No abstract

The trophic dynamics of the Yukon boreal forest have been under investigation at the Kluane Lake Research Station since 1973. We monitored and conducted experiments on the major species in this ecosystem, except the large mammals (for logistic reasons). The central problem has been to determine the causes of the 9 -10 year cycle of snowshoe hares, and to achieve this we carried out several large-scale experiments manipulating food supplies, predator pressure, and soil nutrient availability to test hypotheses that food, predation, or habitat quality regulate populations. The hare cycle is driven top-down by predators, and most hares die because they are killed by predators. Predators also cause stress in female hares, and the stress response seems to be responsible for the loss of reproductive potential in the decline and low phases of the hare cycle. Many of the specialist predators and some herbivores in this ecosystem fluctuate with the hare cycle. Arctic ground squirrels do, but red squirrels do not, being linked closely to white spruce seed masting years. Small rodents fluctuate in numbers in two patterns. Red-backed voles and four species of Microtus voles have a 3 -4 year cycle that seems to be driven by food supplies and social behaviour. Deer mice, in contrast, have fluctuated dramatically in the 38 years we have monitored them, but not cyclically. White spruce seed production varies with temperature and rainfall, but was not affected by adding nutrients in fertilizer. Global warming and reduced hare browsing in the last 20 years have helped to increase the abundance of shrubs in these forests. It will be challenging to predict how this system will change as climatic warming proceeds, because even closely related species in the same trophic level respond differently to perturbations. We recommend continued monitoring of the major species in these boreal forests.

This study demonstrates that cataract removal by clear cornea phacoemulsification in glaucoma patients, glaucoma suspects, and normal patients results in a small but significant decrease in IOP that is sustained at 3 years and a mean of 5 years in all groups. This study does not imply that cataract removal by phacoemulsification is a substitute for a combined procedure but may be an appropriate procedure for certain patients based on medication requirements and extent of optic nerve damage.

Clear corneal phacoemulsification was associated with a statistically significant long-term reduction in IOP.

Coyotes and lynx are the two most important mammalian predators of snowshoe hares throughout much of the boreal forest. Populations of hares cycle in abundance, with peaks in density occurring every 8-11 yr, and experimental results suggest that predation is a necessary factor causing these cycles. We measured the functional responses of coyotes and lynx during a cyclic fluctuation of hare populations in the southwest Yukon, to determine their effect on the cyclic dynamics. We used snow-tracking and radio telemetry to examine changes in the foraging behavior of the predators.Coyotes and lynx both fed mostly on hares during all winters except during cyclic lows, when the main alternative prey of coyotes was voles, and lynx switched to hunting red squirrels. Both predators showed clear functional responses to changes in the densities of hares. Kill rates of hares by coyotes varied from 0.3 to 2.3 hares/d, with the most hares killed one year before the cyclic peak, while those of lynx varied from 0.3 to 1.2 hares/d, with the highest one year after the peak. Maximum kill rates by both predators were greater than their energetic needs. The functional response of coyotes was equally well described by linear and type-2 curves, and that of lynx was well described by a type-2 curve. Kill rates by coyotes were higher during the increase in density of hares than during the cyclic decline, while the reverse was true for lynx. Coyotes killed more hares early in the winter, and cached many of these for later retrieval. Lower densities of hares were associated with longer reactive distances of both predators to hares, but with little apparent change in time spent searching or handling prey. In summary, our data show that the two similarly sized predators differed in their foraging behavior and relative abilities at capturing alternative prey, leading to different patterns in their functional responses to fluctuations in the density of their preferred prey.