The Effect of Temperature on the Life-Cycle of Four Species of Lake-Dwelling Triclads

Abstract

Temperature is unquestionably an important factor in the ecology of most animals especially poikilothermous species. Recent accounts of the various ways temperature may influence the distribution and abundance of organisms are given in Allee, Emerson, Park & Schmidt (1949) and in Andrewartha & Birch (1954). However, Odum (1953) points to the danger of over-emphasizing the role of this factor and also to the problem of interpolating from the constant temperatures generally used in the laboratory to the complex regimes found in nature. Bullock (1955) is critical of the failure of many ecologists to recognize the extent to which homeostasis may operate in narrowing the range of temperature changes within poikilothermous animals. The present paper deals with the effects of relatively constant temperatures upon speed of development in the various stages of the life-cycle of lake-dwelling triclads and with their tolerance ranges. We are concerned with two levels of approach, one strictly utilitarian in the sense that the data are necessary to interpret the life-history in the field, a second of more general implication concerns the possible effects of temperature upon distribution and abundance. Here there is both the complex problem of the part which temperature changes in the past may have played in determining present-day distribution, and the part temperature may be currently playing in maintaining it. It will be clear, therefore, that we are concerned with ecological aspects rather than physiological, with tolerance limits and life-cycles rather than enzymes and hormones. The unreality and difficulty of applying results obtained from a study based on constant temperatures to nature, are real both at the physiological level and at the practical level of measuring the temperatures to which the animals are exposed in the field. This problem is discussed admirably by Andrewartha & Birch (1954) who indicate several studies which show that development usually proceeds faster under conditions of varying temperature than under constant temperatures when both regimes have the same mean. However, despite this, a knowledge of the effect of temperature upon the range and speed of development at all stages of the life-cycle, even at constant temperatures, is a useful tool in the hands of the ecologist as shown for example by the application of thermographs to the study of insect pests (Bodenheimer 1938). Indeed, this factor must be included in the majority of studies dealing with population events in nature, particularly comparative studies of several species. Temperature has already been identified as a factor of over-riding importance in the distribution of stream-dwelling triclads (see Dahm 1958 for discussion) although tolerance limits are still in dispute and further work on indirect effects is needed. Despite all the attention paid to temperature, Macan (1961) is still able to say justifiably that its modus operandi at the ecological level is imperfectly known except for some species of fish studied-by Fry and his school at Toronto. It is against such a background that the effects of temperature upon lake-dwelling triclads have been studied. This work deals with the effect of relatively constant temperatures selected over a range of 1%5-24' C upon cocoon production, speed of egg