BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL. 38 PT. 11(8)


compared to other small mammals. For example, high densities caused population
declines in Rattus species (Davis 1953; Calhoun 1962), due to increased aggressive
interactions, and decreased reproduction.
 The bottomland hardwood swamp consistently supported the highest densities
of woodrats and may represent optimal habitat. The bottomland swamp is a large
continuous tract of forest, and vegetation was the most structurally complex on
horizontal and vertical planes. Plant species composition in the three habitats was
very similar, but the mesic hammock with saw-palmetto understory and hardwood
hammock with open understory were structurally less diverse. These habitats also
are fragmented. Because of this fragmentation, woodrat densities may be
restricted. The bottomland swamp may also serve as a 'source' to other connected
but marginal habitats.
 Declines in density may also have been due to predation. The most
significant amount of predation was attributed to barred owls (Strix varia) and
great hored owls (Bubo virginiana). Snake predation was the second most
commonly observed incidence of predation, attributed to the eastern diamondback
rattlesnake (Crotalus adamanteus) and the yellow rat snake (Elaphe obsoleta).
 Of the incidences of predation, all known predation occurred to animals in
the mesic hardwood hammock with open understory, and the mesic hardwood
hammock with saw-palmetto understory. No predation was observed in the
bottomland hardwood Mill Creek Swamp, which is dominated by extremely dense
fetterbush that is impenetrable to large, avian predators, but presents no difficulty
to reptilian predators. Timmerman (1989) and Franz (pers.comm.) suggested that
reptile populations may be relatively abundant in the bottomland hardwood swamp.
Wright (1989) considered owls to be one of the top predators to opossum
(Didelphis virginiana) in the mesic hardwood and xeric oak forests. If true for
Neotoma, then near absence of owl predation in Mill Creek Swamp as observed in
this study, could offer a partial explanation for the consistent higher densities
found there. Conversely, the joint impact of snakes and owls in mesic forests,
where both predators occur, may be partially responsible for the observed sharp
fluctuations and declines in density.
 This study occurred in the middle of a severe drought in north-central Florida
and may have been another factor influencing densities. During this period, Dodd
(1992) documented a dramatic decline in densities of upland amphibians and
reptiles on the Ordway Preserve, which was positively correlated to the reduction
in rainfall. Additionally, Jones (1990) documented dramatic declines in Florida
mice (Podomys floridanus) in the sandhill community which was positively
correlated to declining rainfall levels. Woodrat densities at Goose and Ashley
lakes crashed, and water levels in these two lakes dropped sharply. These two
study sites also are isolated from other mesic forest types which could potentially
limit woodrat dispersal.
 Negative effects from climatic changes such as drought on small mammal
populations can result from numerous factors including a decrease in food supplies,