177 SWEETAPPLE: POSSUM IMPACTS ON MISTLETOE New Zealand Journal of Ecology (2008) 32(2): 177-185 ©New Zealand Ecological Society Available on-line at: http://www.newzealandecology.org/nzje/ Spatial variation in impacts of brushtail possums on two Loranthaceous mistletoe species Peter J. Sweetapple Landcare Research, PO Box 40, Lincoln 7640, New Zealand (Email: sweetapplep@landcareresearch.co.nz) Published on-line: 8 October 2008 ___________________________________________________________________________________________________________________________________ Abstract: Browsing by introduced brushtail possums is linked to major declines in mistletoe abundance in New Zealand, yet in some areas mistletoes persist, apparently unaffected by the presence of possums. To determine the cause of this spatial variation in impact I investigated the abundance and condition (crown dieback and extent of possum browse cover) of two mistletoes (Alepis lavida, Peraxilla tetrapetala) and abundance and diet of possums in two mountain beech (Nothofagus solandri var. cliffortioides) forests in the central-eastern South Island of New Zealand. Mistletoe is common and there are long-established uncontrolled possum populations in both forests. Mistletoes were abundant (216–1359 per hectare) and important in possum diet (41–59% of total diet), but possum density was low (c. 2 per hectare) in both areas. Possum impacts were slight with low browse frequencies and intensities over much of the study sites. However, impacts were signiicantly greater at a forest margin, where possum abundance was highest, and at a high-altitude site where mistletoe density was lowest. Mistletoe crown dieback was inversely proportional to intensity of possum browsing. These results suggest that the persistence of abundant mistletoe populations at these sites is due to mistletoe productivity matching or exceeding consumption by possums in these forests of low possum-carrying capacity, rather than low possum preference for the local mistletoe populations. ___________________________________________________________________________________________________________________________________ Keywords: Alepis lavida; diet; mistletoe condition; Peraxilla tetrapetala; Trichosurus vulpecula Introduction The Australian brushtail possum (Trichosurus vulpecula) was introduced to New Zealand in the 1850s (Pracy 1974) and is now considered one of the most damaging of a suite of introduced herbivores in the country’s indigenous forests (Cowan & Tyndale-Biscoe 1997). Possums modify indigenous forests by selectively browsing, and sometimes killing, mature trees of preferred species (Batcheler 1983; Payton 1987; Leutert 1988; Rose et al. 1992; Rogers & Leathwick 1997). The intensity of possum impacts varies between forest communities, largely due to inherent species differences in vulnerability to possums (Rose et al. 1992, 1993; Rogers & Leathwick 1997; Payton 2000). However, there is also marked spatial variation in possum impact on vulnerable species, within and among populations, both on local and regional scales (Batcheler 1983; Payton 2000). These intraspeciic spatial patterns in possum impacts are largely unexplained, although geomorphic processes appear to explain at least some of the regional variation in possum impacts in southern rātā–kāmahi (Metrosideros umbellata – Weinmannia racemosa) forests (Stewart & Rose 1988). 1 Provenance variation in palatability to possums has been proposed to explain regional differences in possum impacts on tree fuchsia (Fuchsia excorticata), but a test of this hypothesis (Sweetapple & Nugent 1999) failed to support it. Loranthaceous mistletoes provide one of the best- documented New Zealand cases of spatial variation in possum impacts. These hemiparasitic plants were once widespread and abundant throughout much of New Zealand, particularly in Nothofagus (beech) dominated forests, but have undergone dramatic decline in many regions since European colonisation; one species, Trilepidea adamsii , is extinct (Norton 1991) and populations of the other ive species have suffered local extinctions (Ogle & Wilson 1985; de Lange & Norton 1997; Bockett & Knightbridge 2004). Possums have been widely implicated as the causal agent of mistletoe decline, although much of the evidence is circumstantial (Ogle & Wilson 1985; Ogle 1997). A few quantitative studies have demonstrated high possum preferences for mistletoes at sites where mistletoes are rare (Sweetapple et al. 2002, 2004; Sweetapple 2003), or substantial possum impacts on mistletoes in some regions (Wilson 1984; Sessions ___________________________________________________________________________________________________________________________________ 1 Nomenclature follows Allan Herbarium (2000)