Responses to specific points raised by Colin O'Donnell
My comments refer to numbered locations marked on Appendix 4 (attached - see rhs column), starting at Section 2 before returning to the Summary & Recommendations (Section 1) at the end.
Protection of significant habitats
1. There is clear evidence of ongoing declines in areas where there has been little or no habitat deterioration, and several demographic predictions that predation is a primary cause (Elliott 1996, Elliott et al. 1996a, Innes et al. 1996; James & Clout 1996; McLennan et al. 1996; O'Donnell et al. 1996; and other references cited in the DoC Critique). For such species habitat is clearly not a current limiting factor and slight modifications proposed from sustainable logging practices are unlikely to have any impact on such species, based on deduction from ecological first principles. The DoC Critique is correct in emphasising that these other habitat values ( e.g. sufficient nesting / roosting holes and food sources) must also be retained, especially if TWC predator controls eventually enhance bird and bat numbers. The point of difference at the heart of the matter is what controls populations now, c.f. what might be important later. Some of reduction in current food and nest holes may be tolerated without further depressing the number of these populations from current levels, but too great a reduction in foods and tree cavities may limit the extent of recovery possible in future, and therefore should be avoided for long-term conservation of biodiversity.
2. The DoC Critique is correct in emphasising that they have specialised food and habitat requirements. But this is not the key question; instead we need to focus on whether the logging proposed would change habitats in ways that reduce, do not affect, or increase the abundance and/or survival probabilities of the threatened biota. In no case is there unequivocal evidence that the nationally threatened biota present in the TWC forests are at critically low abundance ( i.e. below the "Minimum Viable Population") in ways that threaten long-term local persistence. Thus we do not know if low abundance itself is a threat. Nevertheless the "Environmental Precautionary Principle" suggests that TWC would be prudent to set a proximate goal to not do anything that lowers threatened wildlife numbers below its current level, or below its current trajectory of ongoing decline. Nor should TWC do anything that will greatly reduce the prospects of population recovery later, assuming that effective and widespread predator controls can be achieved.
3. This statement is only logical if habitats limit the population in some way. If predation or competition from introduced mammals and wasps suppresses the population or is causing an ongoing decline, the population will be below the carrying capacity of the forest set by habitat variables such as food and nest/hole availability. As pointed out in 1 above, there is ample evidence of the pervasive impacts of predation, and these predatory impacts have been operating for well over a century. In such circumstances we would expect the current populations to be very much below the habitat carrying capacity . For example, in the only detailed study of its kind, Elliot, Dilks & O'Donnell (1996) calculated that mohua had nine times more holes than they needed, During the first four years of their study, "the density of mohua appeared to be about carrying capacity" (note that this was assumed but not tested), "yet it was clearly not limited by the availability of nest sites nor by competition for nest sites with parakeets". The authors go on to say: "In the absence of information about parakeet density, we can not say whether nest sites are limiting, though given the high density of holes available it seems unlikely." p. 276). Colin O'Donnell ( in litt., 5 October 1998) points out that bats have very strict thermal properties for their roost holes, and that kaka and bats are probably able to use very many fewer holes than can mohua. Accordingly he believes that the data for mohua do not provide the best example of the potential limitation by hole capacity. The critical question is whether the logging proposed will lower the carrying capacity below that which the current populations occur in the face of persistent predation. I think it is very unlikely indeed that this will occur, but a more formal model along the lines suggested by the DoC Critique would help check this. As in all such debates, the critical question will come down to who carries the burden of proof. Strict testing of overall tree cavity abundance as grouped-tree extraction proceeds is recommended to safeguard TWC biodiversity goals as stated in their plans.