TERRPSTRIAL ENVIRONMENTS AND ECOLOGY 



197 



1400 crabs during the rainy wet season. Because of the 

 life history characteristics of Birgus emigration, immigra- 

 tion, natality, and morality are not considered to affect 

 these estimates Adult crabs cannot move between islands. 

 Small, young crabs are secretive, slow growing, and not 

 numerous in the data. Large adult crabs are estimated to 

 be 30 to 40 years of age. Therefore, the differences in 

 population estimates arc postulated to result from foraging 

 behavior. The best explanation of the data is that during 

 ideal conditions of moisture and lush vegetation, the crabs 

 forage every night, while under adverse, dry conditions, 

 they forage only every third to fourth night. 



The ratio of females to males is nearly even. Males are 

 larger than females. Crabs are solitary. Small crabs defer 

 to large crabs when feeding. 



Four species of the genus Coenobita occur at 

 Enewetak. Coenobita perlatus is the most abundant and 

 conspicuous species. The large red adults remain in the 

 forest during the day but go to the beach at night to for- 

 age, to replenish the water in their shells, and to repro- 

 duce. They prefer dark nights. A full moon tends to inhibit 

 their activity. Small C. perlatus occur at all times closer to 

 the beach; and their numbers are associated with the 

 amount of debris and cover present on the beach. They 

 may be tied closer to the beach by osmoregulatory 

 demands. Coenobita rugosus is common too, and both 

 large and small individuals tend to occur farther inland. 

 They are less common on the beach, and these were gen- 

 erally females engaged in releasing their larvae into the 

 sea. Coenobita breuimanus is not common. It tends to 

 occur deeper in the forest often closely associated with 

 Birgus and indeed has been confused v;ith Birgus. Gross 

 (1964) suggested the C breuimanus may be more depend- 

 ent on fresh water for shell reservoir replenishment than 

 the other two species. Coenobita cavipes is rare. The avail- 

 ability of suitable, empty gastropod shells appears to limit 

 the population of crabs on smaller islands such as Bokan- 

 dretok when compared with larger islands like Ikuren. 



Coenobitid crabs are scavengers like Birgus, and Page 

 and Willason (1983) demonstrated that they play an impor- 

 tant role in reducing carrion and thereby potential fly 

 breeding sites. They also feed on fruits, flowers, roots, and 

 seedlings of a wide variety of plants. 



Land crabs play an extremely important role in atoll 

 ecosystems. Alexander (1979) and Fosberg (Wiens, 1962) 

 observed that crabs carried seeds from the beach into their 

 burrows, thereby effectively planting them, providing they 

 were not eaten. They also noted that on atolls where 

 Coenobita scavenged and removed carrion, flies were not 

 abundant, whereas on atolls where Coenobita were scarce, 

 flies were common. Flies lay their eggs in rotting organic 

 material, especially carrion. In addition, the burrowing 

 behavior of land crabs tends to mix and aerate the rubble 

 and poor soil of the atoll. 



On uninhabited islands adult Birgus latro reign as the 

 dominant terrestrial animals. They are vulnerable to rats 

 and insects at time of molting only, but this is done under- 

 ground affording some measure of protection. Man is the 



principal predator on adult coconut crabs and probably 

 adult Coenobita as well in the atoll ecosystem. Where 

 human populations are high, crab populations, especially 

 populations of Birgus. considered a delicacy, are low. 



Rats 



Although the biology of Enewetak rodents is treated 

 elsewhere in this volume (Chapter 12, Jackson et al), it is 

 important to attempt to evaluate the impact of rodents on 

 the atoll ecosystem. In general, rats are considered de- 

 structive to island ecosystems (Smith, 1969; Wodzicki, 

 1969, 1972). They are a major problem in coconut and 

 sugar cane plantations. The least offensive is the Polyne- 

 sian rat, Rartus exulans, considered a commensal with 

 man; it probably accompanied, most likely as a stowaway, 

 the early Micronesians on their voyages of discovery. The 

 roof rat, Rartus rartus, and the Norway rat, Rattus norueg- 

 icus, are larger and do more damage. Fortunately, at the 

 time Jackson and his colleagues conducted their surveys 

 (1964 to 1978) only the roof rat was present at Enewetak. 



Temme (1982) examined the stomach contents of 602 

 Polynesian rats collected in the northern Marshall Islands 

 during the wet season, October and November 1978, 

 including 243 from five islands of Enewetak. By estimated 

 volume, about 98 to 99% of the diet of R exulans is of 

 plant origin. The remaining 1 to 2% is animal matter, prin- 

 cipally insect remains. It should be noted, however, that 

 the cellulose of plant material and the chitin of insect parts 

 are more readily detected in stomach contents than, for 

 example, the remains of a bird's eggs. Temme noted that 

 those islands which were free of rats had the largest bird 

 populations. 



At best, rats may contribute to the atoll ecosystem by 

 digging burrows, thereby helping to aerate the soil, and by 

 feeding on carrion, including human excrement, thereby 

 reducing the potential breeding sites for flies. At worst, 

 because they enter human habitations, they may be vec- 

 tors for disease, they compete with man for plant food, 

 and, in all probability, they attack the eggs and young of 

 nesting seabirds. The latter may be their most destructive 

 role because seabirds play such an important role in pro- 

 viding organic replenishment of atoll soils. Seabirds arc 

 perhaps the single most important group of organisms pro- 

 viding an energy bridge from the marine to the terrestrial 

 atoll ecosystem. 



MAN AND THE ISLAND ECOSYSTEM 



Carrying Capacity 



Man is the dominant biotic component of the terrestrial 

 ecosystem. This is especially true for small, isolated, 

 self-sustaining ecosystems such as islands. Indeed, the 

 concept of carrying capacity of Pacific islands for human 

 papulations is the subject of considerable interest and con- 

 cern (Bayliss-Smith, 1975; Carroll, 1975; Kirch, 1980; 

 Kiste, 1974). The consensus is that island populations had 

 reached their full potential size before contact with Euro- 

 peans. In the Marshall Islands, ownership of land is of 



