Ecology. Michael Begon

Ecology - Michael  Begon


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that hundreds of dams across the USA, whether originally built for public or private benefit, have been removed in river restoration projects in recent years.

Graph depicts the ordination contrasts the multidimensional niches of native and invasive fish. Plot of results of an ordination technique called canonical correspondence analysis showing native species of fish, introduced invasive species 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

An illustration of a map depicting the location of fossil bones of the takahe in the South Island of New Zealand.

      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.

      2.3.1 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


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