Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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from pools in a few accessible cave systems in the Baliem Valley, revealing that certain insect species occurring above the surface in surrounding areas, particularly the genus Microvelia in the Heteroptera, will colonize pools in the twilight zones of such caves. To date, no blind cave fishes, crustaceans, or insects have been recorded from Papua, although such taxa are known from adjacent Papua New Guinea (Allen 1996; Holthuis 1980; Chapter 5.13).
SEAWARD INTERFACES
The inland water ecosystems described above intergrade into marine systems at several points, the most important being saline marshes and swamps, and the estuaries that form at the seaward ends of the terminal reaches of perennial streams. Recent classifications of marine environments in the insular tropical Pacific address such estuarine ecosystems in a manner similar to that used above for freshwater systems.
The estuarine transition zone between limnetic and euhaline waters is primarily one in which mixohaline waters in delineable basins exhibit continuous or periodic surface connection to the ocean, allowing the entry of a diverse euryhaline marine fauna, including certain snappers, glassfishes, cardinalfishes, damselfishes, gobies, and gudgeons (this definition excludes waters inhabited by stenohaline marine inshore fauna such as corals, urchins, etc.). The level of the water surface exhibits tidal fluctuations, which may also produce strong inflows and outflows, and there is generally a pronounced stratification of halinity (concentration of sodium chloride), temperature, and (usually) oxygen concentration. Two distinct subtypes of natural estuaries may be recognized based on freshwater inflows and diadromous fauna.
True Estuaries
These are drowned river and stream mouths fed by limnetic water from perennial stream runoff. Their inland extent is determined by measurable tidal fluctuation and topography, such that estuaries of this type tend to be much more extensive on the south coast of Papua than on the north. In Papua such estuaries also tend to be horizontally stratified, with relatively large freshwater inflows relative to channel or basin volumes, leading to pronounced differences in salinity from head to mouth as one progresses along the length of the estuary. Additionally, such estuaries usually have a degree of vertical stratification, resulting from the fact that freshwater has a lower density than salt water, and therefore tends to ‘‘float’’ on top of a salt water wedge as the stream discharges into the ocean. The predominance of horizontal or vertical stratification is dependent on a variety of factors, including the volume and velocity of freshwater inflow, the depth and size of the estuary basin, and the degree of mixing created by winds and currents. In general, estuaries at the mouths large rivers tend to be homiohaline along any particular reach, while those of smaller stream systems tend to exhibit poikilohalinity resulting from wide seasonal fluctuations in freshwater discharge. Along many coasts in Papua true estuaries are often dominated by mangroves, and serve as important migratory pathways for larval and juvenile diadromous fishes and other animals (Erftemeijer et al 1989). They also support unique assemblages of marine water striders in the family Gerridae (Halobates, Rheumatometroides, Stenobates) which partition such estuaries on the basis of horizontal salinity gradients (Herring, 1961; J. Polhemus and D. Polhemus 1996; Andersen and Cheng 2004).
Estuarine Limnocrenes
Estuaries of this type consist of nearshore basins with subterranean limnetic water sources (generally basal springs) and open connections to the sea. In contrast to true estuaries, their waters tend to be uniformly homiohaline, due to annually stable limnetic discharges. The biota is generally similar to that of true estuaries, but may also include submerged vascular plants, and lacks transient diadromous stream fauna.
Threats to Inland Water Ecosystems in Papua
Although the overall condition of freshwater ecosystems in the New Guinea region is excellent, there are still obvious threats to the biota, which tend to manifest themselves on local rather than regional scales. These threats may be grouped into three general categories: physical alteration of habitat, utilization of biotic resources, and invasive species. Each of these threat categories is discussed separately below.
PHYSICAL ALTERATION OF HABITAT
Logging
Large-scale industrial logging, particularly by international timber companies, is a clear threat to watershed integrity throughout the New Guinea region. The obvious and disastrous effects of clearcutting aside, even selective logging by such companies results in an extensive network of poorly-planned and constructed secondary roads that create widespread siltation and stream impoundment problems. Although treefalls are a natural element of the New Guinea rain forest and the small impoundments resulting from them are encountered on nearly every forest stream in the region, particularly in the lowlands, logging tends to greatly increase the number of such channel obstructions, increasing pool habitat and decreasing riffles. Logging roads also tend to employ rudimentary bridges that subsequently collapse, creating further impoundments. Opening the forest canopy also increases insolation (exposure to sunlight) and thereby increases water temperature. The overall effect, then, is to create a stream that is warmer, more slowly flowing, and traps more sediment.
Much of the large-scale logging in Papua is undertaken by foreign companies with poor environmental records, or their local Indonesian subsidiaries. In addition to large-scale operations by companies such as PT Inhutani II and PT Astra, local military garrisons often set up illegal logging operations to subsidize their pay, usually with no consideration of environmental effects.
By contrast, the advent of small-scale logging, utilizing ‘‘walkabout sawmills’’ appears to result in rather light and transient damage to streams and watersheds. Such operations leave a lighter environmental footprint because they usually target only particular tree species, such as rosewood, which are widely scattered in the forest; they do not operate in one area for a long period of time; and they do not require the creation of an extensive road network.
Shifting Cultivation
The impacts of shifting cultivation are similar to those of clearcut logging, but on a far more localized scale. In traditional village settings, the effects of shifting cultivation were mitigated over time by the fact that such garden patches were relatively small in size and widely dispersed. In many cases, if all available garden areas had been used at least once, entire villages simply relocated to alternative sites, allowing the old gardens to go back to forest. As population has increased in many highland areas, however, the number of gardens has proliferated while the number of years they are allowed to lay fallow has decreased, and local governments have discouraged villages from changing location.
In general, shifting cultivation tends to have disproportionate impacts on first order streams (the smallest streams in a given drainage network), which are characteristic of the ridge slopes on which gardens are usually established. Creeks passing through newly cleared garden areas are usually exposed to intense sunlight and high air temperatures, and obstructed by massive tangles of vines and tree branches that in many cases make them nearly impossible to traverse. These ecosystem impacts produce significant faunal changes, with deep forest species that require cooling shade, particularly certain genera of Odonata (Selysioneura, Tanymecosticta), being absent in such areas. Provided that a patchwork of forest and garden plots remains intact, however, such forest biota will eventually recolonize streams in former garden areas once a canopy of native trees is re-established.
Oil Palm
In common with clearcut logging, oil palm plantations result in wholesale ecosystem conversion that has broad impacts across entire stream catchments. The creation of a plantation requires initial land clearing equivalent to clearcut logging (which may in fact be the first step if the proposed plantation area is covered with primary forest), after which a new canopy structure of oil palms eventually becomes established. Nutrient inputs from such plantations into adjacent streams appear to be high, probably due to fertilizer and other agrochemical runoff, leading to a proliferation of algae and consequent impacts on the benthic biota. Because oil palm development is generally undertaken on relatively flat lowland sites, it disproportionately impacts the terminal reaches of streams via clearance and drainage channelization of alluvial and swamp forests, with consequent impacts on diadromous biota similar to those described subsequently for mining.
Mining
Large-scale