Cyclic dynamics of snowshoe hares on a small island in the Yukon

Abstract

Snowshoe hare (Lepus americanus) populations were monitored from 1977 to 2001 on Jacquot Island (5 km 2 ) in Kluane Lake, southwestern Yukon, and on nearby mainland sites. Jacquot Island hares averaged twice the density of mainland control populations and, although they show 10-year cycles, fluctuate with much lower amplitude than mainland populations. Three separate intensive studies over 6 years attempted to determine what caused these differences. We tested two hypotheses to explain the dynamics. Reproductive rates of hares were similar on Jacquot Island and the mainland. Adult survival rates were higher on the island in most years, with the exception of years of population decline. Juvenile survival rates from 0 to 30 days of life were much higher on the island than on the mainland except for decline summers. The adult-and juvenile-survival differences between the island and the mainland were explained most consistently by predation. Improved survival on the island is correlated with a reduction in the numbers and types of predators found on Jacquot Island compared with the mainland. In particular, red squirrels were rare on Jacquot Island, arctic ground squirrels were absent, and the larger predators, like lynx and great-horned owls, were sporadic in occurrence on this small island. 2 ) au Lac Kluane (sud-ouest du Yukon), ainsi qu'à des sites sur la terre ferme, à proximité. Les densités de liè-vres sur l'Île Jacquot équivalent en moyenne au double de celles des populations témoins sur la terre ferme et, quoique les premières fluctuent selon un cycle de 10 ans, leurs fluctuations sont d'une amplitude de beaucoup inférieure à celles des populations de terre ferme. Les causes de ces différences ont été examinées au cours de trois études intensives différentes s'échelonnant sur 6 ans. Nous avons testé deux hypothèses quant à la dynamique de ces populations. Les taux de reproduction sur l'île Jacquot et sur la terre ferme sont similaires. La plupart des années, le taux de survie des adultes est plus grand sur l'île que sur la terre ferme, sauf durant les années où les populations sont en déclin. Le taux de survie de 0 à 30 jours chez les juvéniles est beaucoup plus grand sur l'île que sur la terre ferme, excepté durant les étés de déclin. C'est l'effet des prédateurs résidants qui explique le mieux les différences de taux de survie entre les lièvres de l'île et ceux de la terre ferme, aussi bien chez les adultes que chez les juvéniles. Le meilleur taux de survie des lièvres sur l'île associé à une guilde de prédateurs plus pauvre en nombres et en espèces que celle des sites la terre ferme. Plus particulièrement, les écureuils roux sont rares sur cette petite île, les spermophiles arctiques y sont absents, et les plus gros prédateurs comme le lynx et le grand-duc ne s'y trouvent que sporadiquement. 1450Krebs et al.

Bad hare day: very low survival rate in brown hare leverets

“…Comparing to other lagomorph species, survival of brown hare leverets seems to be low (Table 3). For snowshoe hares -which have a similar lifestyle compared to brown hares -a similar survival rate has been found by O'Donoghue and Boutin (1995) in contrast to a much higher survival rate found by Krebs et al (2002) and O'Donoghue (1994, Table 3). According to these three studies on snowshoe hares, it seems that survival does not change very much after two weeks (day 14-28) whereas in brown hares there was a more or less constant increase of mortality from day 14 to day 28 (Table 3).…”

Tree Recruitment Limitation by Introduced Snowshoe Hares, <em>Lepus americanus</em>, on Kent Island, New Brunswick

“…The absence of significant predators allows introduced herbivores to reach high population densities and exert intense grazing pressure on poorly defended plants (Krebs et al 2002;Donlan et al 2002). Because woody plants are most vulnerable to browsing during their early stages of development and growth (Clark et al 1999;Liang and Seagle 2002), intense browsing may limit tree regeneration and allow less palatable species to proliferate (Rossow et al 1997;Heinen and Currey 2000).…”

Climate Change Effects on Plant-Pest-Natural Enemy Interactions

“…The classic example is the outbreak of locust when the confluence of rains increases host plant abundance in space and time, and favorable winds occur that carry building locust populations from one favorable place to another until massive swarms occur (Roffey and Popov, 1968). Rapid buildups in response to host plant availability may occur in some species (e.g., cowpea aphid; Gutierrez et al (1974); and African armyworms, see Janssen (1993)), while in other cases outbreaks develop more slowly due to cyclic changes in host plants over time [e.g., spruce budworm (Thompson et al, 1979); bark beetles (Rebetez and Dobbertin, 2004); some vertebrate species (e.g., snowshoe hare/lynx, Krebs et al, 2002) and others]. The capacity to model such systems realistically is critical if we are to assess the impact of climate change on native and invasive species, but these require sound models of the plant dynamics.…”