Chapter 1 1 



Terrestrial Environments and Ecologx; of 

 Eneivetak Atoll 



ERNST S. REESE 



Department of Zoology, University of Hawaii 

 Honolulu. Hawaii 96822 



INTRODUCTION 



Enewetak Atoll is a necklace of 39 coral islands sur- 

 rounding a circular lagoon. The atoll is a coral limestone 

 cap about 1400m thick sitting on a pedestal of volcanic 

 basalt rising abruptly some 5000 to 6000 m from the sea 

 floor. Enewetak is about 50 to 60 million years old, having 

 its birth in the Eocene of the Tertiary Period. It is notewor- 

 thy that during this time, sea level was about 50 m below 

 the present level and that during the Wisconsin glaciation, 

 about 20,000 years ago, the sea level may have been as 

 much as 150 m below present sea level. What we do not 

 know is to what extent vertical tectonic movements cou- 

 pled with rates of coral growth match these sea water level 

 changes. There is evidence that at one time the atoll was a 

 raised coral limestone island. The physiography and geol- 

 ogy of Enewetak are discussed by Colin and Ristvet, 

 respectively, in Chapters 3 and 4 of this volume. 



The dry land area of Enewetak Atoll is only about 2.5 

 mi^, about 6.5 km^, about 1600 acres, or about 647.5 

 hectares. The total land area is less than 4 m above sea 

 level. The 39 islands which comprise this dry land area are 

 distributed along the north, east, and south perimeter of 

 the atoll (Fig. 1). A single, small island, Biken, occurs iso- 

 lated on the west rim. The islands range in size and biotic 

 diversity from extremely small patches of coral rubble colo- 

 nized by sparse vegetation to the larger islands of Enjebi, 

 on the north rim, which is triangular in shape and mea- 

 sures about 1.2 km in size, and Enewetak, on the south- 

 east corner, which is elongate and measures about 1.3 

 km in size. The larger islands on the south rim of the 

 atoll — Ikuren, Mut, and Boken — support a forest of 

 mature coconut and Pisonia grandis trees and a 

 correspondingly richer biota; however, these three islands 

 together constitute only 0.45 km^ of land area. It is impor- 

 tant to note that the area of the lagoon, approximately 

 925 km^, is about 138 times larger than the total area of 

 dry land. 



The terrestrial ecosystem of Enewetak Atoll is the 

 result of the dynamic interaction between the biota associ- 

 ated with the small dry land area and physical parameters 

 of the environment, especially the climate, soil, and 

 groundwater. The shrubs and trees, man, birds, rats, and 

 land crabs are among the more conspicuous elements of 

 the terrestrial biota, whereas climatic events, the soils, and 

 the availability of groundwater are the most important 

 physical components of the ecosystem. 



CLIMATE AND WEATHER 



The climate of Enewetak Atoll is determined by its 

 geographical location in the north central Pacific. The atoll 

 lies well within the northeast trade wind area. The meteo- 

 rological events affecting Enewetak and details of the cli- 

 mate are discussed by Merrill and Duce (Chapter 6, this 

 volume). 



With respect to ecology, there are a number of highly 

 relevant aspects to the weather at Enewetak that deserve 

 mention. There are two seasons, the dry season of approx- 

 imately 4 months duration, December through March; and 

 the wet season of approximately 8 months duration 

 extending from April through November. All aspects of the 

 weather are tied into this seasonal pattern. 



Unvarying high temperatures, high humidity, moderate 

 rainfall, steady easterly and northeasterly tradewinds, and 

 partial cloudiness are all modulated seasonally. Even tropi- 

 cal storms and typhoons, which are otherwise unpredict- 

 able from year to year, occur more frequently in the wet 

 season. 



The average minimum temperature in the dry season is 

 23.6°C and 23.7°C in the wet season. The average maxi- 

 mum temperature in the dry season is 30.6°C and 31.8°C 

 in the wet season. The variance around these means is 

 very small (Table 1 in Merrill and Duce). The minimum 

 temperatures occur at night or during storms, whereas the 

 highest temperatures occur during the afternoon of the rel- 

 atively few cloudless days, particularly during the months 

 of August and September in the wet season. 



Average temperatures, of course, are less important to 

 organisms than are extreme temperatures which may exert 



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