to be identified that meet the species’ requirements even though the species is currently absent. The ability to project into geographic space can be used to predict species distributions in previously unexplored parts of the native range (checking how good the model is) or in new, often quite distant locations of interest (e.g. predicting places where a potentially invasive species may prove problematic; Figure 2.5).
Figure 2.4 Ecological niche modelling. The first step is to characterise a species’ distribution in two‐dimensional geographic space. Then the niche is modelled in ecological space, in terms of a number of influential dimensions of the n‐dimensional hypervolume (such as temperature, precipitation, humidity, soil pH, etc.). Finally, the occupation of ecological space is projected back into geographic space.
Source: After Peterson (2003).
Figure 2.5 Modelling the potential range of an invasive starfish. (a) Current distribution records for the sea star Asterias amurensi in its native (northern hemisphere) and invasive (southern hemisphere) range. (b) Modelled distribution in its invasive range. Red regions represent areas with suitable mean winter and summer seafloor temperatures for the benthic adult stage (light red suitable, dark red highly suitable). Blue regions represent areas where the sea surface temperature is suitable for the pelagic larval stages (dark blue optimal). Isotherms represent mean sea surface temperature (°C) during winter. Boxes show islands that might provide a stepping‐stone habitat for invasion of A. amurensis into Antarctica, especially the Macquarie, Heard and Kerguelen Islands, which are ice‐free year‐round. Currently the Balleny Islands are only ice‐free in summer but this may change with global warming.
Source: From Byrne et al. (2016).
APPLICATION 2.1 Ecological niche modelling and ordination as management tools
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
