James L. Dooley

Predation hazard and seed removal by small mammals : microhabitat versus patch scale effects

Predator avoidance may involve response strategies of prey species that are time and space specific. Many studies have shown that foraging individuals avoid predators by altering microhabitat usage; alternatively, sites may be selected according to larger-scale features of the habitat mosaic. We measured seed removal by two small mammal species (Peromyscus leucopus, and Microtus pennsylvanicus) at 474 stations over an experimentally created landscape of 12 patches, and under conditions of relatively high (full moon) and low (new moon) predatory hazard. Our objective was to determine whether predator avoidance involved the selection of small-, medium-, or large-scale features of the landscape (i.e., at the scale of microhabitats, habitats, or habitat patches). We found rates of seed removal to vary more with features of whole patches than according to variation in structural microhabitats within patches. Specific responses included: under-utilization of patch edge habitats during full moon periods, and microhabitat effects that were only significant when considered in conjunction with larger-scale features of the landscape. Individuals residing on large patches altered use of microhabitats/habitats to a greater extent than those on smaller patches. Studies just focusing on patterns of microhabitat use will miss responses at the larger scales, and may underestimate the importance of predation to animal foraging behavior.

DEMOGRAPHIC RESPONSES TO HABITAT FRAGMENTATION: EXPERIMENTAL TESTS AT THE LANDSCAPE AND PATCH SCALE

We tested the ecological consequences of habitat fragmentation by comparing the density, population growth rate, survivorship, and recruitment of Microtus pennsylvanicus populations within a 20-ha fragmented landscape with those of populations in a 20-ha unfragmented landscape. We also tested for fragment-size effects by comparing the same measures of demographic performance across three fragment sizes (0.06, 0.25, and 1.0 ha). During 17 censuses between June 1993 and October 1994, we recorded 10020 captures of 3946 individuals and found strong landscape differences but weak fragment-size effects. Although fragmentation reduced the habitable area by 72%, density and adult recruitment were significantly higher on the fragmented landscape relative to the control. With the exception of adult recruitment (higher on small relative to medium and large patches), no significant demographic differences existed 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 enhanced local reproduction rather than from immigration. Thus, despite the fact that populations in the fragmented landscape experienced severe habitat loss, some individuals on fragments accrued important reproductive 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 that illustrates the importance of using hierarchical field designs in tests of population responses to large-scale habitat alteration. We conclude that controlled, large-scale field tests can serve as an important intermediary between the inherent abstraction of simulation modeling and what is observed in the real world.

Use of space and habitats by meadow voles at the home range, patch and landscape scales

Using capture/recapture methods, we examined the spatial usage patterns of Microtus pennsylvanicus within and between experimentally created habitat patches 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 tested the prediction that home ranges near patch edges would be qualitatively different from those in patch interiors, and that the edge:interior habitat ratio could be used to make predictions concerning the dispersion and spatial use of individuals occupying different sized patches and between landscapes with different habitat structure. We found adult females on patch edges to have larger and more exclusive home ranges, larger body sizes, longer residence times, and to reproduce at a higher frequency than those in patch interiors. These “edge effects” also appeared to be largely responsible for the greater proportion of larger, reproductive females we found in small than larger patches and in the fragmented than in the continuous habitat (control) landscape. The selection of higher quality edge habitats by dominant females and the relegation of sub-dominants to patch interiors provides an explanation for the observed differences in the distribution and performance of females over patches and between landscapes.

Controlled experiments of habitat fragmentation: a simple computer simulation and a test using small mammals

Habitat fragmentation involves a reduction in the effective area available to a population and the imposition of hard patch edges. Studies seeking to measure effects of habitat fragmentation have compared populations in fragments of different size to estimate and area effect but few have examined the effect of converting open populations to closed ones (an effect of edges). To do so requires a shift in spatial scope-from comparison of individual fragments to that of fragmented versus unfragmented landscapes. Here we note that large-scale, “controlled” studies of habitat fragmentation have rarely been performed and are needed. In making our case we develop a simple computer simulation model based on how individual animals with home ranges are affected by the imposition of habitat edges, and use it to predict population-level responses to habitat fragmentation. We then compare predictions of the model with results from a field experiment on Peromyscus and Microtus. Our model treats the case where home ranges/territories fall entirely within or partially overlap with that of sample areas in continuous landscapes, but are restricted to areas within habitat fragments in impacted landscapes. Results of the simulations demonstrate that the imposition of hard edges can produce different population abundances for similar-sized areas in continuous and fragmented landscapes. This edge effect is disproportionately greater in small than large fragments and for species with larger than smaller home ranges. These predictions were generally supported by our field experiment. We argue that large-scale studies of habitat fragmentation are sorely needed, and that control-experiment contrasts of fragmented and unfragmented microlandscapes provide a logical starting point.

Influences of Patch Size and Microhabitat on the Demography of Two Old-Field Rodents

We used capture/recapture methods to test the responses of two small mammal species (Peromyscus leucopus and Microtus pennsylvanicus) to small- (microhabitat) and large- (patch) scale habitat variation. Analyses examined the responses of individuals to microhabitat variation among trap stations as well as differences in the density and persistence time of adults and juveniles, and the proportion of reproductive females on experimentally created patches of three sizes (0.0625, 0.25, and 1.0 ha). With the exception of transient P. leucopus, all groups shared significant correlations with microhabitat at the scale of trap stations. By contrast, only juvenile P. leucopus exhibited a response to patch-size (i.e., higher densities on small relative to larger patches). Microhabitat differences among patches also accounted for variation in M. pennsylvanicus densities (but not P. leucopus) in analyses of covariance. Our results suggest that both individuals and populations of M. pennsylvanicus responded to habitat variation at the microhabitat scale, while P. leucopus appeared to respond to both microhabitat and patch scale habitat variation. We note that species characteristics (particularly relative dispersal ability) may prove critical in predicting the scale of habitat responses. We conclude by noting that current theory that assumes uniform responses of population to homogeneous patches is too simplistic to be of much predictive value in field tests.

Landscape composition and the intensity and outcome of two- species competition

A spatially explicit model of interspecific competition for two species that perform best in alternate habitat types is developed. Computer simulations of the model showed that the outcome of interspecific competition varied according to the make-up of the habitat mosaic. In general, competitive ability of species varied in proportion to the abundance of optimal habitat for that species as well as dispersal ability. Dispersal distance determined to what extent competition occurred locally (within patches) or regionally (at the level of patch networks).These results suggest that the intensity and outcome of interspecific competition reflects dispersal ability and patch composition as much as the intrinsic competitive abilities of species themselves.

An Experimental Examination of Nest‐Site Segregation by Two Peromyscus Species

The role of interspecific competition for nest sites in the coexistence of the white-footed mouse (Peromyscus leucopus noveboracensis) and the cloudland deermouse (Peromyscus maniculatus nubiterrae) was experimentally investigated in the montane for- ests of southwestern Virginia. Radiotelemetry was used on control grids, with both species present, and reciprocal removal grids to contrast nest-site selection with and without con- traspecifics present. On control grids, Peromyscus maniculatus used only arboreal nests while P. leucopus used both ground and arboreal nests. In addition, relative to P. manicu- latus, those P. leucopus which did select arboreal nests, used lower cavities and trees of smaller diameter at breast height (dbh). These results provide strong evidence for vertical nest-site segregation, and confirmed observations from previous studies. Though both species demonstrated shifts in nest selection between treatments in the removal experiment, the analysis of the P. maniculatus response was statistically suspect and both responses appeared to be ecologically insignificant (that is, shifts were not towards the type of nest site used by the removed species). Additionally, home range analysis and examination of nest availability suggest that large portions of suitable habitat on each grid may not have been used and that nest sites were not a limiting resource. We suggest, therefore, that interspecific competition for nest sites was probably not important to co- existence during the course of the study.

EMS Studies at the Individual, Patch, and Landscape Scale: Designing Landscapes to Measure Scale-Specific Responses to Habitat Fragmentation

Ecologists have long recognized that the habitats of species are patchy at some spatial scale, that habitats vary in quality, and that most populations exhibit substantial demographic and spatial structure that is, in some way, related to this variability. Inequalities in the capacity of local habitat patches to produce or absorb dispersing individuals creates a regional population dynamic with a hierarchical demographic component—birth and death schedules often reflect local conditions, and the emigration and immigration of individuals over sites reflects regional or landscape ones (Pulliam 1988, Tilman et al. 1994). Differences in the quality of local habitats tend to increase the variability in species abundances over sites, whereas dispersal (immigration and emigration) of individuals among sites can even it out (Vance 1984, Bowers and Dooley 1991). Connecting processes working at the local, metapopulation, and regional scales has become a goal of landscape ecology (Forman and Godron 1986, Turner 1989, Noss 1991, Wiens 1995). Hence, despite the original intent of landscape ecology to be holistic (Forman and Godron 1986, Pickett and Cadenasso 1995), predictions about landscape-level processes are becoming increasingly specific and mechanistic (Wiens et al. 1993).

Female Aggression in Microtus pennsylvanicus: Arena Trails in the Field

-The relative importance of siteand individual-specific aggression between resident and nonresident adult female Microtus pennsylvanicus was investigated using encounter trials in the field. Residents within their home range exhibited significantly greater aggression when paired with nonresident females and a trerid towards greater aggression when paired with resident females that were removed from their home range. Nonresident females exhibited significantly more aggression than residents when the trial was conducted at a neutral site. These results suggested a dual basis for female aggression with evidence for both siteand individual-specificity.