Project: 2001-03. Report - ARE PLANTATION FORESTS WITH INCREASED BIODIVERSITY MORE RESISTANT TO PEST OUTBREAKS?
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Date: 2003 Author: Eckehard Brockerhoff, Hervé Jactel, Publication: Report Project reference: 2001-03 Full report is available from: |
Executive summary:
This study confirms that exotic plantation forests can provide habitat for a diverse understorey and insect fauna with a large proportion of indigenous species, including some rare or threatened species. Despite the homogeneous character of the land occupied by Eyrewell Forest and the almost identical stand structure and age distribution, there is considerable variation in the understorey among stands.Overall, our study suggests that more diverse stands have a more diverse natural enemy fauna (i.e., carabid beetles). More diverse stands also had on average lower populations of P. suavis, suggesting that there could indeed be a relationship between forest diversity and pest abundance. By contrast, no such clear pattern was observed for the leafrollers we studied, and some species (e.g., gorse) appear to even promote leafroller populations. This could be caused by the fact that gorse and possibly other understorey species could boost populations because they are alternative host plants (see Jactel et al., in press, for a discussion of these mechanisms).
More work is required to confirm the patterns we observed in the first year of this study, and it is critical to verify the mechanisms at work to gain confidence in the relationship between habitat diversity and pest abundance. In particular, more work is needed to determine the level of parasitism in different stands. The observations so far have been surprising because such low parasitoid populations are unusual. In future studies we will also determine whether the level of parasitism is higher in adjacent habitats such as shrublands dominated by indigenous species. Because P. suavis and the two leafroller species of interest are polyphagous, it is important to determine the suitability of kanuka, gorse, and other species as hosts. To this end we are planning to undertake host specificity tests with these defoliators.
The occurrence of the threatened carabid Holcaspis brevicula is also of great interest, and future work will aim at elucidating its distribution in Eyrewell Forest and adjacent habitats and at determining its population size. This information is critical for the future management of this threatened species.
Full report is available from:
Forest Research
Private bag 3020
Rotorua
New Zealand
