THE GENERAL ENVIRONMENT 



85 



and then decrease. As we have seen, how- 

 ever, the lunar periodicity of spawning in 

 L. fucata is bimodal, and the maxima he 

 about the first and last quarters, when the 

 duration of illumination is first increasing 

 and later decreasing. Something more than 

 a simple quantitative relationship is in- 

 volved. One factor that varies as does 

 swarming is the rate of change in the dura- 

 tion of moonlight. This reaches a maximum 

 near the time of the new and the full moon 

 and a minimum at the first and the third 

 quarters. Descriptively, then, for the At- 

 lantic palolo, the eflFectiveness of moonUght 

 seems to be correlated with some aspect of 

 the daily rate of change of duration. The 

 swarming of other annelids may be initiated 

 by other factors, such as a variation in the 

 intensity of moonlight or a change in some 

 direct efi^ect of the tidal cycle. At this point, 

 as in many other aspects of ecology, we 

 await further field and laboratory analyses. 



The angles made by the moon and sun 

 with the plane running through the earth's 

 equator vary independently, and so does 

 the distance of each from the earth. The 

 resultant forces exerted by the two bodies 

 on the waters of the earth vary in a complex 

 fashion, one result of which is that in 

 addition to the daily and lunar tidal cycles, 

 seasonal high tides also exist that have 

 their due eflFect on organisms. Other tidal 

 compUcations may be important locally 

 or along long reaches of the seacoast; dis- 

 cussion of these does not fit into our 

 crowded outfine (Harvey, 1928). Some of 

 the complications, as well as the funda- 

 mentals, are treated simply and with in- 

 sight by Coker (1947). 



Seasonal cycles in tidal amplitude and 

 their efiFects on littoral marine communities 

 are insignificant in comparison with the 

 seasonal changes on terrestrial communities. 

 As stated in Chapter 2, the study of phe- 

 nomena associated with seasonal appear- 

 ance, or phenology, has a long history. In 

 much of the tropics, the annual changes 

 are governed by rainfall and associated 

 factors rather than by temperature, which 

 exerts a controlhng influence in higher 

 latitudes. An intermediate climate, dom- 

 inated by winter rains and summer drought, 

 occurs typically around the Mediterranean 

 Sea and in much of CaUfornia. Many other 

 seasonal variations in climate produce dis- 

 tinctive efiFects upon biotic communities. 



Seasonal appearance does not necessarily 



follow the four conventional seasons even 

 in a region where temperature is a major 

 element in the annual cycle. In woodlands 

 associated with the prairie peninsula in 

 IlUnois, it is often possible to recognize six 

 seasonal subdivisions of the biotic com- 

 munity; these are outfined on page 53. 

 On the south side of the equator, in cut- 

 over and primeval mountain forests in the 

 state of Rio de Janeiro, Brazil, Davis ( 1945, 

 p. 294) also found the year divided into 

 six comparable seasons. The time hmits 

 in such subdivisions are only approximate 

 and may vary widely in diflferent years. 

 The exact number of seasonal subdivisions 

 may also differ according to the community, 

 the geographic and physiographic location 

 of the community, according to the organ- 

 isms used as index species and according to 

 individual judgment as to the time hmits 

 (Clements and Shelford, 1939; Wilhams, 

 1936). 



Other Cycles 



More than fifty environmental periodic- 

 ities had been listed by 1925; these varied 

 in length from a few days to nearly two 

 centuries. Others have been added since 

 that time. Cycles of solar radiation are fre- 

 quently discussed and are highly variable 

 in duration and intensity. Among others, 

 they include recurring periods of seven, 

 eight, eleven, twenty-one, twenty-five, forty- 

 five, and sixty-eight months' duration. 

 The last-mentioned runs for about 5.7 years 

 and is approximately half the length of the 

 sunspot cycle of eleven ^- years. All may be 

 regarded as submultiples of the cycle of 

 magnetic change on the sun that has a 

 periodicity of 276 months, or twenty-three 

 years. A still longer cycle, that of Briickner, 

 lasts from seventeen to fifty years, with a 

 mean length of about thirty-five years. This 

 may be thought of as a threefold multiple 

 of eleven + years or as an effect of inter- 

 ference between this particular sunspot 

 cycle and another of somewhat shorter 

 duration. 



The literature on such cycles continues 

 rich in quantity and varied in quality. There 

 seems to be some evidence of mind-set in 

 discussing these problems, and judgments 

 differ concerning the ecological importance 

 of many of them. Clements and Shelford 

 (1939), Elton (1942) and Huntington 

 (1945), to mention only a few mature stu- 



