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.
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.
2.3 Responses of individuals to temperature
2.3.1 What do we mean by ‘extreme’?
It seems natural to describe certain environmental conditions as ‘extreme’, ‘harsh’, ‘benign’ or ‘stressful’. It may seem obvious when conditions are ‘extreme’: the midday heat of a desert, the cold of an Antarctic winter, the salinity of the Great Salt Lake. But this only means that these conditions are extreme for us, given our particular physiological characteristics and tolerances. To a cactus there is nothing extreme about the desert conditions in which cacti have evolved; nor are the icy fastnesses of Antarctica an extreme environment for penguins. It is lazy and dangerous for the ecologist to assume that all other organisms sense the environment in the way we do. Rather, the ecologist should try to gain a worm’s‐eye or plant’s‐eye view of the environment: to see the world as others see it. Emotive words like harsh and benign, even relativities such as hot and cold, should be used by ecologists only with care.
2.3.2 Metabolism, growth, development and size
exponential effects of temperature on metabolic reactions
Individuals respond to temperature essentially in the manner shown in Figure 2.1a: impaired function and ultimately death at the lower and upper extremes (discussed in Sections 2.3.4 and 2.3.6), with a functional range between the extremes, within which there is an optimum. This is accounted for, in part, simply by changes in metabolic effectiveness. For each 10°C rise in temperature, for example, the rate of biological enzymatic processes often roughly doubles, and thus appears as an exponential curve on a plot of rate against temperature (Figure 2.8a, b). The increase is brought about because a higher temperature increases the speed of molecular movement and speeds up chemical reactions. The factor by which a reaction changes over a 10°C range is referred to as Q10: a rough doubling means that Q10 ≈ 2, and animals generally conform quite closely to this value (Figure 2.8c) as do microbial organisms (Kirchman, 2012) and plants (Lange et al., 2012).
