Ecology. Michael Begon

Ecology - Michael  Begon


Скачать книгу
– an essential resource for animals but lethal at high concentrations – and for the many elements that are essential micronutrient resources in the growth of plants and animals (e.g. copper, zinc and manganese), but that can become lethal at the higher concentrations sometimes caused by industrial pollution.

      The term ecological niche is frequently misunderstood. It is often misused to describe the sort of place in which an organism lives, as in the sentence: ‘Woodlands are the niche of woodpeckers’. Strictly, however, where an organism lives is its habitat. A niche is not a place but an idea: a summary of the organism’s tolerances and requirements. The habitat of a gut microorganism would be an animal’s alimentary canal; the habitat of an aphid might be a garden; and the habitat of a fish could be a whole lake. Each habitat, however, provides many different niches: many other organisms also live in the gut, the garden or the lake – and with quite different lifestyles. The word niche began to gain its present scientific meaning when Elton wrote in 1933 that the niche of an organism is its mode of life ‘in the sense that we speak of trades or jobs or professions in a human community’. The niche of an organism started to be used to describe how, rather than just where, an organism lives.

      niche dimensions

Schematic illustration of 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. (b) A niche in two dimensions for the sand shrimp 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.

      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

Schematic illustration of the use of ordination to facilitate understanding of the multidimensional niche. (a) Weights along two ordination axes of seven environmental factors used to characterise the ecological niche of 35 phytoplankton taxa in French coastal seas. (b) Space occupied by two of the taxa, Leptocylindrus and Skeletonema spp., along the first and second axes of the ordination <hr><noindex><a href=Скачать книгу