Ecology. Michael Begon

Figure 2.3 The use of ordination to facilitate understanding of the multidimensional niche. (a) Weights along two ordination axes of seven environmental factors (TEMP, water temperature; PAR, photosynthetically active radiation; SALI, salinity; TURB, turbidity; PO4, phosphates; DIN, dissolved inorganic nitrogen; SIOH, silicates) used to characterise the ecological niche of 35 phytoplankton taxa in French coastal seas. (b) Space occupied by two of the taxa, Leptocylindrus (LEP) and Skeletonema (SKE) spp., along the first and second axes of the ordination analysis. The yellow to red colour gradient represents phytoplankton density from low to high. (c) Space occupied by each taxon along axis 1 of the ordination. The diameter of the circle is proportional to the total occurrence frequency of each taxon. Axis 1 is positively related to nutrient concentrations and negatively related to temperature, salinity and photosynthetically active radiation.

      Source: From Farinas et al. (2015).

      ecological niche modelling approach to the multidimensional niche

Another approach to characterising a multidimensional niche makes use of ecological niche models (also known as climate matching or climate envelope models) (Jeschke & Strayer, 2008). A species’ niche characteristics, defined largely by its physiology, are fairly constant, so that it is not unreasonable to expect that the details of a species’ niche in one location may be broadly transferable to another. This is the basis for ecological niche modelling (Peterson, 2003), where occurrence patterns in a species’ native range are used to build a model that can be projected to identify other areas that are potentially habitable, using one of several available software packages: BIOCLIM, GARP, MAXENT and others (Elith & Graham, 2009). The basic process of niche modelling is outlined in Figure 2.4. As much environmental information as possible is taken from all of the locations where a species is currently found and from a range of locations where the species has not been recorded, allowing those locations 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