Lesson 5 - Tolerance

Question 1

Environmental Gradients

Answer 1

An environmental gradient is a gradual change in abiotic factors through space (or time). Environmental gradients can be related to factors such as altitude, temperature, depth, ocean proximity, and soil humidity.

Species abundances usually change along environmental gradients in a more or less predictive way. Species abundance along an environmental gradient is not only determined by the abiotic factor, but also by the change in the biotic interactions along the environmental gradient.

Question 2

Tolerance

Answer 2

Tolerance is an organism’s capacity to survive variation in environmental conditions. Environmental factors affecting an organism do not remain constant. Any organism must be able to tolerate changes in the environment if it is to survive. The tolerance of the organism influences how the organism is distributed in its environment.

For any factor there will be an optimum range —> conditions are ideal. For any factor there will also be a zone of stress and limits of tolerance.

Question 3

Zones (different to tidal zonation)

Answer 3

Optimal range:
- Where the organism is comfortable with the environmental condition (individuals abundant).

Zones of physiological stress:
- Found on either side of the optimum and where the organism feels stressed and uncomfortable (individuals infrequent).

Upper and lower limits (zones of intolerance):
- Where an organism dies due to too much or too little of the abiotic factors (individuals absent).

Question 4

Liebig’s Law of Minimum

Answer 4

The extent of a species’ habitat and its distribution pattern in the environment are determined by its tolerance of the least favourable abiotic factor (limiting factor).

Growth is dictated not by total resources available, but by the scarcest resource (limiting factor). If just ONE factor is varied beyond the limits of tolerance, then the species could not survive. Such factors that have exceeded the limits of tolerance are called limiting factors.

Limiting factor - the only factor that is missing from the organism’s environment that prevents it from thriving e.g. temperature too cold is the only limiting factor for trees on a mountain at a high altitude.

—> The scarcest amount of a certain abiotic factor (called the limiting factor) determines the survival of an organism.