Environmental History

ENVIRONMENTAL HISTORY AND GEOGRAPHY

Figure 1. Sahul. Source: Bourke 2009

During glacial maxima, when sea levels are up to 200m lower than at present, Australia and the island of New Guinea form a single landmass, known as Sahul (Fairbairn et al 2006).  During interglacial time periods, a land bridge that connects Australia and New Guinea falls below the sea surface.  This bridge was last present over 8500 years ago (Bourke 2009).  As a result, Australia and New Guinea share similar, yet distinct flora and fauna.  There is a relatively low degree of genetic isolation between like species in Australia and New Guinea, especially for flora, as a gene flow exchange between plant species during glacial maxima (MacQueen 2010).

Figure 1. Vegetation types. Source: Fairbairn et al 2006

The colonization of Sahul by humans, during the time period 40-50ka, is notable, because it was, most likely, the earliest known example of a homo sapien open ocean crossing (Fairbairn et al 2006).  When humans landed in New Guinea, they found several large extinct mammal species, including grassland wallaby grazers (Flannery 2003; Fairbairn et al 2006).  The niche usually filled by large mammalian predators  on continental landmasses, was filed by large snakes and birds (Fairbairn et al 2006).  Between 51200ka and 39800ka, all Sahul land mammals, reptiles and birds weighing more than 100kg went extinct, with human induced land use change and hunting pressure the most likely culprits (Roberts et al 2001; Fairbairn et al 2006).

Figure 2. Long-Beaked Echidna. Source: Arkive.com

Habrele (2007) notes a collapse of forest ecosystems during the mid-late Holocene, with human colonization of New Guinea.  Between 49ka and 36ka, forest patches were first cleared to promote the growth of exploitable plant and animal species (Summerhayes 2010).  The Pandanus tree was likely one of the first candidates for exploit, a species which has been actively managed for over 10000 years (Bourke ; Summerhayes 2010).

Figure 3. Pandanus Fruit. Source: Tree-Species.blogspot.com

Climatically, New Guinea is dominated by the Austral Summer Monsoon and the ITCZ, which make the island cloudy year round and particularly wet.  Average precipitation rates across the island are between 2700mm-3450mm per year (Fairbairn et al 2006: Haberle 2006).  New Guinea is transected by a thin mountainous chain, with a maximum height of roughly 5000m at Mt. Jaya (Fairbarin et al 2006).  This mountain range develops five vegetation types across New Guinea: Alpine grasslands, Sub-alpine Forest/shrub grasslands, Upper montane moss/cloud forests, mid-montane forests and lower montane forests (Fairbairn et al 2006; See figure 2).

The highlands extend from 700-3000m above sea level; the sub-alpine environment exists from 3000-4000m above sea level; and the intermontane valleys exist between 1400-1800m above sea level (Fairbairn et al 2006).  Temperature along the mountain chain changes rapidly, with an average temperature at 1500m of 19 degrees Celsius, 3500m of 8 degrees Celsius and 4300m of 6 degrees Celsius (Fairbairn et al 2006).  Above 2500m frost is common.  Hope (1993) believes that above 780m, vegetation is highly sensitive to climate change, and vegetation patterns frequently shift.