How does cold constrain life cycles of terrestrial plants and animals?

Short summer seasons with low thermal energy input characterise polar ecosy stems. Climatic variables and physical isolation of terrestrial habitats ac t as selective filters which must be passed to allow colonisation, establis hment and survival in these extreme environments. Life history studies of t he terrestrial biota of such ecosystems give little evidence of adaptive re sponses to low temperatures having evolved in situ, even though behavioural , ecophysiological and biochemical features allowing tolerance of the likel y extremes are well-developed. Observed life history strategies are often c onsistent with the general predictions of "adversity" (A) or "stress" (S) s election. However, biota successful in these extreme environments may be be tter-grouped by the lack of particular life history features, rather than c ommon possession.