Tolerance Range and Species Distribution

A tolerance range refers to the optimum levels of environmental factors that a species needs to survive and thrive. Every species can cope with changes in factors like temperature, soil nutrients, salinity etc. within a certain range. Beyond the upper and lower limits of this range, individuals cannot live.

For example, tropical fish species can only survive in water temperatures between 22°C and 28°C. Temperatures dropping below 16°C or exceeding 30°C would kill them. On the other hand, goldfish have a wider temperature tolerance range between near-freezing up to 24°C. As long as changes in water temperature are gradual, goldfish can adapt.

Within the tolerance range, there is an optimum sub-range of each factor where organisms operate at peak efficiency. A species abundance is highest within this optimum zone. Towards the upper and lower limits, smaller populations subsist, although not thriving.

What Governs Species Distribution?

The law of tolerance states that the existence, numbers and spread of a species depends on environmental factors remaining within the tolerance limits. Every individual has slight variations in tolerance, based on genetic differences. But most members of a species share an average tolerance range.

This is why certain species are restricted to particular ecosystems and parts of the world. For example, magnolia trees cannot handle freezing winters and are absent from northern forests. Cacti and other desert plants need hot and dry conditions to survive, not damp tropical forests. Factors like temperature, rainfall, soil type limit which species can inhabit an area.

How Do Limiting Factors Impact Growth?

Even when most environmental conditions are ideal, single limiting factors can curb population growth. This limiting factor principle applies when there is too much or too little of something, relative to a species’ specific needs.

For example, farmland may have adequate water, space and sunlight for growing corn. But corn growth stops upon using up the limited available phosphorus in the soil. Here phosphorus scarcity directly limits how much corn survives, irrespective of other favorable factors. In this ecosystem, phosphorus becomes the limiting factor for that species.

Can Gradual Change Lead to Sudden Disasters?

Organisms able to adapt to gradually changing environments have an important survival mechanism. For instance, people can gradually get used to increased ambient temperatures. But there is a tolerance threshold beyond which health sharply declines.

This threshold effect means that the last small change after a series of gradual changes can have huge detrimental effects. Multiple environmental problems like deforestation, pollution, climate change etc. keep worsening gradually. Their cumulative impact builds up over years and then suddenly entire forests die off or species go extinct.

We often fail to take preventive actions in time due to the delay in visible effects. But once threshold limits are crossed, the resulting damage may be irreversible.

Flexible Species Have Wider Tolerance

Species native to fluctuating habitats like coastal regions generally have wider tolerance ranges and adaptability. For them, environmental factors like water level, temperature and salinity keep varying within months and years. Compared to this, species in tropical rainforests and the deep ocean evolve within relatively stable conditions.

When environmental conditions remain quite constant over evolution, organisms cope best within a narrow range around that optimum zone. Minor or sudden variations in their habitat prove detrimental. Consequently, stable ecosystems often have a lot of endemic species found nowhere else.

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