Birds of New Zealand, Hawaii, Central and West Pacific. Ber Perlo van

      The Environment

      Some Basics for Oceania

      The following paragraphs give a short introduction to the factors that determine the presence and distribution of bird species in the area. The main factor that dominates all others is that the area is composed of islands, varying between very large and very small, lying either far apart from or close to each other. All these islands have been uplifted from the ocean bottom by tectonic activity; those in the tropics are modified by reef building. Almost all islands, except the most southern ones, were once covered by forest. The final stage in the formation of habitats was the way in which people transformed and used the environment.

      TECTONICS The outer mantle of the earth is formed by solid rock (the lithosphere), covered by an accumulation of sediments, volcanic products and changed basic rock (the crust). The lithosphere overlays the asthenosphere, a mantle of plastic flowing rock.

      The lithosphere is horizontally subdivided into seven or eight major plates and many minor plates, which ride on the asthenosphere. Some of these plates and parts of them are denser and heavier, lay lower and form the floor of the oceans. The plates move in relation to each other:

      • at spreading (divergent) boundaries (A1);

      • at collision (convergent) boundaries (A2); and

      • at transform boundaries (A6), where two plates move in opposite directions.

      The area covered by this book is dominated by a convergent border between the oceanic Pacific Plate and the continental Australian and Filipino Plates (see map ‘PLATE TECTONICS’).

      Tectonic plates separate (or diverge) from each other along a 80,000km long, mainly mid-ocean network (A1) that encompasses the earth. Nearest to the area is a network segment along the west coasts of North and South America. A typical spreading (or divergent) zone (C) can be described as a pair of parallel ridges on both sides of a rift. The rift bottom fills itself with upwelling, red-hot magma, which drives the plates apart and forms new ocean floor.

      Collision boundaries are zones of subduction, where heavier oceanic plates dive under lighter continental plates as shown in A2 and D. These zones are marked by a deep trench (D3). When the crust, together with lithospheric material, sinks into the asthenosphere it is heated to such a high temperature that magma chambers (D4) are formed, which float to the surface forming rows of volcanoes (D5) arranged in island arcs (the Kermadecs and Northern Marianas are typical island arcs). These arcs form a sort of perforation, along which the edge of the overlaying plate is often torn off and dragged under itself on the back of the submerging plate.

      Other types of conflicting boundaries are also possible (A6), for example, where plates or plate fragments rub along each other under a sharp corner.

      The speed of spreading is unevenly dispersed along the mid-ocean ridge system. Tensions are solved by many fissures (A7) perpendicular to the rift. The rift segments (B8) shift in relation to each other; the parts of fissures between rift segments are called transform faults. The movement at their sides is in opposite directions, which may cause volcanic activity. The outer parts of the fissures are called fracture zones; these separate areas moving in the same direction, which causes no or only low volcanic activity.

      Here and there, far from the edges, magma penetrates through the ocean plate. These places are known as hotspots (E); hot magma wells up via these holes giving birth to volcanic islands at the surface. Because the ocean plate moves in a north-westerly direction the hot spots keep drilling holes, forming chains of islands, the youngest being the most eastern one. The Hawaiian islands are a good example.

      The map ‘PLATE TECTONICS’ also shows a transform fault (green line on map), running from the Nazca Plate (near South America) via the Pitcairns to the Line Islands, which could have produced the many islands of the Pitcairn, Tuamotu and Line Islands. However, their origin could also have been a hot spot near Easter Island.

      REEF BUILDING There are many species of coral organisms. The group that can build a reef is only found:

      • in clear salt waters;

      • at depths shallower than 50m (beneath this depths the coral skeletons change to coral limestone, darker yellow-green in figures);

      • with an optimum temperature of 26–27°C; and

      • strong currents and/or heavy agitation (otherwise food particles are unable to reach the tentacles of the polyps).

      Most if not all tropical Pacific islands have a volcanic origin. Reef building starts as soon as a new volcano has emerged (A) and coral larvae have been carried in by ocean currents.

      The first stage is a fringing reef (B) at a short distance from land and normally en-compassing a shallow lagoon. It takes about 10,000 years from stage A to reach stage B.

      In

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