S. G. Haberle
forces may have included direct forest clearance or alteration of the forest disturbance regime, perhaps through selective forest clearance practices, burning activity or climate change. For example, a shift in fire regimes may be human or climate driven. Climate changes are generally considered to be relatively minor during the past 10 000 years, though it has been suggested that the impact of short-term climate variability, such as increased drought stress and fire associated with El Nin˜o-Southern Oscillation (ENSO) events, has had a significant influence on vegetation dynamics in the New Guinea region over the past five millennia (Haberle & Chepstow-Lusty 2000; Haberle et al. 2001). This has been supported further by records showing landscape destabilization from the South American coast (Sandweiss et al. 1996) and increased disturbance of vegetation in Australia (Shulmeister & Lees 1995) between 5000 and 4000 years ago. Neoglacial advances of highland glaciers during the past 3500 years have been reported by Hope & Peterson (1976), suggesting that minor temperature fluctuations were probably experienced in the occupied valleys.
Recent studies have shown that, under increasing pressures from external forces of change, ecosystems are vulnerable to loss of resilience which may lead to switches to alternative states with consequential species losses (Molinari et al. 2005). In the Tari Basin example (figure 3), the swamp forest appears to have responded to an increase in fire-related disturbance after 3000 yr BP with a shift from Myrtaceae to Dacrydium– Pandanus dominated swamp forest. The reason for the final loss of forest cover and its replacement by grass-sedge swampland by 1700 yr BP may have been direct clearance or a switch to an alternate state under continued high fire-related disturbance frequencies. Either way, human activity appears to be the primary driver of ecosystem change, though it remains difficult to exclude the possibility that El Nin˜o-related climate change may have played a partial role in enhancing the rate of swamp forest loss during the Mid–Late Holocene.
(d) What has been the overall impact on biodiversity and are there key taxa now missing as a result of past human activities? None of the pollen records from New Guinea indicates an extinction event in the swamp forest plant community as a result of past human activities. What is evident is that widespread extirpation of diverse swamp forest communities has occurred at different times over the past 7000 yr BP. The implications for vulnerability to extinction of the plant and animal species within swamp forest communities are the same as for any species whose spatial distribution is reduced through human activity: the vulnerability to extinction may increase with fragmentation and aerial reduction of the community. The current lack of knowledge about the nature of species diversity within swamp forests of the highlands also hinders any definitive measure of potential extinction. However, given the extremely high diversity and local endemism in families such as the Orchidaceae, it may be reasonable to assume that loss of swamp forest habitat in a single
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valley may have led to extinctions in a number of plant species that are not registered in the pollen records.
Extinctions in the animal kingdom have been recorded in the highlands of New Guinea during the past 50 000 years. Fossil fauna have been recovered from swamp forest sites in the Tari Basin and elsewhere in New Guinea. The now extinct large forest-browsing mammal group known as Hulitherium tomasettii is believed to have browsed within swamp forest environ- ments and may have been a target for early human hunting, which is considered the primary cause of their extinction during the Last Glacial Period (Menzies & Ballard 1994).
(e) What were the consequences of biodiversity loss for human populations in the highlands during the Mid–Late Holocene? The story of agricultural development in the highlands of New Guinea is one of the continued indigenous innovation in agricultural techniques in the face of increased land degradation and climate change (Haberle & David 2004). The adoption of Casuarina agroforestry techniques around 1200 years ago as a strategy to alleviate the local loss of forest resources previously available in swamp forests is an example (figure 2). Furthermore, Bayliss-Smith & Golson (1992) believe that the widespread planting of Casuarina facilitated the rehabilitation of soils after gardening through nitrogen fixation, its use for fire- wood and building material and the elimination of taro beetle infestation in gardens where it was planted, and that this practice was successful enough in the dryland for labour-intensive wetland cultivation and swamp forest clearance to be reduced or given up. Bourke (1997) cautions placing too much emphasis on the importance of Casuarina tree-fallowing alone through- out the highlands and suggests that the initial rise in Casuarina pollen may have been related to people planting Casuarina trees in and near villages to provide timber as supplies became scarce with the continued clearance of forests. Either way, the innovation of planting of utilitarian forest products close to settlements would have reduced the need for further primary forest clearance and perhaps reduced further biodiversity loss.
The remaining highland swamp forests should be considered as a conservation priority as their current relict distribution within densely populated valley floors makes them an extremely vulnerable plant community. Part of this conservation effort has been underway in the indigenous community as some swamp forest patches are conserved as significant sacred sites by land owners. Although agrarian populations in the highlands of Papua New Guinea have almost doubled over the past 25 years, the area of agricultural land has expanded by only 11% (MacAlpine & Freyne 2001). This implies ongoing intensification of existing agricultural areas through the incorporation of new crop species and agricultural techniques. While this is a positive indication of potential buffers for forest resource management, ongoing popu- lation increases will lead to further pressures to clear forested land and particularly valley floor swamp forests. The impacts of continued intensification on land degradation and the long-term sustainability of relict forest communities have yet to be realized.