Karl W. Larsen

Movements, survival, and settlement of red squirrel (Tamiasciurus hudsonicus) offspring

Movement and settlement patterns of animal offspring, along with the costs of occupying familiar and unfamiliar habitats, have been inferred frequently, but rarely have they been documented directly. To obtain such information, we monitored the in- dividual fates of 205 (94%) of the 219 offspring born over 3 yr in a population of the North American red squirrel (Tamiasciurus hudsonicus), at Fort Assiniboine, Alberta, Canada (54.20? N, 114.45? W). We located neonates by radio-tracking mother squirrels, and there- after we documented the movements, survival, and settlement patterns of the offspring, using a combination of telemetry, live-trapping, and visual observations. Prior to settle- ment, offspring made forays of up to 900 m (X = 126 m) off the natal territory, but they did not abandon the natal territory until they had settled on their own territory. Foray distance was not related to the age or size of the offspring. We used the locations of offspring kills to show that the risk of predation significantly increased when the offspring were travelling off of their natal territories. Just under half of the 73 offspring that acquired territories did so on or immediately adjacent to their mothers; the farthest settlement distance was only 323 m from the natal territory, or about the distance of three territory widths. Movement data from adults in the population showed that all offspring settled within potential contact of their mother (and possibly their father). Offspring that settled relatively farther away from their natal territory were more likely to obtain larger territories, with traditional hoarding and overwintering sites (middens). These offspring also had higher overwinter survival, suggesting that the costs of making forays off the natal territory may be balanced by the advantages of locating a superior territory.

DOES DENSITY REFLECT HABITAT QUALITY FOR NORTH AMERICAN RED SQUIRRELS DURING A SPRUCE-CONE FAILURE?

Abstract Predicting animal populations over time often is done using models of density–habitat relationships that assume animal density is a reflection of habitat quality. We explored whether this assumption was true for the North American red squirrel (Tamiasciurus hudsonicus), a species present at different densities in 3 conifer habitats: white spruce (Picea glauca; high resource quality, unstable availability), mixed conifer, and lodgepole pine (Pinus contorta; lower resource quality, stable availability). We documented density, body condition, survival, reproduction, and turnover in these 3 habitats during a failure of spruce-cone crop, when the potential for the relative importance of habitats to change was greatest. During this time squirrel densities in white spruce decreased by 66% to match with those found in pine and mixed-conifer forests. Red squirrels in spruce forests experienced lower survival and fewer females successfully weaned young, and juvenile production was lower. Adult and juvenile immigration was more important than local juvenile production in replacing squirrel mortality in spruce and mixed-conifer forests. Our results indicate that density does not always reflect habitat quality for red squirrels, and we question the historical high-quality rating of white spruce habitat for this species. Further, our findings suggest that movement of ostensibly highly territorial adults in late winter and early spring is an important mechanism in determining annual squirrel densities regardless of habitat type.

Does food availability affect growth and survival of males and females differently in a promiscuous small mammal, Tamiasciurus hudsonicus?

In dimorphic mammals, males grow faster than females, but often suffer higher mortality during periods of resource shortage. We compared growth and survival of males and females in a promiscuous small mammal with a relatively small degree of dimorphism (male/female body mass ratios ranged from 1.05 to 1.11). We compared two geographically distinct populations over 4 years during which natural food levels varied considerably. We also experimentally supplemented and reduced food levels

ECOLOGY OF NORTH AMERICAN RED SQUIRRELS ACROSS CONTRASTING HABITATS: RELATING NATAL DISPERSAL TO HABITAT

Abstract Because natal dispersal affects both individual fitness and population persistence, it is important to understand how dispersers are affected by habitat heterogeneity. To explore the effect of habitat on dispersal, we compared the ecology and natal dispersal of red squirrels (Tamiasciurus hudsonicus) originating from mature forest and adjacent commercially thinned forest. Because individuals living along the edge between the 2 forest types were more likely to have experience in both habitats, we classified squirrels according to habitat type (mature or thinned) and position (edge or deep within forest). Using livetrapping and radiotelemetry, we compared 4 habitats in terms of juvenile settlement patterns, surrogate measures of fitness, and population demography. Mature forest appeared to represent the highest quality habitat: mean density, mean overwinter survival, probability of surviving the field season, and success at raising ≥1 juveniles to emergence were higher in mature forest. However, the majority of juveniles from all habitats settled close to their natal territory, and with the exception of juveniles living along the edge of mature forest, juveniles settled within their habitat of origin. Juveniles living along mature edge biased their settlement for deep within mature forest. It appears that dispersal outcomes were affected by a combination of experience and opportunity. There are few, if any, other studies that have simultaneously compared demography, dispersal movements, and settlement patterns across contrasting habitats. While rare, studies such as this that link individual behavior and population theory are vital to effective population and landscape management.

Constraints on first reproduction in North American red squirrels

Age at first reproduction influences lifetime reproductive success of individuals and growth rates of populations, and is thus of general interest to ecologists. In red squirrel (Tamiasciurus hudsonicus) populations, yearlings and older nulliparous females (age two and above) are less likely to have a litter than multiparous females (i.e. those that have bred before). To explain these life history traits we tested several hypotheses for why particular females fail to breed in a given year. In a five-year study of red squirrels in a jack pine (Pinus banksiana) forest in central Alberta, Canada, the probability of primiparity (i.e. producing a litter for the first time) was correlated with cone crop size, but doubling the caches of cones available to females failed to increase the probability of first time reproduction. Cones were never completely depleted on any territory, so first reproduction rarely appeared to be restricted by absolute food availability on a territory. We hypothesize, that first reproduction is constrained by the squirrels ability to obtain seed energy enclosed in serotinous cones that vary in distribution on a territory. We found that the probability of primiparity is lower and more variable between years than that of multiparity, because foraging and cone-handling efficiencies vary more in younger than in older squirrels. Nulliparous females given sunflower seeds to reduce handling time were two to six times more likely to produce a first litter than controls extracting seed from jack pine cones. Small body size, inexperience with seed extraction from serotinous cones, and a lack of strategies for gathering and using cones from a new territory are supported as mechanisms that constrain primiparity, especially in yearlings. Cold spring temperatures added a further constraint in one year, as did absolute cone crop size. Multiparous females strip the bracts off cones in the autumn, reducing handling costs in winter and spring, while young nulliparous females lacked such behavioural compensation. In jack pine, reproductive success of red squirrels depends upon behaviour adapted to a seed supply that is abundant but costly to extract from serotinous cones.

Anticipatory parental care: acquiring resources for offspring prior to conception

Many organisms acquire and defend resources outside the breeding season and this is thought to be for immediate survival and reproductive benefits. Female red squirrels (Tamiasciurus hudsonicus) acquire traditional food cache sites up to four months prior to the presence of any physiological or behavioural cues associated with mating or offspring dependency. They subsequently relinquish these resources to one of their offspring at independence (ten months later). We experimentally show that acquisition of these cache sites cannot be explained by conventional arguments such as enhanced survival of the parent or increased reproductive output. Instead this behaviour functions as a form of parental care.

Songbird communities in a pyrogenic habitat mosaic

Wildfires play a key role in shaping the boreal forest landscape, yet the response of wildlife to the patchy mosaics they create is poorly understood. We studied songbirds 5–6 years post-fire in a large burn (9600 ha) in the boreal mixed wood forest of north-eastern Alberta. In the spring of 1995 and 1996 we estimated abundance of songbirds in four areas, each with four replicate sites: unburned patches within the fire (Isolates); burned patches (Burns); patches that had been clear-cut prior to burning by wildfire (Cut-Burns); and the unburned, continuous forest adjacent to the burn (Peripherals). We also sampled shrub-based arthropods with sweep-nets at each site. To investigate the role of Isolates, we compared them to Peripherals and to Burns. We compared Cut-Burns to Burns to examine the effect of logging prior to burning. In general, Isolates supported higher numbers of species and individuals than Peripherals, mainly due to higher numbers of aerial-foraging birds. Isolates and Burns had similar species richness and abundance, but Burns supported more aerial foragers while Isolates had more foliage gleaners. Cut-Burns generally supported fewer individuals than Burns, in particular fewer shrub-nesters, aerial foragers, and neotropical migrants. More bird species reached their highest densities in Burns than in any other area. Songbird species richness and abundance were positively related to dry arthropod biomass, with Burns and Isolates having the highest arthropod biomass. These patterns indicated that, 5–6 years post-burn, the patchy mosaic created by this wildfire supported more species than the mature forest surrounding the fire. Clear-cut logging prior to burning resulted in a diminished songbird community compared to that found in burned stands.

Effects of Hoard Manipulations on Life History and Reproductive Success of Female Red Squirrels (Tamiasciurus hudsonicus)

We conducted a series of hoard manipulations on the red squirrel ( Tamiasciurus hudsonicus ), a solitary species that constructs central food caches, to determine if the size and content of the hoard influenced the life-history parameters and reproductive success of females. Larderhoards of jack pine ( Pinns banksiana) cones were experimentally increased in autumn of 1989 and 1990, and decreased in autumn 1989. In both years we also supplied sunflower seeds (a food item with high energy content and low handling time) to individual females. Our experiments had no significant effect on the survival of females (prior to parturition or after weaning), prepartum or postpartum body mass, size of litters, number of offspring emerging from nests, or number of offspring weaned. Parturition date was significantly earlier ( P = 0.003) in females that received additions of seeds in the 1st year of the experiment. We also determined that control females harvested only a small fraction of the jack pine cones available on their territories. Our observations suggest that availability of food was not placing constraints on females during the years of our experiment. We also discuss possible reasons why dates of parturition (and estrus) were relatively more responsive to levels of food than our other measurements.

Aspen Stands as Small Mammal “Hotspots” Within Dry Forest Ecosystems of British Columbia

Forest stands of trembling aspen (Populus tremuloides) have been shown to support relatively abundant and diverse faunal communities, but this potential has not been extensively explored within the dry interior forests of British Columbia, Canada. These forests are primarily composed of conifers, particularly stands of Douglas-fir (Pseudotsuga menziesii), with only a small proportion consisting of trembling (or quaking) aspen. During 2005 and 2006, we live-trapped and compared small mammal assemblages within rare aspen stands to those in neighboring Douglas-fir and mixed-wood (aspen + Douglas-fir) stands. We captured a total of 4246 individuals of 10 small mammal species during 15,761 trap nights—with 54% of individuals captured in aspen stands. Commonly captured species included the deer mouse (Peromyscus maniculatus), southern red-backed vole (Myodes gapperi), long-tailed vole (Microtus longicaudus), montane vole (Microtus montanus), meadow vole (Microtus pennsylvanicus), yellow-pine chipmunk (Tamias amoenus), and common shrew (Sorex cinereus). Small mammal densities were most often higher within aspen stands than mixed-wood and Douglas-fir stands, as were species richness indices. Aspen stand communities also had consistently higher mean proportions of reproductive adult females and a higher proportion of juveniles. These results illustrate the importance of aspen stands as small mammal “hotspots” within dry forests, such as those found in British Columbia.

Small mammal communities in a pyrogenic habitat mosaic

We studied the small mammal community across a mosaic of habitats created by a large wildfire in the mixed-wood boreal forest of Alberta, Canada, 5 years after the fire occurred. We focussed on four habitat types within this landscape mosaic, namely burnt stands, stands of unburnt forest within the burn, unburnt forest on the periphery of the fire, and areas harvested before the fire (and subsequently burnt). The abundance of the two most common species – red-backed voles (Clethrionomys gapperi) and deer mice (Peromyscus maniculatus) – often differed inside v. outside the burn’s perimeter; however, reproduction, survival and abundance showed little to no correlation with habitat. Year-to-year changes in the relative abundance of these two species appeared greater within the burn’s periphery; the heterogeneity of the burnt landscape also supported a higher diversity of small mammal species than seen at the periphery. Comparison of our results with those collected by a coincidental study of forest harvesting suggests that the responses of the communities and populations of the animals to the two disturbance types were relatively similar. The value of long-term and chronosequence studies notwithstanding, detailed study of the wildlife communities shaped by individual wildfires improves our overall understanding of the ecological effects of natural and anthropogenic disturbances.