996), through processes such as exchange rates between local populations (Ims and Yoccoz 1997), have received most of the current research. More small-scale fragmentation changing the spatial structure of local populations may, however, be important too (Rolstad 1991, Wiens 1995). This latter situation applies the concept of patchy populations (sensu Harrison 1991), where fragmentation may change the behavior of individual animals. Indeed, characteristics of patchy populations such as intrinsic rate of growth, dispersal rate, and social organization (Foster and Gaines 1992, Ostfeld 1992, Diffendorfer et al. 1995, Bowers et al. 1996) are expected to be emergent properties of how the individuals exploit patchily distributed resources and how interactions among the individuals (competitive or co-operative) are modified by the spatial configuration of suitable habitat patches (Cockburn 1988, Lomnicki 1988, DeAngelis and Gross 1992, Ostfeld 1992, Dunning et al. 1995, Lima and Zollner 1996, Bowers et al. 1996), However, how plastic individual traits are to varying habitat patch configurations and how such plasticity translates into population-level phenomena (e.g., social organization) are largely unexplored (Sutherland and Dolman 1994, Sutherland 1996). Such relations can probably best be uncovered in studies where the responses of the individuals to habitat manipulations are measured in experimental designs with replication at the population level (Ims and Stenseth 1989, Wiens et al. 1993).

One of the most important individual responses to investigate in the context of habitat fragmentation is space use (Ims et al. 1993). Space use is usually described by the size of home ranges, how the intensity of use and movement patterns varies within individual home ranges, and the extent to which a home-range is shared with other individuals (see Andreassen et al. 1993 for a review on animal space use descriptors). Whether habitat fragmentation will influence such space use descriptors obviously depends on its spatial scale. Crucial points in this context are the relationship between fragment size and intrinsic home range size requirements such as minimum territory size (Rolstad 1991) and the relationship between interfragment connectivity (Forman and Godron 1986) and trade-off between costs and benefits associated with interfragment movements. However, because individuals make their space use decisions in a social context, intrinsic propensities for social behavior also matters.

Based on intrinsic space requirement and social behavior, Ims et al. (1993) predicted three possible scenarios for space use responses to habitat fragmentation (see also Ostfeld 1992 for similar scenarios). A fission response with less overlap is expected for territorial animals when the fragment size approaches the minimum individual area requirement as "surplus individuals" will be expelled from fragments of limiting size. If a habitat fragment becomes less than the minimum individual space requirement, an expansion response may be induced. In this scenario individuals may expand their home range to include several patches and thus engage in frequent interfragment movements. Interfragment movements (and thus an expansion response) will only be possible given sufficient connectivity between fragments depending on the degree of hostility of the matrix, interfragment distances and presence of habitat corridors. Socially induced expansion may result from subordinate individuals forced to move between fragments when the habitat is saturated with socially dominant individuals. In the latter case, a mixed-space use response would be observed within the population. Finally, a fusion response (also termed a crowding effect by Lovejoy et al. 1986) with shrinking home ranges and increased overlap may be expected for social individuals tolerating more overlap and less space as fragment size is reduced.

None of these scenarios have previously been tested adopting an appropriate experimental approach, i.e., using individuals as the observation unit and populations as the experimental (test) unit. Observational studies and more small-scale habitat manipulations have, however, indicated expansion responses to relatively fine-grained habitat fragmentation (sensu Rolstad 1991) for owls (Carey et al. 1992, Redpath 1995), forest grouses (Rolstad and Wegge 1987), squirrels (Wauters et al. 1994) and foxes (Geffen et al. 1992).