Ecology. Michael Begon

Figure 2.2 The ecological niche in one, two and three dimensions. (a) A niche in one dimension showing the thermal range of passerine birds in southern Canada and the contiguous USA recorded during the North American Breeding Bird Survey 2002–06 in relation to minimum and maximum thermal limits of an average of 10 occurrence locations for each species (measured in each case as mean springtime breeding season temperature). (b) A niche in two dimensions for the sand shrimp (Crangon septemspinosa) showing the fate of egg‐bearing females in aerated water at a range of temperatures and salinities. (c) A diagrammatic niche in three dimensions for a stream‐dwelling alga showing a volume defined by temperature, pH and water velocity; in reality, the niche would not appear as a neat cuboid defined by the three tolerance ranges because, for example, temperature tolerance may be reduced when pH is low.

      Source: (a) Data from Coristine & Kerr (2015). (b) After Haefner (1970).

      the n‐dimensional hypervolume

      It is easy to visualise the early stages of building such a multidimensional niche. Figure 2.2b illustrates the way in which two niche dimensions (temperature and salinity) together define a two‐dimensional area that is part of the niche of a sand shrimp. Three dimensions, such as temperature, pH and current velocity in a stream, may define a three‐dimensional niche volume of a stream alga (Figure 2.2c). In fact, we consider a niche to be an n‐dimensional hypervolume, where n is the number of dimensions that make up the niche. It is hard to imagine (and impossible to draw) this more realistic picture. Nonetheless, the simplified three‐dimensional version captures the idea of the ecological niche of a species. It is defined by the boundaries that limit where it can live, grow and reproduce, and it is very clearly a concept rather than a place. The concept has become a cornerstone of ecological thought.

      ordination as an aid to conceiving the n‐dimensional niche

The difficulties of interpreting the multiplicity of relevant niche dimensions can be reduced by a mathematical technique called ordination. This is one of several methods used by ecologists to condense information from many dimensions into a much smaller, more manageable number, in this case allowing us to simultaneously display species and several influential environmental variables along one or more ‘ordination axes’. Species with the most similar niches appear closest together, and the direction of increase or decrease of environmental variables along each axis reveals how the species’ niches are arranged in relation to these variables. In their study of marine phytoplankton along the French coast, Farinas et al. (2015) related abundance data for 35 taxa to seven environmental factors: temperature, salinity and turbidity (conditions), and photosynthetically active radiation and the concentrations of three inorganic nutrient concentrations (resources for phytoplankton). The relationships of these variables along two ordination axes derived by the method are displayed in Figure 2.3a. Note, for example, that nutrient concentrations are positively related to the first axis, while salinity, solar radiation and temperature are negatively related to this axis. Figure 2.3b illustrates for two taxa, Leptocylindrus spp. and Skeletonema spp., the space occupied along the first and second axes of the ordination. Leptocylindrus has a narrower niche than Skeletonema, and Leptocylindrus is displaced towards the negative end of axis 1 and the positive end of axis 2, being more strongly related than Skeletonema to temperature and photosynthetically active radiation. Figure 2.3c shows the space occupied by all 35 taxa along axis 1: those with more negative positions are associated with higher temperatures, salinity and photosynthetically active radiation levels; those with more positive positions are associated with higher nutrient concentrations. While a hypervolume with more than three dimensions cannot be visualised, ordination allows it to be more readily comprehended, and hence allows us to see how much species’ niches overlap, which of them are quite distinct, and so on.