Figure 2.6 Ordination contrasts the multidimensional niches of native and invasive fish. Plot of results of an ordination technique called canonical correspondence analysis (CCA) showing native species of fish (purple circles), introduced invasive species (red triangles) and five influential environmental variables. Note how the native and invasive species occupy different parts of multidimensional niche space.

      Source: After Marchetti & Moyle (2001).

      fundamental and realised niches

      Provided that a location is characterised by conditions within acceptable limits for a given species, and provided also that it contains all the necessary resources, then the species can, potentially, occur and persist there. Whether or not it does so depends on two further factors. First, as we have just seen, it must be able to reach the location, and this depends in turn on its powers of colonisation and the remoteness of the site, or on human agency in spreading invasive species from one area to another. Second, its occurrence may be precluded by the action of individuals of other species that compete with, prey upon or parasitise it.

      Usually, a species has a larger ecological niche in the absence of enemies than it has in their presence. In other words, there are certain combinations of conditions and resources that can allow a species to maintain a viable population, but only if it is not being adversely affected by enemies. This led Hutchinson to distinguish between the fundamental and the realised niche. The former describes the overall potentialities of a species; the latter describes the more limited spectrum of conditions and resources that allow it to persist, even in the presence of competitors, predators and parasites. One of the acknowledged shortcomings of the modelling of niches based on distributions in species’ native ranges, described earlier, is that it is the realised niche that is under consideration (on the assumption that competitors, predators and parasites are present and exert an effect). When a species invades a new area, there is every possibility that some or all of its native enemies will be absent, so that it may be able to occupy an expanded niche, closer to its fundamental niche. Modellers need to beware this possibility (Jeschke & Strayer, 2008).

      Just as negative interactions can play a role in determining species’ distributions (leading to a realised niche smaller than the fundamental niche), so can the positive effects of mutualists that we discuss in more detail in Chapter 13 (potentially producing a realised niche larger than the fundamental niche). Take, for example, the tropical anemone fish Amphiprion chrysopterus, which retreats between the stinging tentacles of the sea anemone Heteractis magnifica when predators threaten, but protects the anemone against its grazers, increasing anemone survivorship, growth and reproduction (Holbrook & Schmitt, 2005). Either species may tolerate the conditions at a location, but their success also depends on the presence of the other. In similar vein, most higher plants have intimate mutualistic associations between their roots and fungi (mycorrhiza; Section 13.9) that capture nutrients from the soil and transfer them to the plants, as well as improving water uptake and disease resistance, while receiving photosynthetic products from the plant (Delavaux et al., 2017). Many plants can live without their mycorrhizal associates in soils when water and nutrients are in good supply, but in the highly competitive world of plant communities the presence of the fungi is often necessary if the plant is to prosper.

      APPLICATION 2.2 Judging the fundamental niche of a species driven to extreme rarity

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.