Use of Life Tables in a Bioclimatic Study of an Experimental Aphid‐Braconid Wasp Host‐Parasite System

to environmental extremes of drought and flash floods. 2. The natural life span for most Rhinichthys oscults in the Chiricahua Mountains is less than 3 years. None was found to reach 4 years. 3. High mortality rates during summer drought must be interpreted, in part, as an acceleration of death among older fish since most fish are expected to die before they are 3 years old. Among younger fish, drought directly elevates the mortality rate. 4. Mortalities during a drought are caused directly by the disappearance of water and indirectly by starvation of the fish which are crowded into reduced habitat with inadequate food. 5. Flash floods are an important cause of mortality among fish of the year if they occur while the fish are very small. The greatest potential loss of fish of the year occurs in late summer when the initial flash flood induces major reproduction and is then followed by another flash flood. 6. Flash floods are not a significant cause of mortality among older fish. 7. Temporary waters may persist in some sections through a period of wet years. The populations of fish inhabiting them are derived from upstream sections during a flash flood. All fish that were located in downstream temporary sections in the spring of 1960, were one-year-olds. 8. Temperatures do not rise to lethal levels in any of the flowing streams. In exposed shallow isolated pools, the temperatures may become lethal for oldcr fish, but not for fish of the year. 9. Predators play a minor role. Only one, the garter snake, Thamnophtis cyrtopsis, is common and apparently important. 10. Although fish populations are greatly reduced by a year of drought, there is no immediate threat of their local extinction.