Tolerance is a classic concept of ecology that has been heavily employed in explaining and predicting species distribution in Earth ecosystems. Much of our current understanding of species distribution owes to the concept of single limiting factors (law of the minimum), but multiple environmental limitations co-occur in most Earth ecosystems and organisms performance is often limited by several environmental factors simultaneously. Current global climate change with unique blend of environmental stresses has led to explosive interest in the organism responses to multiple limitations and tolerance of multiple limitations. In this article, we review the classic concepts of environmental limitations, stress, and tolerance, and provide an overview of current understanding of tolerance to single and multiple environmental stresses and to simultaneous tolerance to several environmental factors (polytolerance). While the concepts of tolerance, stress, and environmental limitations are broad concepts of ecology, we have chosen the examples in this article from plant kingdom for consistency and also because, plants as sessile organisms are exposed to the widest spectrum of single and combined environmental stresses, and exhibit, thus, the richest range of responses to environmental variability.
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