Ecological niches

The term ecological niche is frequently misunderstood and misused. It is often used loosely 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.

The modern concept of the niche niche dimensions was proposed by Hutchinson in 1957 to address the ways in which tolerances and requirements interact to define the conditions (this chapter) and resources (Chapter 3) needed by an individual or a species in order to practice its way of life. Temperature, for instance, limits the growth and reproduction of all organisms, but different organisms tolerate different ranges of temperature. This range is one dimension of an organism's ecological niche. Figure 2.2a shows how species of plants vary in this dimension of their niche: how they vary in the range of temperatures at which they can survive. But there are many such dimensions of a species' niche - its tolerance of various other conditions (relative humidity, pH, wind speed, water flow and so on) and its need for various resources. Clearly the real niche of a species must be multidimensional.

It is easy to visualize 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 the availability of a particular food, may define a three-dimensional niche volume (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. None the less, 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.

Provided that a location is characterized 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, it must be able to reach the location, and this depends in turn on its powers of colonization and the remoteness of the site. Second, its occurrence may be precluded by the action of individuals of other species that compete with it or prey on it.

Usually, a species has a larger ecological niche in the absence of competitors and predators 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 realized 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 and predators. Fundamental and realized niches will receive more attention in Chapter 8, when we look at interspecific competition.

The remainder of this chapter looks at some of the most important condition dimensions of species' niches, starting with temperature; the following chapter examines resources, which add further dimensions of their own.

the «-dimensional hypervolume fundamental and realized niches

Ranunculus glacialis

2600

Oxyria digyna

2500

Geum reptans

2500

Pinus cembra

1900

Picea abies

1900

Betula pendula

1900

Larix decidua

1900

Picea abies

900

Larix decidua

900

Leucojum vernum

600

Betula pendula

600

Fagus sylvatica

600

Taxus baccata

550

Abies alba

530

Prunus laurocerasus

250

Quercus ilex

240

Olea europaea

240

Quercus pubescens

240

Citrus limonum

80

(m)

(c)

Temperature

100% mortality 50% mortality m15

m 10

100% mortality 50% mortality

Zero mortality

Temperature

Figure 2.2 (a) A niche in one dimension. The range of temperatures at which a variety of plant species from the European Alps can achieve net photosynthesis of low intensities of radiation (70 W m-2). (After Pisek et al., 1973.) (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. (After Haefner, 1970.) (c) A diagrammatic niche in three dimensions for an aquatic organism showing a volume defined by the temperature, pH and availability of food.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment