Читать книгу Ecology - Michael Begon - Страница 38

The takahe (Porphyrio hochstetteri), one of only two remaining species of large, herbivorous, flightless birds that dominated the pre‐human New Zealand landscape, was itself believed extinct until rediscovery in 1948 of a small population in the remote and climatically extreme Murchison Mountains in the south‐west of the South Island (Figure 2.7). Intense conservation efforts have involved captive breeding, habitat management, predator control, wild releases into the Murchison Mountains and nearby ranges as well as translocations to offshore islands that lack the introduced mammals that are now widespread on the mainland (Lee & Jamieson, 2001). From just a handful of individuals, there are now more than 300 in existence. Some ecologists believed that because the takahe is a grassland specialist, feeding mainly on tussock grasses in the genus Chionocloa, and adapted to the alpine zone, they would not fare well elsewhere. Others noted that fossil evidence indicated that takahe were once widespread in New Zealand and occurred at altitudes below 300 m, including coastal areas that were a mosaic of forest, shrubland and grassland (Figure 2.7), and that they might therefore be well suited to life on offshore islands that lack the mammals that have caused their demise. Indeed, they have formed self‐sustaining populations after introduction to four offshore islands, although the island habitat may not be optimal (with poorer hatching and fledging success in island than mountain populations) (Jamieson & Ryan, 2001). The fundamental niche of takahe probably encompasses much of the South Island, but it became confined to a much smaller realised niche because of the effects of predators (human hunters and introduced stoats, Mustela erminea) and competitors for food (introduced red deer, Cervus elaphus scoticus). The removal of these mammalian interlopers would enable takahe to occupy something closer to their fundamental niche, as they did before humans and the other invaders arrived in New Zealand.

Figure 2.7 Location of fossil bones of the takahe in the South Island of New Zealand. The population had become restricted to a single site in the Murchison Mountains, but was this a true reflection of its niche requirements?

Source: After Trewick & Worthy (2001).

The remainder of this chapter looks at some of the most important condition dimensions of species’ niches, starting with temperature; the following chapter examines resources, which add further dimensions of their own.