14.2 Applications and skills

14.2.1 Using models: Limits of tolerance and the niche

Students often find it difficult to understand what is meant by ‘ecological niche’. A niche is not a physical place or role. Rather, a niche considers the sum of the habitat requirements, both biotic and abiotic, for each species.

Limits of tolerance and zones of stress

A single limiting factor influences the distribution and success of species. An example is shown in Figure 14.2.1a. Here, temperature affects the metabolic activity of three species of mussel of the genus Mytilus. There is more energy available for growth (scope for growth) at different temperatures. The shape of the graph is characteristic of the zone of stress model.

Limit of tolerance example

Figure 14.2.1a – Limit of tolerance examples
Source: www.asnailsodyssey.com 

Activity 1: Analysing data

Study Figure 14.2.1a and answer the following:

  • Which species has the highest scope for growth at 25°C?
  • Which species has the lowest overall scope for growth?
  • Does the data show evidence of niche overlap? Explain.
  • Compare the shapes on the graph to Figure 14.1.1a. How useful is the model in explaining the phenomenon?

A niche considers all limiting factors

Imagine that we now put together all of the tolerance data for all of the limiting factors in a single graph. It would have too many dimensions to represent on a flat surface, but it would represent a species’ niche more realistically.

three-dimensional nicheFigure 14.2.1b – A three-dimensional niche

Figure 14.2.1b shows the distribution of one plant species based on three limiting factors: latitude, elevation and mean annual temperature. A niche tells us about species distribution based on n limiting factors.

Fundamental vs Realised niche

  • Removal experiments demonstrate that most species are limited in natural environments by interspecific competition for resources. An example is shown in Figures 14.1.2c and d.

Paramecium cultured separatelyFigure 14.1.2c – Paramecium cultured separately
When three species of Paramecium are cultured separately, they show a normal pattern of sigmoid population growth.

Paramecium cultured togetherFigure 14.2.1d – Paramecium cultured together 
When different species of Paramecium are cultured together, one species is limited.

  • Because competition excludes species from their full range of habitat, the realised niche is smaller than the fundamental niche.
  • We use the terms ‘narrow’ and ‘broad’ to describe a species niche. Niches are narrowed as a result of competition.

water fowl population

Figure 14.2.1e – Waterfowl populations
Distribution of waterfowl according to foraging habits. Foraging area has been partitioned as a natural consequence of competition.

Activity 2: Analysing data

Study Figures 14.2.1e and 14.2.1f and answer the following questions:

  • Which species of waterfowl show niche overlap?
  • Consider pairs of species and match each pair to the level of resource partitioning shown in Figure 14.2.1f.
  • Extension: How does resource partitioning relate to evolution?

overlaps

Figure 14.2.1f – Levels of resource partitioning

woodland duck

Figure 14.2.1g – Woodland duck
The woodland duck's (Cairinini sp.) foraging range is limited by competitive exclusion.

Key questions

  • Define niche
  • Distinguish between fundamental and realised niches
  • Explain how competitive exclusion leads to narrowing of realised niches

musselsFigure 14.2.1h – Mussel
In the genus Mytilus, metabolism and growth are limited by temperature.

P. bursariaFigure 14.2.1i – P. bursaria
All three species of protists exploit the same type of resources and environment. 

P. caudatumFigure 14.2.1j – P. caudatum limits the realised niche of the other paramecium species.

P. aureliaFigure 14.2.1k – P. aurelia has a wider realised niche when it is not in competition with other protists.

Nature of Science

A good model explains phenomena and has predictive power. Discuss ways in which the limits of tolerance model can predict interactions between species in a community.