A controlled, hierarchical study of habitat fragmentation: responses at the individual, patch, and landscape scale

We compared the performance of individuals and whole populations of meadow voles, Microtus pennsylvanicus, within and between experimentally created h abitat fragments of three sizes (1.0, 0.25, and 0.0625 ha) and between a 20 -ha fragmented and a 20-ha continuous habitat landscape. We recorded 10,020 captures of 3946 individuals over 17 censuses between June 1993 and Octobe r 1994. Five demographic parameters showed significantly different populati on responses between the two landscapes but no difference in tests comparin g fragment size: i.e., mean and peak population densities (the latter, in e ach of the two growing seasons) averaged 149 to 172% higher, population gro wth rate averaged 219% higher, and adult recruitment 170% higher in fragmen ted than in the continuous control landscape. Observations at the individua l level (body sizes, rates of reproduction, residence times) suggested that these landscape differences involved enhanced performance of adult females associated with edge habitats rather than differential immigration or emig ration. If this turns out to be a common response to fragmentation, the det ection of such responses will be greater when comparing fragmented and unfr agmented landscapes with qualitatively different structure than for fragmen ts of varied size with differing proportions of edge. That responses to hab itat fragmentation may be more evident at the very small (individual) and v ery large (landscape) scales, but may be obscured at the intermediate spati al scale of fragments, is a proposition that clearly requires more attentio n.