Concepts of stress and limitation

Possibly the most widely used term in ecology, stress (a special, extreme form of limitation), is often used in a very misleading way. Note:

• What is stressful to humans is not necessarily stressful for other organisms (the anthropocentric bias).
• What is too stressful to some wild organisms, opens life conditions for others, although these conditions may be far from those permitting maximum growth rates (the optimality bias).
• A relief from what is considered stressful, commonly leads to the extinction of those suspected of having suffered from stress, mostly because of competitive exclusion.

Remember:

• Only those which are not fit are stressed.
• Evolution selects for fitness.
• Old succession communities which have passed the sieve of evolutionary selection hardly ever experience fatal stress conditions.

1 - Moving plants from the snowline to montane life conditions will be fatal for these plants. Most plants adapted to life at extreme high elevation do not survive in lowland botanical gardens, because the warmth exerts 'stress' and because they will be overgrown (outcompeted) by local species (Sajama volcano, Bolivia, 6542 m).

2 - Fit to survive and reproduce under demanding environmental conditions (Ranunculus glacialis, Alps, 3150 m).

This picture was taken in the middle of the short nival zone summer (8 August). What looks fatal from a lowland perspective is not really problematic for high altitude specialists. These plants need only a few days of good weather in a 5 - 6 week summer to complete their seasonal life cycle. As perennials they can also survive years in which snow cover does not disappear. Surprisingly, these plants can even cope with substantial losses of green leaves due to snow-vole grazing, which suggests that they are not constrained by a shortage of photoassimilates.

2 - An example for the European Alps: Ranunculus glacialis in midsummer at 3150 m rapidly dies once transplanted to low elevation.