TERRESTRIAL ENVIRONMENTS AND ECOLOGY 



189 



direct physiological stress on thern. Extreme temperature 

 values for Enewetak are 21 °C and 34.4°C, and these 

 values are rare (Blumenstock and Rex, 1960). 



These extreme temperatures, taken by themselves, 

 probably do not impose severe physiological stress on any 

 of the terrestrial organisms at Enewetak, providing water 

 and shade are available. Seeking shade and the intake of 

 water are well-known active behavioral processes of many 

 terrestrial animals. Moisture and soil conditions determine 

 the distribution and abundance of plants. Studies of the 

 physiological ecology of terrestrial organisms were not 

 undertaken at Enewetak. 



There are, however, supporting observations. For 

 example, Coenobita land crabs and especially Birgus, the 

 coconut crab, discussed later, tend to be nocturnal or at 

 least crepuscular in their activity. On dark, wet, overcast 

 days, however, they are occasionally observed out forag- 

 ing. Conversely, during the dry season they are more 

 active on dark, humid nights of new moon or cloud cover, 

 little wind, and brief rain showers. 



Humidity is affected by temperature and moisture. It is 

 maximal in the morning and decreases in the afternoon as 

 temperature increases. It is higher during the wet season. 

 Physiologically high humidity may have an ameliorating 

 effect on high temperature through evaporative cooling, 

 providing the organism can situate itself in a microhabitat 

 of shade and exposure to wind. Unfortunately supporting 

 data do not exist for terrestrial organisms at Enewetak. 



Brisk, steady winds characterize the weather at 

 Enewetak perhaps as much as the high, unvarying temper- 

 ature and humidity. Trade winds blow from the east or 

 northeast about 95% of the year. During the latter part of 

 the wet season, August through October, wind direction is 

 more likely to shift from the southeast around to the 

 north. Wind speed is about 5.8 to 10.4 ms^^ (or 13 to 24 

 mih~ ). Winds are more brisk in the dry season and tend 

 to weaken in the wet season. Again selective exposure to 

 wind by an organism can ameliorate the effects of high 

 temperature and humidity. Seabirds while nesting on land 

 may position themselves to take advantage of wind direc- 

 tion (Lustick, 1984). 



Partial cloudiness is the rule at Enewetak even during 

 the dry season, when, however, the degree of cloudiness is 

 more variable. The sky is seldom clear. Cloudiness 

 decreases solar radiation and affects the duration of time a 

 plant will be exposed to direct sunlight and, therefore, has 

 a moderating effect on terrestrial ecology. 



The average annual rainfall at Enewetak is 1470 mm. 

 It is not distributed uniformly throughout the year. About 

 85% of it falls in the wet season starting in April and end- 

 ing in mid-November. October is the wettest month. The 

 remaining 15% falls in the dry season. There is consider- 

 able yearly variation. Needless to say, rain affects both 

 temperature and humidity, and cloud cover is greatest dur- 

 ing rainy periods. Situated in the extreme northwest, at 

 11°N latitude, Enewetak is one of the driest of the 

 Marshall Islands. Kwajalein Atoll (9°N latitude) averages 



about 2400 mm, and the average annual rainfall at Jaiuit 

 Atoll (6°N, latitude) exceeds 4000 mm. 



Perhaps the single most important aspect of rainfall in 

 the terrestrial ecosystem is the replenishment of ground- 

 water. The hydrography of Enewetak is discussed by 

 Ristvet (Chapter 4, this volume). The larger islands of the 

 atoll have a lens of fresh water of varying quality and 

 volume. Probably the distribution and abundance of vegeta- 

 tion and related biota on the larger islands are directly 

 related to the availability of groundwater. The correlation, 

 however, is not possible due to drastic alterations of the 

 vegetation resulting from activities during World War II 

 and subsequent events at Enewetak. 



Although on average the weather at Enewetak is both 

 predictable and benign, at least in the general patterns 

 described previously, there are two aspects of the weather 

 which are remarkably variable and unpredictable. Both 

 have profound effects on the terrestrial ecosystem. 



First, wind and rain squalls, wind shifts, periods of little 

 or no wind are of short duration and seem to occur almost 

 at random. They are not detected within the larger 

 weather pattern as measured periodically by conventional 

 weather recording instruments. Nevertheless, these events 

 are probably of great importance to terrestrial organisms in 

 modifying the effects of high temperature, desiccation, 

 humidity, and solar radiation. To my knowledge, measure- 

 ments to substantiate this statement have not been made 

 for the terrestrial biota of atolls. 



Second, the occurrence and severity of tropical storms 

 and typhoons are highly unpredictable. Tropical storms of 

 greatest strength are called typhoons in the Western 

 Pacific. Of eight tropical storms and disturbances which 

 impacted Enewetak from 1959 to 1979, only one attained 

 typhoon strength (Table 2, Merrill and Duce, Chapter 6, 

 this volume). This was Alice, which struck Jan. 5 and 6, 

 1979. Three were tropical storms, whereas the remaining 

 four were classed on the basis of their severity as distur- 

 bances or depressions. What is even more remarkable is 

 that no weather disturbances occurred during the 7-year 

 period 1959 to 1966 and for a 3-year period 1973 to 

 1975. Of the eight tropical storms and disturbances which 

 did occur between 1967 and 1979, six occurred in the 

 wet season with three of these occurring in October, the 

 wettest month (no doubt these data are correlated), but 

 two occurred in January in the dry season. 



The severest of these, Alice in January 1979, caused 

 the greatest amount of damage to the terrestrial environ- 

 ment of any storm that I observed over the 19 years, 

 1960 to 1979, that I visited Enewetak Atoll. Storm waves 

 coming from the east and northeast washed over the entire 

 north end of Enewetak Island (see frontispiece) carrying 

 away vegetation and flooding the laboratory buildings. 

 Wind speeds reached 145 km hr~^ Strand vegetation of 

 Ipomoea vines, Lepturus grass, and Tournefortia and 

 Scaevola shrubs was either washed away by high seas on 

 low-lying, small islets or denuded of leaves by the wind. 

 Massive rearrangement of sand and coral boulders 



