Temperature

Temperature — the degree of hotness or coldness of an environment — is one of the most far-reaching abiotic factors, because nearly every biochemical process an organism runs, from metabolism to reproduction to development, is temperature-sensitive, and extreme temperatures can be directly lethal. Several classical ecological 'rules' capture temperature's influence on body form and physiology. Van't Hoff's rule states that metabolic (reaction) rate roughly doubles for every 10°C rise in temperature and roughly halves for every 10°C drop — this ratio is formalised as the Q10 value, and for most living systems Q10 is close to 2.0. Bergmann's rule observes that within a broadly distributed group of related species, populations in colder regions tend to be larger-bodied than populations of the same lineage in warmer regions, since a larger body has a lower surface-area-to-volume ratio and therefore loses heat more slowly. Allen's rule is the complementary observation that warm-blooded animals in colder climates tend to have shorter limbs, ears and other extremities than their relatives in warmer climates, again to minimise heat loss from exposed body surfaces. Jordan's rule notes an inverse relationship in fish between water temperature and the number of vertebrae (meristic count) — colder water tends to produce more vertebrae. Organisms also differ in their temperature tolerance range: eurytherms can survive across a wide temperature span (an evolutionary advantage that let species survive past ice ages and colonise new regions), while stenotherms — such as many fish, frogs, lizards and snakes — can only tolerate a narrow …