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In any dimension the realised niche may be defined as we have said in terms of the set of resources exploited by an organism (or by a population of organisms) and the relative extent of use made of each of the different portions of that resource set. We can portray this diagrammatically.

If a set of resources is visualised in terms of some continnum (perhaps prey size in mm) along one axis of a graph and extent of exploitation of various portions of that resource continuum is represented by the other axis, the niche may be seen upon this plot as a defined area, enclosing the range of the resource continnum used, and showing relative proportion of resource use at each point.

Ecologically speaking the most important features of a niche are its position on the resource continuum, its spread, and its overall shape and form. The niche position and form relate to the role of the organism within its surrounding community, describing the relationship of that organism to all others which may utilise the same resource continuum, while one of the most important descriptors of the niche itself it its ‘spread’.

This niche width is a measure of the breadth of exploitation of a given resource by an organism; it is a complex function of how much of a generalist or specialist the organism may be in that particular element of its ecology. In the diagrammatic representation of the niche in Fig. these same niche parameters of position, width and form may be defined quite precisely in terms of the distribution mean (x), the standard deviation (s) and the actual distribution of y.

(Note; These niche descriptors-position, with, nich shape-must of course be separately defined for the niche of a species, or population, and that of any individual organism within such population, and we must distinguish clearly between the separate concepts of individual niche and population niche. Niche position and niche form of the individual are determined by its own individual abiotic tolerances, and its own competitive or other interactions with other organisms around it, of its own or different species.

The niche position and niche form of the population or species is derived as the envelope of the separate niches of its component organisms. And because of variability between different individuals within such a population, the niche described for the entire population may have different limits and a different form from those described for any one individual. It is important to stress this distribution between individual and population niche, for as we shall see the two do not behave in the same way).

Graphical representations of the niche, as that in Fig. frequently, through not invariably, display the characteristic bell-shaped curve typical of normal distributions. Such normal distribution may in fact be expected at least in consideration of the fundamental niche, for both individuals and populations. Individuals within any species will not all operate at exactly the same point on the resource continuum.

Because of variations within the species or within a population in various physical characteristics (let us say, for example, bill-length in insectivorous birds) there will be variation between individuals in the optimum position selected along the resource continnum of prey size in Fig.

Such physical characteristics within a population are normally distributed; the relative variation in individual’s niche-use will thus also show a normal distribution, and population niche will show the characteristic bell-shape. Similarly, any one individual within the population may also be expected to show a normal distribution of niche-use.

While any individual may have an optimum point on the resource continuum (related to the precise length of its bill) it will of course tend to operate over a certain range around this optimum (rather than only taking insects of exactly 0.337 mm every time!); the extent of its operation around that optimum may also be expected to show a normal distribution and as a result. Niche form will show for the individual, too, the characteristic normal curve.

Note, however, that the realised niche may not necessarily show such a clear normal distribution. Interaction with other organisms through predation or competition may well skew and distort this ideal niche curve.

Any niche of individual or population may thus be defined in one resource dimension in terms of its position and spread, with regard to that particular resource continuum, and the extent of use of different portions of that distribution.

Various factors combine to determine the shape of this niche, combine to influence niche width and position. As already noted, the position and range of the fundamental niche are determined by physiological characteristics of the organism itself in relation to abiotic features of the environment.

Thus the temperature range and actual temperature of fresh water which may be occupied by, let us say, a platyhelminth flatworm is determined exclusively by its physiological tolerances. Individuals within the population will show. Normal variation in temperature tolerance and the extent of use of each part of the temperature gradient overall by the population will thus show a normal distribution over its full range.

However, in practice the organism may never occupy that full niche. Other factors-largely biotic-may restrict the range of temperatures used, and shift the mean of the actual realised niche expressed. Introduction to another species of flatworm into our freshwater system, offering competition to the first, may radically affect the actual niche occupied.

In more general terms there are a number of factors which thus determine the breadth and position of the realised niche within this overall potential which may be summarised as predation, distribution and predictability of resources, and perhaps most importantly, inter-and intra-specific competition. Each of these and their likely effects in translation of fundamental into realised niche will be considered below.

Competition

As illustrated in Fig, interspecific competition can have a profound effect in restricting niche width and in determining actual position of the realised niche of any organism. If there are a large number of species trying to exploit the same resource there will be a strong tendency for each to specialise to utilise one particular position of that resource not utilised by the other competing species.

In other words there will be a tendency for each to restrict their own niche until the species occupies only that part of its which does not overlap with the niches of competing species.

This will have the effect of compressing the realised niche (i.e., narrowing the niche). In addition unless competition is so to speak equal on both sides, the niche will not necessarily ‘shrink evenly’. Intense competition from other species on one side or another may result in ‘uneven’ shrinkage and thus, effectively, a shift in niche position (fig.)

Such competition may also result in extremes, in a ‘skewing’ of the normal distribution of niche form and resource use. If the organism is pressed by extreme competition to the limits even of the fundamental niche, it can clearly ‘withdraw’ no further; under such circumstances- if it remains within the community at all-the extent of occupation of the end of the niche most under competition may well be reduced, while that at the opposite end may be increased in compensation. Interspecific competition in general, thus acts to compress’ or reduce niches.

By contrast, intraspecific competition tends to cause expansion of the niche. Where competition with other species is not severe, intraspecific competition will force individuals to try and exploit those parts of their environment where competition with their fellows is reduced: there will be a tendency for individuals to diversify.

The Species as a whole thus becomes more generalised and its niche broadens. Note, however, that while the niche occupied by the species as a whole is broadened byintraspecific competition, the individual organisms of the population may still occupy very restricted niches within this.

And here it becomes clear how important is the distinction drawn earlier between the niche of an individual and that of the population to which it belongs. Competition of whatever sort causes specialization by the individual towards using that part of a set of resources where least competition with others may be experienced.

It has specific competition causes a specialization of both individuals and of the spaces as a whole; intra­specific competition causes the specialization of individuals alone; the resulting diversification actually broadens the niche of the species as a whole.

This is why, as emphasised on page the different scales of individual or population niche must be distinguished, for as noted, they respond differently to give pressures. Thus interspecific competition causes a narrowing of the niche of both individuals and populations.

Unpredictability of resources, as we shall see; results in an expansion of both individual and population niches, and predation, too, causes a change for both in the same direction. By contract intraspecific competition narrows the niche of the individual, but broadens that of the population.

Stability and Predictability of Resources

Intraspecific competition acts to broaden the population niche; both inter-specific competition and predation pressures act to restrict the niche realised.

The actual niche width finally expressed is a product of these interacting effects. But this is one further factor which may be involved in the final shaping of the make predictability of resources used.

In simple terms, no organism can afford to become too much of a specialist in an unstable environment where conditions and the availability of resources may be unpredictable and changeable: here is a death case for remaining a generalist and preserving a broad niche.

In more general terms the final niche width expressed by any organism must be in part a function of resource predictability. The effects of this may be viewed, rather than in itself as expanding or contracting the niche as instead providing a lower limit to niche width, providing an ultimate ‘stop’ below which other factors, of competition or predation, may not further reduce the niche.

There must in effect be a basic minimum size for a realised niche, below which the niche is no longer tenable: a basic minimum width to which an organism or population can reduce while still remaining viable.

That ‘minimum niche must in fact be determined at that level where its resultant resource utilization curve encloses sufficient ‘area’/amount of resource to maintain the organism or population as a viable unit. And this ‘basic minimum’ or lower limit will vary in relation to resource predictability.

The niche can be restricted to its absolute minimum size only if resources are entirely predictable and wide-spread. As soon as there is any uncertainty in the continued availability of any set of resources, the niche must be maintained considerably wider, in order to include sufficient ‘area of resources’ that even under the worst conditions the resources available to it still remain sufficient to support that niche and its occupants.