Jl. Dooley et Ma. Bowers, DEMOGRAPHIC RESPONSES TO HABITAT FRAGMENTATION - EXPERIMENTAL TESTS AT THE LANDSCAPE AND PATCH SCALE, Ecology, 79(3), 1998, pp. 969-980
We tested the ecological consequences of habitat fragmentation by comp
aring the density, population growth rate, survivorship, and recruitme
nt of Microtus pennsylvanicus populations within a 20-ha fragmented la
ndscape with those of populations in a 20-ha unfragmented landscape. W
e also tested for fragment-size effects by comparing the same measures
of demographic performance across three fragment sizes (0.06, 0.25, a
nd 1.0 ha). During 17 censuses between June 1993 and October 1994, we
recorded 10 020 captures of 3946 individuals and found strong landscap
e differences but weak fragment-size effects. Although fragmentation r
educed the habitable area by 72%, density and adult recruitment were s
ignificantly higher on the fragmented landscape relative to the contro
l. With the exception of adult recruitment (higher on small relative t
o medium and large patches), no significant demographic differences ex
isted among patches of different size. Low rates of between-population
movement and differential juvenile growth rates suggested that higher
recruitment rates on the fragmented landscape likely resulted from en
hanced local reproduction rather than from immigration. Thus, despite
the fact that populations in the fragmented landscape experienced seve
re habitat loss, some individuals on fragments accrued important repro
ductive advantages (possibly as a result of diminished social costs or
enhanced food resources). That population and individual responses to
fragmentation could differ so dramatically provides a novel result th
at illustrates the importance of using hierarchical field designs in t
ests of population responses to large-scale habitat alteration. We con
clude that controlled, large-scale field tests can serve as an importa
nt intermediary between the inherent abstraction of simulation modelin
g and what is observed in the real world.