Responses of litter-dwelling arthropods and house mice to beech seeding in the Orongorongo Valley, New Zealand

This study investigates further the possibility that eruptions of house mice in forests of southern beech (Nothofagus spp.) in New Zealand after mast seedings are triggered by increases in the populations of some arthropods, especially Lepidoptera larvae and spiders that are common foods of mice, rather than by the beech seed. It reports on a 5-year study of arthropods of the forest floor in hard beech and silver beech forest in the Orongorongo Valley, near Wellington, in relation to (1) litter and seedfall, and (2) the numbers and diet of mice. Litter-feeding larvae of Lepidoptera in both the litter and fermentation layers of the forest floor feed on the fallen male flowers of beech, and most species were more common after heavy flowering of the beeches. A few of the common spiders (including Miturga sp., the main spider eaten by mice) were also more abundant after beech seeding. Analysis of the long-term records of hard beech seeding, numbers of mice, and numbers of adults of the moth Gymnobathra tholodella (Oecophoridae) showed that the number of mice was positively correlated with both the number of beech seeds and the number of moths. These results indicate a more complex web of interactions in beech forest than was earlier suggested, but because the intensity of flowering in spring largely determines the numbers of both Lepidoptera larvae and beech seeds, the role of each in the population increase of mice can not be determined.