New Guinea is one of the most biodiverse regions on the globe as it is believed to harbour over 5% of the world's biodiversity in less than 1% of the land area (Johns 1993
; Miller et al. 1994
; Heads 2001
). In the current century, the greatest threat to regional New Guinea–Australian tropical biodiversity is an ever-increasing human population and pressures from global economic activity that have led to accelerated forest clearing, ecological degradation and climate change (Sekhran & Miller 1994
; Haberle 2003a
; Hilbert et al. 2004
). In contrast to conditions in Southeast Asia to the west, at least 70% of the natural environment of the island remains intact and represents the third most significant expanse of tropical rainforest wilderness on Earth after the Amazonian and Congoli forest blocks (Mittermeier et al. 2005
). While large-scale short-term assessments of the region's vulnerability to biodiversity loss have generally excluded New Guinea from the category of ‘biodiversity hotspot’ (Mittermeier et al. 2005
), this approach ignores the potential that small-scale processes and long-term influence of human activity may also have a significant impact on biodiversity change.
The broad changes in vegetation since the arrival of humans in the highlands1
of New Guinea around 30
000 calendar years before the present (yr BP; Kosipe record, White et al. 1970
) have been established from 19 swamp and lake sites (Haberle 2003b
). These records show that at the time of early deglaciation, beginning around 14
500 yr BP, the forest limit rises and montane forests invade valley floor grasslands in response to warmer temperatures and rising atmospheric CO2
, though conditions were not uniformly suitable for forest development until after 9000 yr BP. The palaeoecological records from highland valleys point to a sustained and gradual intensification of forest clearance and burning from at least 7000 yr BP (Haberle 2003b
), though evidence from the archaeological site of Kuk Swamp in the Wahgi Valley () suggests that at least here the valley floor was never completely forested during the Early Holocene. During this time, people were using fire to increasingly disturb and modify the montane forest and soils on the valley floor for the purpose of managing and harvesting significant food plants (Denham et al. 2004
The island of New Guinea and location of sites mentioned in the text.
The development of agriculture as an indigenous innovation during the Early Holocene is considered to have resulted in increased population pressures, rapid loss of forest cover and increased land degradation over thousands of years (Haberle & David 2004
). A review of the evidence for early agriculture in New Guinea supported by new data from Kuk Swamp demonstrates that cultivation had begun there by at least 7000 yr BP and probably much earlier (Denham et al. 2004
). The focus of early agricultural activity was in the intermontane valleys between 1000 and 1900
m above sea level, where the Early Holocene organic-rich soils and swamp forest cover provided a suitable environment for agricultural development. Today, these intermontane valleys have been substantially transformed through human activities associated with agriculture, leading to the original organic-rich soils and swamp forest cover being replaced by organic-depleted soils supporting grasslands ().
Remnant swamp forest on the valley floor surrounded by anthropogenic grasslands south of the Kainantu Valley, Papua New Guinea.
While swamp forests persist in the intermontane valleys, today the community is restricted to small patches that fringe the grass or sedge peat swamps of the valley floors (Paijmans 1976
). Most swamp forest trees grow on hummocks separated by pools of water, creating a sparse or open canopy with dense layer of small trees and shrubs (). The common trees include Syzygium
and other Myrtaceae, conifers (especially Dacrydium
are often dominant as their stilt roots are ideally suited to an ever-wet soil environment. The conifers (e.g. Dacrydium nidulum
) are also an important component in disturbed swamp forests where they form dominant stands in early stages of swamp forest recovery towards a diverse mixed swamp forest assemblage (Johns 1980
). These swamp forests are assumed to represent the remnants of a once much more extensive forest community that covered the wet peaty soils of the intermontane valley floors. Here, I review the palaeoecological evidence from five major intermontane valleys in New Guinea () and use two measures of past biodiversity to address the following questions:
- Are the present day swamp forests remnants of a once much more extensive forest community?
- How rapidly did the transformation from swamp forest to grassland occur and was the change ubiquitous in time and space?
- What influence did anthropogenic fires and forest clearance activities have upon shaping the present day landscape?
- What has been the overall impact on biodiversity and are there key taxa now missing as a result of past human activities?
- What were the consequences of biodiversity loss for human populations in the highlands during the Mid–Late Holocene?
Pandanus and conifer (Dacrydium and Araucaria) dominated swamp forest in the Erave District of the Southern Highlands, Papua New Guinea.