Southwestern Yukon Research Articles

Effects of food and predators on the home-range sizes of Arctic ground squirrel (Spermophilus parryii)

We used radiotelemetry to study the effects of food addition and predator reduction on the home-range sizes of adult Arctic ground squirrels (Spermophilus parryii) on large-scale experimental grids in the boreal forest of the southwestern Yukon Territory. Home ranges were 2-7 times smaller on food-supplemented grids than on nonsupplemented grids, regardless of whether large mammalian predators were present. Similarly, core areas (where 50% of activities occur) were 8-11 times smaller on food-supplemented grids. Food availability rather than predator presence primarily determined the sizes of home ranges and core areas of Arctic ground squirrels.

Population Limitation in Arctic Ground Squirrels: Effects of Food and Predation

1. We examined the relative importance of food and predators in limiting Arctic ground squirrel (Spermophilus parryii plesius Richardson) populations in the boreal forest of the southwestern Yukon during the peak and early decline of a snowshoe hare cycle (Lepus americanus Erxleben). 2. Squirrels were live-trapped from 1990 to 1992 on two control grids and three experimental treatments (food addition, mammalian and avian predator exclosure, and food addition plus mammalian predator exclosure). Adult squirrels were radiocollared on all areas in 1992. 3. Food addition increased densities 3-8 times, generally increased reproductive traits (increased proportion of females lactating, doubled recruited litter sizes, resulted in earlier emergence of juveniles), increased immigration rates (but only in 1992), resulted in heavier females though not males at emergence in spring, and resulted in more rapid growth rates of juvenile males, but not of juvenile females. It had no effect on active season or overwinter survival rates. 4. Exclusion of predators had virtually no effect on any demographic variable measured, except for population densities in 1991 when they were approximately double those of the control populations. 5. Food addition plus exclusion of mammalian predators resulted in demographic changes that were comparable to those of food addition alone. 6. Thus, it appeared that food, not predators, limited ground squirrel populations at this stage of the hare cycle. However, independent of experimental treatment, active season survival of adult squirrels declined markedly from 1990 (high hare numbers) to 1992 (low hare numbers). Most of the radiocollared squirrels disappearing in 1992 were killed by predators and this was coincident with high densities of predators. In the predator exclosures, all predation mortalities resulted from avian predators which we could not exclude. 7. We conclude that both food and predators interact to limit Arctic ground squirrel populations during the peak and early decline of the hare cycle.

First-year survival of great horned owls during a peak and decline of the snowshoe hare cycle

Most bird species have low survival rates in their first year of life, and the highest losses occur when juveniles become independent and disperse. Young great horned owls (Bubo virginianus), monitored by telemetry in the southwestern Yukon, Canada, survived well during the peak of the population cycle of snowshoe hares (Lepus americanus). Subsequently, juvenile survival collapsed parallel to the decline in hare densities. The proportion of starving owls did not increase, but there was a significant increase in mortalities involving parasitism and predation, probably as an interaction with food shortage. The mortality rates of juvenile great horned owls peaked before, not during, dispersal. We propose that extended parental care makes the postfledging stage safe during optimal conditions, but that the relatively slow development during this stage incurs the cost of increased susceptibility to disease and other mortality factors under environmental stress.

The Numerical Response of Great Horned Owls to the Snowshoe Hare Cycle: Consequences of Non-Territorial `Floaters' on Demography

The numerical response of great horned owls (Bubo virginianus Gmelin) to the 10-year population cycle of snowshoe hares (Lepus americanus Erxleben) in the boreal forest was examined during 1988-93 in the south-western Yukon, Canada. Demographic parameters were estimated based on censuses (territorial pairs), nest visits (productivity), and radio-telemetry (survival, emigration, and integration of young birds into the population). Hares rose to peak densities in 1990, and almost all resident owl pairs bred and raised large broods during years of increasing and highest prey abundance. In 1991, the first year of hare decline, all breeding parameters including post-fledging survival were reduced, and recruitment in autumn was very low. In 1992 and 1993, reproduction was completely suppressed. Survival of young owls in their first 2 years of life was high during the peak of the hare cycle, and a large number of non-territorial 'floaters' were present. These birds were silent, and moved more than territorial owls. Their ranges overlapped broadly with defended territories, and floaters were affected by the hare decline before territory holders. Most ecological studies on birds are based on the territorial fraction of a population. The results of this study show how a large proportion of secretive floaters can delay the detection of population declines in traditional censuses of territorial birds, and can lead to serious underestimates of the impacts of predation.

Brood size manipulations in Great Horned Owls Bubo virginianus: are predators food limited at the peak of prey cycles?

The reproduction of raptors strongly depends on food resources. It is unclear whether predators experience superabundant food during cyclic peaks of prey populations. In order to test this hypothesis, four pairs of Great Horned Owls Bubo virginianus with two young were subjected to brood size manipulations during high densities of cyclic Snowshoe Hare Lepus americanus populations in southwestern Yukon, Canada. Broods older than 35 days were temporarily enlarged by one, and then by two, young. No effects were observed when one owlet was added, but the addition of two young resulted in significant weight losses in manipulated broods. Females with enlarged broods moved farther from their nest sites at night, presumably reflecting increased hunting effort, and also spent less time near the nest during the day. Food additions to enlarged broods returned the parental behaviour to normal. We conclude that these large predators did not experience superabundant food at this stage of the breeding season during a peak in cyclic prey.

Tree-climbing by Arctic Ground Squirrels, Spermophilus parryii, in the southwestern Yukon Territory

Tree-climbing by Arctic Ground Squirrels, Spermophilus parryii, in the southwestern Yukon Territory

Efficacy of ivermectin against nematodes infecting field populations of snowshoe hares (Lepus americanus) in Yukon, Canada.

From July 1990 to February 1991, nematode numbers in free-ranging snowshoe hares (Lepus americanus) at Kluane Lake, southwestern Yukon, Canada, were manipulated by subcutaneous injection (0.4 mg/kg) of ivermectin. Three field experiments were conducted to determine the degree of helminth loss associated with a single administration of ivermectin; the length of time that ivermectin was effective in reducing worm numbers; and the effect of repeated ivermectin administration in reducing worm numbers. Numbers of the nematodes, Protostrongylus boughtoni and Nematodirus triangularis were reduced by approximately 80% 2 wk after treatment with a single dose of ivermectin, and were still significantly lower than controls at 4 wk. However, beyond 2 wk, ivermectin did not affect the rate of acquisition of new worms of either species. All treated groups contained one or more hares in which numbers of P. boughtoni and N. triangularis were not reduced. In addition, ivermectin had no effect on numbers of Trichuris leporis or Passalurus sp. Overall, ivermectin was not as effective against the nematodes of free-ranging hares as has been reported for nematodes of domestic and laboratory animals.

Analyses minéralogiques multivariées de sédiments du Wisconsinien supérieur au sud-ouest du Yukon

For this study, 84 samples of glacial and juxtaglacial sediments were collected in valleys of the Ruby Range and Aishihik Basin (southwestern Yukon). Analyses were conducted to quantify the heavy minerals in the sand fraction and to assess the petrography of the gravel fraction. A cluster analysis performed on the heavy mineral results showed five groups of deposits, three of which are related to local glaciers inside the Ruby Range, the other two being related to regional ice lobes of Kluane and Aishihik, respectively. The three groups of local sediments correspond to the three lithologies of the Ruby Range, i.e., the granitic batholith, the schists, and the alaskite. The sediments from the regional ice lobes are characterized by highly diversified mineralogy and petrography and the relative abundance of minerals from the Saint Elias Mountains, the main source of the ice lobes. A correspondance factor analysis performed on the heavy mineral results organizes samples and minerals into a triangular cloud, the three vertices corresponding to biotite, carbonate, and titanite–garnet. These are the key elements of local glaciers, Kluane ice lobe, and Aishihik ice lobe, respectively. Finally, the study shows the extension of Kluane lobe in one valley of the Ruby Range, the ice flow pattern in another valley, as well as a transfluence from Kluane lobe inside the Range.

Nitrogen fertilization stimulates herbivory by snowshoe hares in the boreal forest

We fertilized small plots in the boreal forest of the southwestern Yukon, Canada, to see how herbivory was affected. We assessed snowshoe hare (Lepus americanus) activity by counting pellets, and estimated grazing on grass as the proportion of blades eaten. After 2 years of fertilization, snowshoe hares spent more time on and near the test plots with more fertilizer. More grass was grazed on those plots but not near them. Thus, herbivores were attracted to the general area of the fertilized plots but only grazed directly on the plots.

Evidence for bottom-up effects in the boreal forest: Do passerine birds respond to large-scale experimental fertilization?

Boreal plant communities are strongly nutrient limited, and the animals of the boreal forest may therefore experience bottom-up nutrient limitation. We conducted a 5-year experimental study of the impact of aerial nitrogen fertilization on birds of the boreal forest near Kluane Lake, southwestern Yukon, to test for such bottom-up effects. Specifically, we tested if avian abundance and species richness increased after fertilization. Variable circular-plot point counts were made to estimate bird numbers and species richness each summer from 1988 to 1992. Fertilization had no effect on abundance for the first two summers, but total abundances of the seven commonest passerine bird species increased by an average of 46% over the final 3 years. Fertilization had no effect on bird species richness. Population densities and species richness were both low at Kluane compared with patterns seen in temperate forest. Yellow-rumped warblers (Dendroica coronata), dark-eyed juncos (Junco hyemalis), and Swainson's thrushes (Catharus ustulatus) dominated the passerine community at Kluane. There was only moderate spatial and temporal variation in songbird numbers on control plots over the 5-year study period.

Prédation on red squirrels during a snowshoe hare decline

We examined the extent and impact of predation on red squirrels (Tamiasciurus hudsonicus) during a cyclic decline of snowshoe hares in the southwestern Yukon, Canada. We monitored survival of squirrels on three control grids and a predator exclosure from March 1991 through August 1993. On controls, adult survival during the breeding season decreased from 1991, when snowshoe hare populations were high, to 1992, when hare populations declined rapidly. Survival increased slightly in 1993, when hare and predator populations were very low. Similarly, adult survival during winter was lower in 1992 – 1993 than in 1991 – 1992. Adult survival on the exclosure remained similar in each breeding season but declined during winter 1992 – 1993. Adult survival was similar on the controls and the exclosure in each year except during winter 1991–1992 and the 1992 breeding season, when it was lower on the controls. There was no difference in juvenile survival between the controls and the exclosure. Despite the decrease in adult survival due to predation, there was no population decline on any of the control grids. We conclude that predation did not have a measurable impact on red squirrel densities at Kluane and that it is unlikely red squirrels show 10-year population cycles in conjunction with snowshoe hares.

Radiometric processing of aerial and satellite remote-sensing imagery

This paper describes some radiometric problems in remote-sensing imagery. Analysis methods are presented to correct or account for atmospheric, illumination, view-angle, and topographic effects prior to the application of image interpretation techniques such as reflectance mapping or automated classification. A key feature of some of these image corrections is the use of a digital elevation model (DEM) created from multispectral, stereoscopic System Probatoire d'Observation de la Terre (SPOT) High Resolution Visible (HRV) satellite imagery using an automatic DEM generation program. The accuracy and use of this model has been tested in automated terrain analysis of a moderate to high relief environment in southwestern Yukon, Canada, and is described here in the formulations for illumination and topographic corrections.

Northern Hawk-Owls in the Nearctic Boreal Forest: Prey Selection and Population Consequences of Multiple Prey Cycles

We studied hawk-owls in the southwestern Yukon, Canada, from 1987-1993. Most information on hawk-owls originates from studies in Europe, and very little is known about the subspecies Surnia ulula caparoch in North America. The boreal forest communities in the two continents differ remarkably in the composition of cyclic herbivore populations. Fennoscandia is dominated by 3-4 year microtine cycles, whereas northern Canada and Alaska experience a 10-year cycle in snowshoe hare numbers, with voles fluctuating at lower levels. We studied the diets of nine nesting pairs by pellet analysis, and we observed prey deliveries at five nests. The proportion of voles in the diets was lower than reported from Fennoscandia, and snowshoe hares made up 40-50% during the peak of the hare cycle. Estimates of prey densities by live-trapping revealed that hawk-owls strongly prefer voles over snowshoe hares and squirrels. Among voles, Microtus were preferred and Clethrionomys were avoided. Hawk-owls showed, however, a functional response not only to voles but also to juvenile hares, and they may be critically dependent on larger prey during certain nesting stages when vole abundance is moderate or low. Breeding densities and winter observations changed concurrently over years of different prey abundance. Prey selection translated into population consequences: hawk-owls did not respond numerically to Clethrionomys outbreaks, but to the combined densities of Microtus and snowshoe hares. We conclude that the Northern Hawk-Owl is less of a vole specialist and more affected by the prey composition in specific systems than commonly assumed, and we discuss this pattern from an evolutionary perspective

Bald Eagle, Haliaeetus leucocephalus, and Northern Goshawk, Accipiter gentilis, nests apparently preyed upon by a Wolverine(s), Gulo gulo, in the southwestern Yukon Territory

Bald Eagle, Haliaeetus leucocephalus, and Northern Goshawk, Accipiter gentilis, nests apparently preyed upon by a Wolverine(s), Gulo gulo, in the southwestern Yukon Territory

New geophysical data from the northern Cordillera: preliminary interpretations and implications for the tectonics and deep geology

In this paper we analyze recently acquired geophysical data from the northern Cordillera and their relation to the mapped geology. A prominent gravity high (> −45 mGal (1 Gal = 1 cm/s2)) coincides with a magnetic low and an aseismic region in west-central Yukon where the underlying geology is dominated by quartzo-feldspathic rocks having moderate densities. Extension (~15%), magmatic underplating, and accretion of the anomalous region onto oceanic crust are three possible explanations.Magnetic, gravity, and seismicity data all show significant differences in the physical state of the crust on either side of the Tintina Fault and, together with geological data indicating large offset, suggest it was once a major crustal-scale strike-slip fault. The new gravity data also delineate an arcuate zone of steep gradients (up to 1.4 mGal/km) in the miogeocline, which may correlate with a west-dipping Proterozoic basement ramp mapped on deep seismic sections farther to the north and a transition from thin (east) to thick sediment cover (west). Seismicity data show that current tectonic activity is concentrated along the Pacific – North America plate margin in southwestern Yukon and adjacent Alaska and, although there is a marked decrease in activity inland of this margin, notable concentrations occur along the Denali Fault System and in the eastern miogeocline. There is a distinct absence of earthquakes in parts of the Selwyn Basin and in the northern Yukon–Tanana Terrane. Limited field studies suggest activity is confined to the upper 10–15 km of the crust.

Effects of nitrogen fertilization on several woody and nonwoody boreal forest species

The effects of three levels of fertilizer were tested on the growth of several woody and nonwoody plants from a boreal forest community in southwestern Yukon. The effects of fertilization were assessed by clipping ground layer vegetation and measuring twig growth at the end of the second summer. Over the 2 years of fertilization there were significant increases in growth over control levels for perennial grasses (Festuca altaica and Calamagrostis lapponica), two herbs (Epilobium angustifolium and Achillea millefolium), and two deciduous shrubs (Salix glauca and Betula glandulosa). However, the growth of white spruce trees (Picea glauca) increased only slightly in response to increasing nitrogen levels, and the evergreen dwarf shrub Arctostaphylos uva-ursi showed no response. Key words: fertilization, boreal forest, shrubs, grasses, herbs, nitrogen.

A 1200-km-long Eocene metamorphic-plutonic belt in the northwestern Cordillera: Evidence from southwest Yukon

Distinctive regional Eocene metamorphic-plutonic units of the Coast Belt in southwestern Yukon match similar units in southeastern Alaska and British Columbia, and possibly in the eastern Alaska Range, and together form a belt originally >1200 km long. The final high-grade metamorphic event in the Aishihik and Kluane assemblages between the Ruby Range batholith and the Denali fault zone occurred in latest Paleocene to early Eocene time. Structural, petrologic, and geochronologic evidence indicates that metamorphism occurred during a major period of west-southwest-vergent deformation and metamorphism. The final phase of consumption of the Kula plate at ∼43 Ma may be linked to the culmination of metamorphism, intrusion, and deformation along the early Cenozoic Cordilleran margin.

Volcanism in an arc-transform transition zone: the stratigraphy of the St. Clare Creek volcanic field, Wrangell volcanic belt, Yukon, Canada

The St. Clare Creek volcanic field in the southwestern Yukon overlies a tectonic transition in the Wrangell volcanic belt between subduction to the northwest in Alaska and transform faulting along the Duke River fault in the southeast. Two large polygenetic volcanic centres dominated the Miocene landscape of the St. Clare Creek field: the 18–16 Ma Wolverine centre and the 16–10 Ma Klutlan centre. The Wolverine centre evolved from a small alkaline shield volcano at 18 Ma, from which alkaline basalts, hawaiites and mugearites erupted, to a larger composite volcano between 18 and 16 Ma composed of transitional basalt, basaltic trachyandesite, trachyte and rhyolite lavas, and pyroclastic rocks. The youngest Wolverine lavas are calc-alkaline basaltic andesites, andesites, and hybrid lavas (transitional–calc-alkaline). This temporal progression from alkaline through transitional to calc-alkaline volcanism is accompanied by a systematic increase in the degree of silica saturation and decrease in Fe/Si, Nb/Y, and P/Y ratios. Klutlan lavas have lower Nb/Y and P/Y ratios and are characterized by an opposite eruption sequence. The earliest Klutlan lavas (16–13 Ma) erupted from a composite volcano and include calc-alkaline andesite, rhyolite, and hybrid trachyandesite lavas, followed by transitional basaltic trachyandesites, trachyandesites, trachytes, and rhyolites. Klutlan vulcanism between 13 and 11 Ma was dominated by basaltic fissure eruptions on the southern flanks of the earlier centre and include early mildly alkaline basalts followed by more voluminous transitional basalts. Volcanism reverted to a more central type of activity between 11 and 10 Ma and includes calc-alkaline dacite lava followed by transitional basaltic trachyandesite, trachyandesite, and trachyte lavas.The volcanic stratigraphy of the St. Clare Creek field and 40Ar/39Ar geochronological data provide the basis for understanding the origin of St. Clare magmas in a regional tectonic context. Early Wolverine alkaline volcanism largely reflects leaky transform faulting, whereas subsequent transitional and calc-alkaline lavas record the onset of subduction-related volcanism at the margins of the then active Wrangell arc. The opposite eruption sequence at the Klutlan centre records the demise of subduction-related volcanism between 16 and 13 Ma, due to northwestward migration of the subducted plate. Upwelling of asthenospheric mantle in place of the subducted slab led to the generation of transitional basalts between 13 and 11 Ma, which resulted in more evolved lavas between 11 and 10 Ma.

Population dynamics of Clethrionomys and Peromyscus in southwestern Yukon 1973‐1989

Clethrionomys rutilus and Peromyscus maniculatus occur together in the boreal forest of southwestern Yukon and we studied their population dynamics on unmanipulated live‐trapping grids for 17 yr. Peromyscus showed a regular seasonal change in numbers each year with little interannual variation. Clethrionomys by comparison, showed low density fluctuations during one period (1976‐1982) followed by changes in number in the next period that are consistent with a typical 3‐4 yr microtine cycle. During one of two years of peak numbers adult males had unusually heavy body weights. The sexual maturation of juvenile Clethrionomys varied inversely with density. In most years Clethrionomys showed higher rates of population increase from spring to late summer than Peromyscus and this may be partly attributed to the vole's longer breeding season. Juvenile female Clethrionomys often reached sexual maturity during their first summer when population density was low but Peromyscus never matured during their first summer. Finally, there was no correlation between the two species in year to year changes in overwinter survival rates. This is the first report of a cyclic Norh American population of Clethrionomys and it underlines the need to search for hypotheses of population regulation that explain both annual and multi‐annual cycles within the same population.

Spatial and spectral classification of remote-sensing imagery

Abstract The use of spatial satellite image information and digital elevation models in remote-sensing classification is described for a mountainous region in southwestern Yukon. A three-stage classification method was devised that incorporates a quadtree-based segmentation operator, a Gaussian minimum distance to means test, and a final test involving ancillary topographic data and a spectral curve measure. The overall improvement in accuracy is significant compared to simple multispectral techniques, and the resulting map products are consistent with few unclassified areas. The three-stage classifier can produce an output map in significantly less time than that required for per-pixel maximum likelihood classifiers, and uses a minimum of field or training data which may be difficult and expensive to acquire in complex terrain. Programs to handle spatial and spectral attributes are coded efficiently in the C programming language. They can be adapted to locate homogeneous regions in high resolution aerial imaging spectrometer data sets (down to 0.1 m pixel resolution) or other raster databases.