Short-term responses of native rodents to aggregated retention in old growth wet Eucalyptus forests

Aggregated retention (a type of variable retention) is a silvicultural practice that is being implemented in forests worldwide as an alternative to traditional clearfelling (clearcutting) practices. Aggregated retention retains patches of unlogged forest within the harvested matrix. It has been proposed that it retains biodiversity values better than clearfelling, although, to date, there has been limited research on small mammal responses to this practice, especially in southern hemisphere systems.

This study determined whether aggregated retention provided a "lifeboat" for two native rodent species; the habitat specialist, cover-dependent swamp rat (Rattus lutreolus velutinus), and the habitat generalist, long-tailed mouse (Pseudomys higginsi), in wet Eucalyptus forests. We compared their abundances in three forestry treatments (clearfell, burn and sow, unlogged native forest and aggregated retention (including edges, islands and harvested matrix)) over three trapping periods (one- to three-years post-burn), and assessed the effect of habitat cover (vegetation) on rodent abundances.

The cover-dependent swamp rat was found in highest abundance in unlogged forest, intermediate in aggregated retention and lowest in clearfelling. Within aggregated retention, there was a trend for decreasing swamp rat abundance with increasing disturbance within the different habitat types (highest abundance in edges, intermediate in islands, lowest in harvested matrix). The generalist long-tailed mouse, was found in equal abundance across all forestry treatments and habitat types. Island size within aggregated retention coupes had no effect on the presence of either species. Sex ratios did not differ between forestry treatments or habitat types for either species, although swamp rats showed female dominance irrespective of treatment or habitat type. Within harvested areas, a high percentage understorey cover was an important predictor of swamp rat abundance.

Our results demonstrate that aggregated retention provides a lifeboat for cover-dependent small mammals in comparison to traditional clearfelling practices. The habitat cover provided by the retained forest allows populations to persist in connected and isolated patches within production landscapes. Vegetation density at the lower strata appears to be an important determinant for recolonisation in harvested areas.