Anatomía de un Sistema BES

NZ is an important part of the southern hemisphere puzzle and is interesting on many levels. It has connections with the whole of former Gondwanaland, historically and currently via oceanic wind and sea currents. NZ has more recent links with landmasses, such as Antarctica, Australia and New Caledonia. It also has its own collection of smaller islands scattered within its waters from the Kermadec Islands in the North to the Chatham Islands in the east and the sub-antarctic islands in the south. The NZ biogeographic relationships with all these areas are

Zealandia separated from Gondwanaland 85 mya and the idea of it moving across the Pacific with its own unique biota has spawned such phrases as ‘Moa’s Ark’ (Bellamy, et al., 1990) and ‘Gondwanan Life raft’ (Briggs, 2003; Sparks & Smith, 2005). For some, the theory of moving continents was enough to explain the majority of biotic distribution throughout the world. The unique Gondwanaland fauna of NZ had remained isolated and unaffected by the rest of the world’s biota, allowing us to glimpse life ‘as it was’ 85 mya. However, this vicariance based explanation is not supported by most recent evidence (Goldberg, et al., 2008). Long distance dispersal (LDD) had also been considered a likely source of NZ biota (Fleming, 1962, 1976).

Geological evidence also shows that from 80 mya Zealandia was slowly sinking, culminating in a period of maximum inundation 25-22 mya, the ‘Oligocene drowning’ (Cooper &

Millener, 1993; Landis, et al., 2008). The Oligocene period of Zealandia’s history is currently a fervent topic of debate based over what proportion of the the NZ landmass was eventually flooded. If the sea managed to cover the whole of the land, even for a geologically short time, then all terrestrial life would have been wiped out. This means the loss of all the

Gondwanaland species on Zealandia and thus the subsequent re-population of NZ from the Miocene onwards will have been by LDD from surrounding areas, although these source populations would also have been of Gondwanan ancestry. It has been argued, quite correctly, that there is no geological proof that there was no land in the NZ region 25 mya. Conversely there is no geological evidence that there was land in the region either (Campbell & Hutching, 2007). What the geology does show is that if there was land in the NZ region then it was very much smaller in area than exists today (Campbell & Hutching, 2007). From what we know of island biogeography and species area relationships, a significant loss of biodiversity would accompany a loss of land area. So even if enough land did remain during the Oligocene for some species to survive, the majority of current NZ lineages have arrived and colonised within the last 25 my. From 25-2 mya was a period of marine regression, increasing NZ’s land area. New taxa continued to arrive via dispersal, supplying the continuous cycle of immigration, evolution and extinction in the expanding new land(Stevens, 1980; Stevens, 1985).

Within the last 2 my the cycles of glaciation of the Pleistocene have also had an effect on the biota of NZ. Much of the tropically adapted taxa, survivors from when NZ was a warmer land, died out. NZ’s Pleistocene legacy is a wide range of alpine adapted species of both flora and fauna (Gibbs, 2008). There are two main explanations for how the large alpine biota arose. The first is that some species already present on the wide flat grasslands quickly

adapted to the new niches, probably due to some pre-existing advantageous character trait. Another theory is that taxa already adapted to alpine conditions dispersed to NZ via suitable regions of the world (Raven, 1973). The conditions in the ice age created many isolated refugia in which led to a high rate of allopatric speciation. The fact that NZ has high rates of radiation in some taxa, but is also missing some of the world major taxonomic groups was described by Mueller-Dombois as being naturally depauperate, but secondarily rich (Gibbs, 2008).

NZ holds a unique fascination for biogeographers. It is seen in the imagination somewhere between a large island and a small continent (Daugherty, et al., 1994) and has even been described as the nearest thing to an alien biota on earth (Diamond, 1997). NZ’s biota has long being viewed as of vicariant origin, relicts from ancient Gondwanaland. However, despite the long standing vicariant view of NZ biogeography the NZ biota has many of the characters expected from a dispersal based island ecosystem (Goldberg, et al., 2008). Consolidating the dispersal based position are the ever growing number of molecular studies which show taxa have dispersed and speciated following NZ’s break away from Gondwanaland. Molecular studies also show that NZ biota also has a strong Pacific connection as well as the much vaunted ‘Gondwanan’ link (Miller 2007).

NZ was once the once the archetypal example of a vicariance which is now being seen in a new light as an example of how effective dispersal and rapid speciation can be in creating a unique ecosystem. The story of NZ is slowly unravelling but is a long way off being

completely told. Further molecular studies, an open mind and an eye for new data are urgedto progress this most intriguing biogeographic conundrum (Goldberg, et al., 2008).