New Guinea is a region where biota of two hemispheres collide and intermingle. It is the only place on earth where Laurasian oaks (Fagaceae: Lithocarpus) occur together with the "trans- Antarctic" southern beeches (Nothofagaceae: Nothofagus) (GRESSITT, 1982a). Thus, there is an inherent conflict if one tries to assign it to either one of the regions. It was attempted to solve this conflict by examining the different proportions of species or groups that seemed to belong to one of them: WALLACE (1876) assigns the Papuan region (plus Sulawesi) to the Australian faunal region calling it "austro-malayan subregion". He was followed by most zoogeographers, see for example DARLINGTON (1957). Phytogeographers on the contrary found in the Papuan flora closer affinities with the tropical Asian one and placed New Guinea and its surrounding islands within the Indo-Malesian floral region (STEENIS, 1950).GRESSITT (1982b: p. 732) sees in his summary of Papuan beetle biogeography higher degrees of Oriental derivation than DARLINGTON (1971) had shown for Papuan Carabidae. GRESSITT argues that this could be a result of the close ties of phytophagous insects to the predominantly oriental flora.
Central to zoogeographic studies of selected taxa that seem to have invaded a certain area is the "progression rule" of HENNIG (1966): within a continuous range of a
monophyletic group it is considered possible that a transformation series of characters would run parallel with progression in space, such that the youngest members would be on the geographical periphery of a group. In the absence of phylogenetic analyses, some zoogeographers used an unbalanced distribution of species-numbers to hint on the center of the groups´ origin. In some cases the center of origin may be the area with the highest number of species. However, care must be taken because it may be just as well an area of local diversification.
If dispersal is allowed as an a-priori-means to explain Papuan biogeography (which seems reasonable for an island that has emerged from the sea in the geologically recent past) basically two different faunal elements must be distinguished, an Oriental and an Australian. Both terms resort to the center of origin of a given taxon. As always in biogeography and phylogeny, hierarchies must be taken into account. If for example, an Oriental or an Australian species enters New Guinea and subsequently undergoes a rapid evolution and speciation in this region, the resulting monophylum may be called a "Papuan element". In some cases such "Papuan elements" are highly derived and their origin is unclear.
It is worthwhile to review the zoogeography of some well-studied groups of vertebrates. According to FLANNERY (1990), the indigenous mammal fauna of New Guinea consists of 190 species belonging to only four orders: Monotremata (2 species), Marsupialia (60 species), Rodentia (57 species) and Chiroptera (71 species). The first two of them are clearly Australian elements. Although some subgroups of the Rodentia and the Chiroptera may first have colonized Australia from Asia and subsequently entered New Guinea from the south, they are both originally Oriental elements. Thus, although the mammals of New Guinea are commonly considered being predominantly "Australian" only 33 % of the species belong to the Australian element while 67 % belong to the Oriental one.
The situation is similar in the composition of the frog fauna (ALLISON, 1996) which consists of only four native families which can be equally divided between the regions. Hylidae (76 species) and Myobatrachidae (7 species) arrived from Australia, Ranidae (13 species) and Microhylidae (104 species) originated from SE Asia. Mammals and frogs colonized New Guinea with relatively few taxa, both from Australia and from Asia, which underwent considerable subsequent diversification.
The situation is more complicated in birds which are represented by 66 families in New Guinea (PRATT, 1982). Being mostly capable of migrations over sea it is no surprise that there is considerable faunal exchange with neighbouring regions - the ranges of 44 families encompass both the Oriental and the Australian region. Nevertheless, the proportions of species of some families differ so markedly that the borders of the Sahul- and the Sunda- shelf are especially evident in birds. Families such as Psittacidae or Meliphagidae are good
examples for typical Australian elements. Moreover, there is a bird family which is a good example for a "Papuan element": the birds of paradise (Paradisaeidae). Obviously, this family has evolved within New Guinea, with only two species occurring outside the Papuan region in northern Queensland.
Groups that have diversified within the Papuan region and strictly adhere to its boundaries are especially numerous among the insects: Arachnopus (Curculionidae, Coleoptera), Leptognatha (Cicindelidae, Coleoptera), Polyrhanis (Cicindelidae, Coleoptera),
Promechus (Chrysomelidae, Coleoptera), Stricklandia (Carabidae, Coleoptera),
Tmesisternus (Cerambycidae, Coleoptera).
Some groups enter the rainforests of Australia with a few species, but they do not reach islands to the west such as Sulawesi. As will be explained in chapter 4.3.4 the separation between the Papuan and the Australian biota is mainly achieved by ecological factors. Typically, Papuan elements are adapted to tropical rainforest, whereas Australian elements are restricted to sclerophyl savannah habitats. At the interface, there is an interdigitation (not an intergradation) of both biota. Savannah regions in the south of New Guinea possess a flora and fauna that is predominantly Australian. The vegetation is dominated by Eucalyptus and Melaleuca; the agile wallaby (Macropus agilis), the taipan
(Oxyuranus scutellatus) and the frilled lizard (Chlamydosaurus kingii) occur, just as in
Australia. On the other hand, there are pockets of "Papuan" rainforest in Australia, namely at Lockerbie (tip of peninsula), and at Mc. Illwraith range and Iron range (both mid of peninsula). Many species that are typical of the rainforests in New Guinea are found in these forest isolates: as mentioned above, the birds of paradise (Paradisaeidae); other examples
are Tricondyla aptera (Cicindelidae), Ornithoptera (Papilionidae, Lepidoptera), Pantorhytes
(Curculionidae, Coleoptera). Nevertheless, as shown by KIKKAWA et al. (1981) the majority of Papuan taxa of New Guinea do not reach these isolates of rather depauperate Papuan rainforest. There is no need to include them in the Papuan region as outlined in chapter 1.3.1.
After the widespread acceptance of the continental drift theory, biogeography changed drastically. PLATNICK & NELSON (1978) clearly differentiated between the two principal options causing a given distribution: vicariance and dispersal. Vicariance means that the range of a taxon is disrupted by a barrier. In the case of dispersal, the barrier predates the taxon which evolved in one region and subsequently dispersed into the other one. The evolution of taxa with poor abilities for dispersal may largely follow geological vicariance events, such as the break-up of a continent or of an island arc. Thus, the sequence of the break-up can be traced in the reconstructed phylogeny of suitable taxa. Methods of modern biogeography are summarized by HUMPHRIES & PARENTI (1986). Central to it is the availability of robust cladistic hypotheses on monophyletic groups and the
identification of "areas of endemism". The latter are areas that are characterized by overlapping distributions of endemic species or monophyletic species groups. If the names of the taxa in a given cladogram are substituted by the respective areas they occupy, a scheme of the relationships of those areas is obtained. If the pattern observed is not unique for the group in consideration but repetitive in other groups studied, it is likely that this pattern is the result of historic events that have influenced the phylogeny and the distribution of those groups in exactly the same way. TURNER et al. (2001) examined dispersal pathways of various plant and animal taxa within Southeast Asia employing cladistic methods. However, there was considerable conflict in the data set and the resulting patterns were too weakly supported to allow general conclusions. Another possible explanation for congruent biogeographic patterns can be sought in geology.
As outlined in chapter 1.3.3, New Guinea is a composite of various geological terrains. It is tempting to examine how far the evolution of Papuan biota followed the geological history of island arc accretions or the movement of continental fragments. Unfortunately, only few animal groups have been studied well enough so far to draw conclusions.POLHEMUS & POLHEMUS (1998) have studied aquatic Heteroptera. They found in
Ptilomera, a genus of waterstriders, a pattern that seems to reflect accretion of two different
island arcs to New Guinea. However, most of the species they refer to are yet undescribed and the cladistic analysis they used was in preparation. Most important are the studies on Cicadas summed up by DUFFELS (1986), DUFFELS & BOER (1990), and BOER (1995). They see a coherence of the northern ranges of New Guinea to other fragments of the Outer Melanesian Arc (OMA), such as the Bismarck Archipelago, the Solomon Islands, Vanuatu, Fiji and Tonga. This OMA-region has a sister area relationship to central New Guinea , which forms a part of the Inner Melanesian Arc (IMA). The birdshead region forms another area of endemism which has a sister area relationship to OMA plus Maluku. DUFFELS (1986) and
DUFFELS & BOER (1990) go as far as postulating that New Guinea can not be regarded as an
area of endemism in itself.
4.3.2. Dispersal versus vicariance - the worldwide zoogeographic pattern of Euops