Читать книгу Ecology - Michael Begon - Страница 37

Managers are frequently confronted by problems associated with invasive species and make use of climate envelope models or ordination to develop solutions.

The Arctic sea star, Asterias amurensis, is among the most ecologically influential of marine invertebrates, being a voracious predator with a particular affinity for bivalves (frequently putting it in conflict with bivalve fishers) and capable of dramatically affecting local biodiversity. Its native range extends in the North Pacific from the Arctic to southern Japan (Figure 2.5a). Accidentally introduced in the early 1980s to Tasmania (probably through the release of pelagic larvae in ship’s ballast water), adults became established on the seabed where they caused the extinction of many species. A. amurensis has since spread to Victoria along the coast of mainland Australia (Figure 2.5a) but so far it has not invaded New Zealand or the sub‐Antarctic Islands. One critical dimension of its multidimensional niche is water depth: the species cannot survive below a depth of 200 m. Both summer and winter temperature ranges are also fundamentally important to the success of the sea stars, and so to assess the potential for range expansion, Byrne et al. (2016) used the climate envelope model MaxEnt to characterise the thermal niche of both adults and the dispersive larval stages. Figure 2.5b shows the predicted invasive range, which includes much of New Zealand, together with the sub‐Antarctic Macquarie, Heard and Kerguelen Islands. The red areas are considered suitable for adult sea stars (dark red highly suitable), while the blue zones are suitable for the development of dispersing larval stages (dark blue optimal). That the species may spread to many new locations is alarming enough, but there is also a strong possibility that global warming will put much of the Antarctic coastline in peril. Results of such analyses highlight the importance of vigilance and border biosecurity.

Marchetti and Moyle (2001) used an ordination technique to define how a suite of fish species, 11 native and 14 invaders, are related to environmental factors in a Californian river (Figure 2.6). The native and invasive species clearly occupy different parts of the multidimensional niche space. Most of the natives were associated with higher mean discharge (m³ s^–1), good canopy cover (higher levels of % shade), lower concentrations of plant nutrients (lower conductivity, μS), lower temperatures and a greater percentage of fast‐flowing, riffle habitat (less pool habitat). These are all features of the natural, undisturbed state of streams. The invaders, on the other hand, are favoured by the present combination of conditions where water regulation and damming have reduced discharge and riffle habitat, shady riparian vegetation has been removed leading to higher stream temperatures, and nutrient concentrations have increased because of agricultural and domestic runoff. Restoration of more natural flow regimes and riparian planting will be needed to halt the continued decline of native fish, and it is heartening to note 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.

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.