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.
Figure 14.2.1a – Limit of tolerance examples
Activity 1: Analysing data
Study Figure 14.2.1a and answer the following:
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.
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.
- 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.
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:
Figure 14.2.1f – Levels of resource partitioning
Figure 14.2.1g – Woodland duck
The woodland duck's (Cairinini sp.) foraging range is limited by competitive exclusion.
- Define niche
- Distinguish between fundamental and realised niches
- Explain how competitive exclusion leads to narrowing of realised niches
Figure 14.2.1h – Mussel
In the genus Mytilus, metabolism and growth are limited by temperature.
Figure 14.2.1i – P. bursaria
All three species of protists exploit the same type of resources and environment.
Figure 14.2.1j – P. caudatum limits the realised niche of the other paramecium species.
Figure 14.2.1k – P. aurelia has a wider realised niche when it is not in competition with other protists.