Small Mammal Density Research Articles

Patterns of Small Mammal Density and Survival Following Ski-Run Development

Abstract We investigated short-term effects of ski-run development on the dynamics of small mammal populations at Vail Ski Area, Colorado. We compared a new ski run, an experimental ski run with added woody debris, a forest adjacent to a new ski run, and a control forest outside ski development by estimating density and survival of common small mammals using Pollock's robust design. In 4 summers (1998–2001), 16,800 trap nights resulted in 1,276 captures of 668 individuals. Before ski-run development, Clethrionomys gapperi was most abundant in forested areas, but after, density was greatest in the forested site adjacent to a new ski run and next highest on the experimental ski run. C. gapperi survival was similar across sites and years. Peromyscus maniculatus and Tamias minimus densities were greatest on the ski run without woody debris and lowest on the forested control site. Estimated survival of T. minimus varied more by year than by site. Greatest densities of Phenacomys intermedius occurred on the 2 s...

Population dynamics of small rodents in the southern boreal forest as influenced by furbearer harvest

:We tested four predictions derived from the hypothesis that the guild of avian and terrestrial predators should regulate small mammal populations at low densities in relatively productive ecosystems of the southeastern part of the North American boreal forest. We estimated the density of two small mammal populations and the abundance of their predators for 3 y in two adjacent areas that differed with respect to trapping of furbearers. We predicted 1) that small mammal densities would not show multi-annual fluctuations in the two areas and 2) that densities would be much lower than peak densities reached by cyclic populations. Due to the harvest of furbearers in one area, we also predicted 3) that population densities and 4) survival of small mammals would be higher in the trapped than in the protected area. In agreement with our first two predictions, densities of the two main small mammal species (southern red-backed vole, Clethrionomys gapperi, and deer mouse, Peromyscus maniculatus) showed little variation between 1998 and 2000 in both areas, and additional data from the two previous years in the trapped area indicated that densities remained relatively stable and low over the 5-y period. However, contrary to our last two predictions, densities of the two most abundant species did not vary between areas, nor did survival rate of red-backed voles, the only species for which this variable could be estimated. Our findings show that the effect of trapping did not cascade down to small mammals. We have some evidence based on snow-tracking data that trapping, which focused on large generalist predators, may have relaxed intra-guild predation on small specialist predators and favoured an increase in abundance of specialist predators. Accordingly, we cannot reject the idea that a predation-based mechanism maintains small mammal densities at relatively low and stable levels in productive forests.

Movement distances and density estimation of small mammals using the spool-and-line technique

Edge effect is an inherent problem when using trapping grids to estimate density of small mammals, resulting in a sampling area larger than the area of the grid. Distances between captures of individuals are used to estimateA(W), the effective sampling area of a trapping grid, but grid size sets a limit for the largest detectable distance. The spool-and-line technique is proposed here as a new method to estimateA(W). Movement distances based on the spool-and-line technique were compared to similar movement distances based on capture-recapture of three species of marsupials of the Atlantic Forest of Brazil. Distances based on the two methods were uncorrelated, and only ln-transformed distances based on the spool-and-line were normally distributed. The maximum distance moved (MaxD) estimated by the spool-and-line was chosen as the more accurate and practical distance to estimate edge effect. Estimates of the effective sampling area and densities for the common opossumDidelphis aurita (Wied-Neuwied, 1826), were compared using MaxD based on spool-and-line (MaxDs p o o l ) , capture-recapture (MaxDcap), and also the distance between traps (DT). MaxDspool reflected more accurately density variation between seasons. Movement distances of small mammals based on the spool-and-line technique permit more accurate estimates of density and its dynamics.

Low density of small mammals at Las Cuevas, Belize

Summary Small mammal ( Marmosa mexicana , Heteromys desmarestianus and Ototylomys phyllotis accounted for 68% of mammals ( Didelphis marsupialis dominated captures of mammals >200 g. Trap success increased with litter fall across grids. Trap success and density tended to be higher at the onset of the dry season. More females were breeding and more animals were in poor body condition at the onset of the wet season. Across studies using grid trapping in the neotropics, diversity and density were positively related to trapping effort. Despite this effect, small mammal densities at Las Cuevas were 2.7 per ha for mammals Didelphis densities had a negative effect on densities of all other small mammals across studies; however, this relationship was driven largely by our study. A combination of poor soil, high Didelphis densities, and high predator densities is hypothesized for such low densities of small (

Predation of Gypsy Moth (Lepidoptera: Lymantriidae) Pupae in Three Ecosystems Along the Southern Edge of Infestation

The predation potential of small mammals, in particular mice, Peromyscus spp., and invertebrates, was evaluated from 1992 to 1995 near the leading edge of gypsy moth, Lymantria dispar (L.), spread into the southeastern United States. Two study sites were established in each of three geographic areas: the coastal plain, Piedmont, and mountains. All sites were mixed hardwood stands with varying amounts of oak, Quercus spp., and all were classified for gypsy moth susceptibility. Small mammal density was estimated using Sherman live-traps and pitfall traps within these 4.68-ha sites in early and late summer. Each site contained 75 trapping stations located on a 25-m grid. Predation was measured by offering freeze-dried gypsy moth pupae near trapping stations at four heights (0, 0.25, 1.0, and 2.0 m) on different tree boles. Pupal predation was monitored for three consecutive nights. Vertebrate predation was positively correlated with good mast production in the previous autumn. Predation data showed that when mice were at high densities they were the major source of pupal predation. However, within these southern sites, when densities of Peromyscus spp. were low, predation by invertebrates was occasionally greater than predation by vertebrates. These data suggest that in some years invertebrates may retard gypsy moth buildup when small mammals are scarce due to mast crop failures.

Landscape structure, habitat fragmentation, and the ecology of insects

Landscape structure, habitat fragmentation, and the ecology of insects

Mammal density and patterns of ectoparasite species richness and abundance.

Patterns of species richness, prevalence and abundance of ectoparasites have rarely been investigated at both the levels of populations and species of hosts. Here, we investigated the effects in changes in small mammal density on species richness, abundance and prevalence of ectoparasitic fleas. The comparative analyses were conducted for different small mammal species and among several populations during a long-term survey. We tested the hypothesis that an increase in host density should be linked with an increase in parasite species richness both among host species and among populations within host species, as predicted by epidemiological models. We also used host species density data from literature. We found that host density has a major influence on the species richness of ectoparasite communities of small mammals among host populations. We found no relationship between data of host density from the literature and parasite species richness. In contrast with epidemiological hypotheses, we found no relationships between abundance, or prevalence, and host density, either among host species or among host populations. Moreover, a decrease in abundance of fleas in relation with an increase in host density was observed for two mammal species (Apodemus agrarius and A. flavicollis). The decrease or the lack of increase in flea abundance in relation with an increase in host density suggests anti-parasitic behavioural activities such as grooming.

HABITAT USE BY BLACK RAT SNAKES (ELAPHE OBSOLETA OBSOLETA) IN FRAGMENTED FORESTS

Declining nest success of forest birds in fragmented habitat has been attri- buted to increased nest predation. Better understanding of this problem and potential so- lutions to it require information on why nest predators are attracted to habitat edges. Toward this end we investigated habitat use by black rat snakes (Elaphe obsoleta obsoleta), an important avian-nest predator in eastern deciduous forests. We radio-tracked 52 black rat snakes for periods of 3-41 mo from 1996 to 1999. All black rat snakes exhibited a strong preference for edge habitats. Consistent with edges being used because they facilitate thermoregulation, gravid females associated more strongly with edges than did males and nongravid females, and sites used by snakes when shedding were significantly associated with habitat edges. Gravid females lost an average of >20% of their body mass, while nongravid females and males did not lose mass, suggesting that edges were not used because they offered high success in foraging. Similarly, an increase in use of edge habitat through the season by all rat snakes was inconsistent with the snakes being attracted principally to hunt: avian prey would have been more abundant in spring when birds were breeding, and the density of small mammals in edges did not vary seasonally. Also, snakes moved longer distances and were found traveling.more often when located in forests. Because our results collectively are most consistent with the hypothesis that rat snakes use edges for ther- moregulatory reasons, the negative impact of the snakes on nesting birds may be coinci- dental; the snakes primarily use edges for reasons other than foraging but opportunistically exploit prey they encounter there. Rat snakes appeared to respond to the edge structure rather than to how the edge was created (natural vs. artificial). Thus, fragmentation of forests by humans has created habitat structurally similar to that preferred by rat snakes in their natural habitat, thereby inadvertently increasing contact between the snakes and nesting birds.

Resource availability and population dynamics of Nicrophorus investigator, an obligate carrion breeder

Summary1. Food resources for rearing young may influence insect populations. This is particularly true for insects that breed obligately on rare, ephemeral resources such as dung, fungi, or carrion.2. Beetles in the genus Nicrophorus bury small vertebrate carcasses for rearing their young. Studies reviewed by Scott (1998) have found a positive relationship between carcass mass and total brood size. It is likely that access to carcasses suitable for breeding, and not food or mates, limits reproduction in both male and female Nicrophorus. Thus, small mammal densities could determine Nicrophorus population sizes.3. The work reported here examined the relationship between Nicrophorus investigator (Coleoptera: Silphidae) population size and small mammal abundance at two sites over a 4‐year period.4. Nicrophorus investigator buried and reared young on all the native small rodent species trapped at two sites in south‐western Colorado, U.S.A. (Peromyscus maniculatus, Microtus montanus, Zapus princeps, Tamias minimus, Thomomys talpoides). They preferred to bury and reproduce on rodent carcasses weighing between 16 and 48 g; rodents of this size represented 82% of captures.5. Population sizes of N. investigator and small rodents were estimated simultaneously using mark‐recapture censuses over a 4‐year period. Considering only rodents within the size range used by N. investigator, the estimated small mammal biomass per hectare in one year and the beetle population size in the following year were correlated significantly.

Impacts of woodland deer on small mammal ecology

Summary The possible impacts of increased deer populations on small mammal ecology in British lowland woodlands are reviewed. These impacts occur mainly through two pathways. First, by the modification or removal of habitat which influences food supply, cover and the balance of competition between the species. Secondly, through direct competition for resources, particularly food supplies. Losses of woodland ground vegetation may provoke major changes in small mammal community structure which, in the extreme, may decrease from five to eight species, or more, to just wood mice (Apodemus sylvaticus). Reduction of understorey species such as hazel (Corylus avellana) or loss of shrub diversity, would be expected to lead to declines in yellow-necked mice (Apodemus flavicollis) and common dormice (Muscardinus avellanarius). Reduction of early succession mixed scrub and grassland may reduce colonization by shrews, harvest mice (Micromys minutus) and field voles (Microtus agrestis), although in woodland rides, field voles may benefit from preferential grazing of grasses by deer. Impacts on regeneration which may delay or inhibit canopy/understorey closure may encourage the ground and shrub vegetation and so favour species with a preference for ground cover such as the bank vole (Clethrionomys glareolus). Reductions in the availability of foods (e.g. mast crops) may reduce the potential for high population densities, particularly of bank voles, wood mice and yellow-necked mice, decrease the lengths of breeding seasons, and further affect community structure. If rodent densities are lowered, avian and terrestrial predators are likely to suffer reduced breeding success and tawny owls (Strix aluco) may prey more heavily on bank voles if their favoured ground cover is reduced. Studies at Wytham Woods, near Oxford, from 1949 to 1999, indicate that a significant reduction in bank vole, but not wood mouse, numbers has occurred; this is suggested to be the result of observed changes in the woodland vegetation, including a significant reduction in the ground cover of bramble (Rubus fruticosus agg.). Deer, as well as increased canopy shading, are probably the major factors causing this change in habitat and small mammal density.

INVENTORYING MAMMALS AT MULTIPLE SITES IN THE MAYA MOUNTAINS OF BELIZE

Inventories of mammals in the Neotropics usually are derived from specimens recorded at a single site, about 25 km 2 in area, but this may not represent the diversity of the whole region. To illustrate this point, presence of 42 species of nonvolant mammals was recorded in the Chiquibul Forest Reserve, an area of subtropical wet forest in the Maya Mountains of western Belize. Data on diversity and abundance of mammals at that site were compared with 3 other sites in the Maya Mountains to determine how measurements varied with location of site. Of the nonvolant species, ,33% were found at all 4 sites and .20% were found only at 1 site. Trapping success of researchers and densities of small mammals varied greatly between sites. Although number of species increased with research effort, the most extensively studied site did not encompass all mammalian diversity in the region. To obtain a representative mammalian inventory, effort should focus on sampling mammalian fauna at multiple sites, rather than increasing time at 1 site.

LANDSCAPE-LEVEL CORRELATES OF SMALL-MAMMAL ASSEMBLAGES IN FOREST FRAGMENTS OF FARMLAND

We studied effects of forest fragmentation on 15 species of small mammals, including 6 species of forest-dwelling granivorous rodents, in the Indian Pine watershed of west-central Indiana. Presence‐absence and population densities of small mammals were assessed in spring in 35 woodlots of various sizes (0.1‐150 ha) and 2 continuous forest sites ( .1,000 ha) using live traps in 1992‐1996. Presence‐absence and population density were related to landscape attributes using logistic and multiple linear regression models, respectively. Species richness of forest-dwelling small mammals increased with area and was highest in continuous forest sites. Nested subsets of the full complement of species were found in smaller woodlots. White-footed mice (Peromyscus leucopus) were ubiquitous, and eastern chipmunks (Tamias striatus) were nearly ubiquitous across the landscape; densities of both species were related inversely to forested area. Fox squirrels ( Sciurus niger) were found at 84% of study sites, and they did not respond negatively to isolation of forest patches. Red squirrels (Tamiasciurus hudsonicus) were unevenly distributed across the landscape and were found most often in woodlots with large core areas and simple shapes, possibly indicating sensitivity to edge. Southern flying squirrels ( Glaucomys volans) and gray squirrels (S. carolinensis) were restricted to continuous forest sites and .4.6-ha woodlots adjacent to other wooded habitat. Species of small mammals differ appreciably in their sensitivities to agriculturally induced fragmentation of forests. Interspecific differences within this assemblage were not due solely, or even primarily, to body size. Rather, differential responses of species to fragmentation likely resulted from variation in habitat breadth and ability to move through an agricultural matrix.

Small mammal trapping in tropical montane forests of the upper Nilgiris, southern India: an evaluation of capture-recapture models in estimating abundance.

Capture-mark-recapture was used to study small mammal populations in tropical montane forests in southern India. Eleven plots in six montane forest patches were sampled from February-October, 1994. Six species were captured, including four rodents and two shrews. PROGRAM CAPTURE was used to derive estimates of density of the most abundant species in the study area, Rattus rattus Linnaeus. The coefficient of variation of the density estimate was used as an index of precision. The coefficient of variation decreased exponentially with increasing capture probability and with an increase in trapping duration. The coefficient of variation and the capture probability were not correlated with estimates of density. The density estimate increased with trapping duration, as did trap mortality. The latter may have been due to the trend of increased mortality with recaptures of the same individual, which in turn may have been due to weight loss over consecutive captures. Estimates of density derived using four estimators were different for 2, 3, 4 and 5 days of trapping. The coefficient of variation was highest for the generalized removal estimate and lowest for Darroch's estimate. The models and estimators could not be applied to more than one species, and for this species, only in select habitats in a few seasons. Therefore, models of density estimation developed for temperate areas may not be suitable for tropical habitats due to low densities of small mammals in these habitats.

The influence of forest fragmentation and landscape pattern on American martens

1. We investigated the effects of forest fragmentation on American martens (Martes americana Rhoads) by evaluating differences in marten capture rates (excluding recaptures) in 18 study sites with different levels of fragmentation resulting from timber harvest clearcuts and natural openings. We focused on low levels of fragmentation, where forest connectivity was maintained and non‐forest cover ranged from 2% to 42%. 2. Martens appeared to respond negatively to low levels of habitat fragmentation, based on the significant decrease in capture rates within the series of increasingly fragmented landscapes. Martens were nearly absent from landscapes having > 25% non‐forest cover, even though forest connectivity was still present. 3. Marten capture rates were negatively correlated with increasing proximity of open areas and increasing extent of high‐contrast edges. Forested landscapes appeared unsuitable for martens when the average nearest‐neighbour distance between open (non‐forested) patches was <100 m. In these landscapes, the proximity of open areas created strips of forest edge and eliminated nearly all forest interior. 4. Small mammal densities were significantly higher in clearcuts than in forests, but marten captures were not correlated with prey abundance or biomass associated with clearcuts. 5. Conservation efforts for the marten must consider not only the structural aspects of mature forests, but the landscape pattern in which the forest occurs. We recommend that the combination of timber harvests and natural openings comprise <25% of landscapes ≥9 km2 in size. 6. The spatial pattern of open areas is important as well, because small, dispersed openings result in less forest interior habitat than one large opening at the same percentage of fragmentation. Progressive cutting from a single patch would retain the largest amount of interior forest habitat.

Influence of prairie dogs (Cynomys ludovicianus) on habitat heterogeneity and mammalian diversity in Mexico

Prairie dogs are considered to be both a keystone species and an ecosystem engineer in grasslands. To partially test these hypotheses we evaluated burrow densities, soil removal, and mammal (i.e. rodents and carnivores) species composition, richness, diversity, and abundance in grasslands with and without prairie dogs (Cynomys ludovicianus) in north-western Mexico. We measured habitat heterogeneity as a function of burrow density. As predicted, density of burrows was much higher in areas with prairie dogs. Soil mix was also much higher in prairie dog colonies. Grasslands with and without prairie dogs differed in small mammal species composition, richness, density, and diversity; four species were exclusively found in areas with prairie dogs. Interestingly, carnivore communities were similar in areas with and without prairie dogs. Our results support the hypothesis that prairie dogs and their activities enhance regional species diversity, and thus are an important component of the grassland ecosystem.

A METHOD FOR DETECTING DEW A TERING EFFECTS OF UNDERGROUND MINING ACTIVITIES ON SURF ACE WETLANDS

In 1996 US Energy/Kennecott Uranium Company initiated a large-scale, long-term monitoring program to document whether or not proposed uranium mining activities under Green Mountain in Central Wyoming would cause a groundwater draw-down resulting in changes in the existing riparian/wetland habitats. The monitoring program consisted of establishing 12 study drainages on Green Mountain and six control drainages on an adjacent but hydrologically isolated mountain not scheduled for mining. Baseline data were collected in 1996 and 1997 prior to the commencement of mining. For each drainage, breeding bird densities (birds/km) and richness (species/km), winter wild ungulate fecal pellet group densities (groups/km), small mammal densities (captures/trap night), and density and species composition of aquatic macro invertebrates were measured along permanent, marked transects within each riparian zone. In order to characterize the baseline vegetation and isolate the effects of livestock grazing, species composition, percent cover, production, and type boundary delineation of riparian vegetation were quantified within adjacent fenced and unfenced half-acre sample sites within each drainage. Baseline photographs were taken at permanent marked points from fixed angles at each of the sample sites. Piezometer holes were drilled at each monitoring site for measuring potential changes in ground water levels over time. If, during mining,more » water levels are found to drop significantly from baseline, a new study of wildlife and vegetative parameters would be conducted to determine whether or not significant decreases in wetland function or changes from baseline characteristics have occurred.« less

Fall and winter diet of Martens, Martes americana, in central Labrador related to small mammal densities

Fall and winter diet of Martens, Martes americana, in central Labrador related to small mammal densities

The structure of small mammal communities in some alpine habitats

Abstract We studied the composition of several small mammal communities living in different mountain and forest habitats of the central eastern Italian Alps. The small mammals were then grouped together, by cluster analysis, according to similarities in species and density. From the 22 stations investigated, five groups emerged, each one having also distinct environmental characteristics. We observed that spruce forest communities are grouped separately from those of mixed forests (larch and Swiss stone pine). We must stress the considerable difference existing between the small mammal communities living in different kinds of coniferous forests. The larch and Swiss stone pine forest seem to be able to support a greater density of small mammals, which includes in particular the bank vole ( Clethrionomys glareolus ).

Comparative study of Spanish and Italian terrestrial small mammal coenoses from different biotopes in Mediterranean peninsular tip regions

A comparison of terrestrial small mammal coenoses belonging to nine different biotopes in the tips of the Iberian and Italian peninsulas was carried out using the pitfall trapping method. The influence of both habitat type and peninsular effect on composition of small mammal coenoses was analysed. In Southern Italy, 203 specimens belonging to seven species were trapped: Suncus etruscus (Savi, 1822), Crocidura suaveolens (Pallas, 1811), C. leucodon (Hermann, 1780), Microtus savii (de Sélys‐Longchamps, 1838), Apodemus sylvaticus (L., 1758), A. flavicollis (Melchior, 1834) and Mus musculus domesticus Schwarz & Schwarz, 1943. In Southern Spain 428 specimens belonging to five species were trapped: Suncus etruscus, Crocidura russula (Hermann, 1780), Microtus duodecimcostatus (de Sélys‐Longchamps, 1839), Apodemus sylvaticus and Mus spretus Lataste, 1883. The relative density of small mammals occurring in the nine Spanish sampling stations was twice that recorded in the Italian stations; however the number of species recorded in the different biotopes show similar mean values, ranging from three to five in Andalusia and from three to six in Calabria. Apodemus sylvaticus was the dominant species in the Calabrian stations, whereas Crocidura russula prevailed in Andalusia. The biotic diversity values are very similar in the Calabrian and Andalusian biotopes. By contrast, the Insectivora/Rodentia ratio was always higher in Andalusia. The more xerophytic biotopes showed greater similarities between the communities in Southern Spain and Southern Italy, while the cooler biotopes differed between these two peninsulas.

Range size and habitat use of an adult male caracal in northern Saudi Arabia

Between March 1996 and January 1997 an adult male caracal was radio-tracked in the northern steppe desert of Saudi Arabia. Total home range, using the 95% minimum convex polygon method from 65 locations, increased from 270 km2in winter and spring to 448 km2by the end of summer, reaching 1116km2by January 1997. The caracal fed on camelCamelus dromedarusand gazelleGazella subgutturosacarcasses, and killed and ate a steppe eagleAquila nipalensis; the faeces contained rodent bones (Merionessp. andGerbillussp.). Diurnal resting areas had higher densities of small mammals than random sites. Low prey biomass in a very arid environment probably necessitated the large movements made by this individual.