Distribution and response of rats Rattus rattus, R. exulans to seedfall in New Zealand beech forests

Abstract

Ship Rats Rattus rattus were much more abundant in Silver Beech-dominated Nothofagus menziesii forest in the Hollyford Valley than in Red Beech-dominated Nothofagus fusca forest of the Eglinton Valley, northern Fiordland National Park, in 1974?78. The Hollyford also supported a small population of Kiore (Pacific Rat) R. exulans, but no Norway Rats R. norvegicus. A moderate beech seedfall in 1976 was followed by a short-lived, six to seven fold increase in a relative index of abundance of Ship Rats in both valleys. This increase, not matched by Kiore, showed up more clearly in long lines of Fenn traps set for Stoats Mustela erminea than in standard rodent snap-trap lines. Analysis of 122 carcases showed a significant upward shift in age structure of Ship Rats after the seedfall, consistent with overwinter breeding. Conservation management programmes already aiming to protect threatened species in beech forests from post-seedfall irruptions of Stoats might need to be extended to include Ship Rats. Elsewhere in New Zealand, Ship Rats were much more abundant in mixed podocarp-hardwood forests than in pure beech or pine forests.