We used stable carbon and nitrogen isotope compositions of hair and liver as a way of examining seasonal diet changes and explaining seasonal breeding in deer mice (Peromyscus maniculatus (Wagner, 1845)). Summer and winter d 13 C values differed, which is attributed to the availability of different plant tissues (C 3 plants). The d 15 N values of liver showed a decrease in consumed animal protein during winter, but the difference was not large enough to indicate a full trophic level change in diet from summer to winter. The d 15 N values of hair remained constant across the seasons, which is attributed to a continuous level of moulting throughout the year. Our data indicate that lowered food quality in the form of reduced dietary animal protein intake may play a role in the cessation of breeding in deer mice in winter.
Résumé :Les compositions des isotopes stables de carbone et d'azote dans le poil et le foie nous ont servi à examiner les changements saisonniers de régime alimentaire et à expliquer la reproduction saisonnière chez les souris du crépuscule (Peromyscus maniculatus (Wagner, 1845)). Il y a des différences entre les valeurs estivales et hivernales de d 13 C, ce qui s'explique par la disponibilité de tissus végétaux différents (plantes C 3 ). Les valeurs de d 15 N du foie indiquent une diminution de la consommation de protéines animales durant l'hiver, mais la différence n'est pas suffisamment importante pour représenter un changement complet de niveau trophique du régime de l'été à celui de l'hiver. Les valeurs de d 15 N du poil demeurent stables au cours des saisons, ce qui est expliqué par un niveau constant de mue au cours de l'année. Nos données indiquent que la qualité réduite de la nourriture à cause de la diminution d'ingestion de protéines animales dans le régime peut jouer un rôle dans l'interruption de la reproduction chez les souris du crépuscule en hiver.[Traduit par la Rédaction]
We recorded seasonal moulting in North American Deer Mice (Peromyscus maniculatus) in the Kananaskis River valley, Alberta, Canada, to test the hypothesis that moulting is restricted to the times of year that do not overlap with other high-energy demands, such as reproduction (spring and summer), or with low nutrient availability (winter). Although a seasonal trend showing a peak in moulting in the post-breeding period provided support for our prediction, a low level of continuous moulting did occur throughout the year.
Summary
The causes of bark beetle outbreaks – particularly the role of disturbances – are poorly understood. Stand‐scale disturbances, like fires, can suddenly improve local host susceptibility and may attract beetles; however, whether such increases can lead to outbreaks in post‐disturbance stands is unclear.
Using low‐density Dendroctonus ponderosae mountain pine beetle populations in Pinus contorta lodgepole pine forests in western Canada, we investigated whether prescribed fires promote outbreaks or provide only short‐term resources.
Proportionally more burned than non‐burned trees were attacked. At one site, beetle attacks increased in response to a resource pulse, but the proportions of attacked trees and numbers of attacks per tree declined over four years after fire. Elsewhere, beetle attacks remained very low.
As the resource (phloem) quality of burned trees remained high three years after fire, we propose that post‐fire mortality, resulting in fewer available host trees, can at least partially explain why D. ponderosae did not build up populations in burned stands.
Synthesis and applications. Our study emphasizes the importance of examining long‐term trends in fire–bark beetle interactions, and of understanding low‐density beetle populations. Because fire does not seem to promote mountain pine beetle outbreaks, we recommend the continued use of prescribed fire for the general management of P. contorta forests with low‐density beetle populations.
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