predominantly of coarse coral rubble (75-98% coverage). 

 These liny motus are the simplest ecosystems on the atoll, 

 representing early Stage I in plant succession. The number of 

 plant species per motu averages three, all hardy, sea-dispersed, 

 and salt-tolerant pioneers (Heliotropium, Portulaca, Lepturus, 

 Tournefurtia). The sole plant community is a natural herb mat 

 of varying thickness and extent. Tournefortia, though stunted 

 and scattered, is not sufficiently common to form a separate 

 scrub habitat. Indigenous vegetation covers 2 to 22% of the 

 islet areas. Seabirds. especially brown noddies and red-tailed 

 tropicbirds, may attempt to nest. 



b) Motus with Areas 0.2 to 0.7 ha ( Figs. 28,3 1 : Tables 5.6): 

 There are five leeward motus in this category whose combined 

 areas total 2.21 ha. Their vegetative cover is more extensive 

 and diverse than in size class a, with herb mats and Tournefortia 

 scrub and forest, but open rubble is still abundant (30-55% 

 cover). Plant succession corresponds to late Stage I and 

 Stage II. The average number of species is 8.2 (range 6-11), 

 one-third of Caroline's total. All vegetation on these motus is 

 indigenous except for a few Cocos palms. Seeds are dispersed 

 by sea, wind, and birds. 



With the appearance of shrubs, the number of species 

 increases markedly, and woodlands, primarily of Tournefortia, 

 form and expand to create dense thickets averaging 5 m tall and 

 covering 25% of the land area. Canopies of 10 m occur on 

 motus Nautonga and Kota. Seabird colonies of up to six 

 species (brown and red-footed boobies, great frigatebirds, 

 black and brown noddies, white terns) are present. 



A low herb mat, dominated by Heliotropium, Portulaca, 

 Boerhavia, and, more rarely, Lepturus, develops first, after 

 which Tournefortia quickly becomes established. Shade, 

 producing locally humid conditions, and better "soils" derived 

 from guano, decomposing leaves, and the activities of land 

 crabs and rats, provide appropriate habitat for Laportea and 

 occasional Phymatosorus and Achyranthes. The major tree 

 species — Pisonia, Morinda. Cordia, and Cocos — subsequently 

 appear but are relatively rare. Pisonia, typically an inland 

 species assumed to need companion trees and underground 

 water (Wiens, 1962; Spicer & Newbery. 1979), could well be 

 salt tolerant as it occurs on motus as small as 0.2 ha (Tables 5,6 ). 

 In this size class Pisonia occupies only 2-6% of the total islet 

 areas. 



c) Motus with Areas from 0.8 to 25.0 ha (Figs. 29,31; 

 Tables 5,6): All 27 motus in this category share a similar 

 complement of species and plant communities (Tables 5, 6). 

 Their combined area totals 1 24.35 ha. They are well wooded 

 (Fig. 29), although the leeward motus have a higher proportion 

 of rubble and herb mats, and forests are higher to windward. 

 Unvegetated rubble covers less land area (219? ) than in size 

 classes a and b (87% and 39%, respectively). Within the 

 woodlands of these motus, substrates mature from basic rubble 

 to primitive "soils" with small, but significant, structural 

 development. Then flora shows increasing diversity with size, 

 and almost the lull complement of seabirds may nest. 



All natural ecosystems are firmly established; canopy 

 heights range from 4 to 2 1 m. On Booby Islet (0.84 ha). Pisonia 

 suddenly becomes very common, and the Pisonia forests on 

 North Brothers (1.71 ha) and Pig (7.25 ha), at 21 m. are the 



tallest on Caroline. As rich guano and dead foliage accumulate, 

 a layer of phosphate-rich humus enables those species already 

 present but poorly represented on the small motus (Pisonia. 

 Morinda, Boerhavia, Laportea, Achyranthes) to increase in 

 abundance and stature (Table 6). Additional species are Suriana, 

 Pandanus, Scaevola, Ipomoea, Lepidium, and Species A. 



Plant succession, ranging from Stage III to Stage V in the 

 interior, primarily involves forest maturity rather than the 

 addition of large numbers of species. On the larger islets, the 

 numberof plant species increases by relatively small increments, 

 filling out the subcanopy layers and, in the cases of Cordia and 

 Pandanus, adding variety to the canopy. 



The average number of plant species is 1 1 .0, ranging from 

 4 to 15. If we divide the motus into smaller size classes, we find 

 that their species numbers increase slightly with increasing 

 size: 8.0 species for areas 0.8-1.0 ha, 9.8 species for areas 

 1.1-2.0 ha, 1 1.5 species for areas 2.1-4.0 ha. 1 1.3 species for 

 areas 4.1-10.0 ha, and 12.0 for areas 10. 1-22 ha. An increase 

 in herbs (range 3-9) is primarily responsible for these higher 

 averages (Table 5). 



Despite the large range of motu sizes in this category, plant 

 communities are essentially natural (Table 5). Their overall 

 species composition is 96% indigenous. Seventeen of the 

 motus lack Cocos, the only introduced species in this area 

 category, which is represented by small, isolated clumps or 

 individual palms. 



On the larger motus, and within the taller forests, more 

 species of birds, especially red-footed boobies, great frigatebirds. 

 white terns, and black noddies, nest in increasingly large 

 colonies, furnishing more minerals to the developing soils, 

 especially where Pisonia covers large areas. 



In summary, by the time a motu on Caroline has reached 

 0.8 ha in size, all the natural plant communities, most species 

 of trees, shrubs, and herbs, and most species of seabirds are 

 present. In Caroline's depauperate flora there are few species 

 left to increase floral diversity on the larger islets, regardless of 

 their size. This is very different from the inhabited atolls such 

 as Kapingamarangi. 



Although we do not know when true freshwater lenses 

 develop, they may occur in motus of this size class. If we 

 assume that Pisonia is not salt-tolerant, limited fresh water 

 must be available on motus as small as 0.2 ha. and actual 

 freshwater lenses may begin forming at around 0.7 ha, as 

 indicated by the sudden proliferation of Pisonia forest 

 (Tables 5,6). However, the Falconers have not been successful 

 in finding any underground fresh water on Motu Ana-Ana 

 (2. 16 ha), which suggests that Pisonia may be salt-tolerant. 



d) Motus with Areas >25.0 ha (Figs. 30,31; Tables 5.6): 

 On Caroline, no motus fall between 22 and 75 ha in size. Thus 

 the three motus in this category (Nake. South, Long) cover a 

 limited range of sizes: 75.98 to 107.50 ha. They average 18.0 

 plant species. The floral components and forest heights of 

 these larger motus (Figs. 32,33; Table 5) are essentially the 

 same as for class c. There are no additional ecosystems 

 ( mangrove swamps, salt Hats, grasslands, et cetera ) or understory 

 layers. Ten species, all rare or uncommon, are present only on 

 the larger motus (Table 3): Scaevola. Tribulus, Hibiscus, 

 Thespesia, Ximenia, Psilotum, Tacca (introduced in 1834), 



24 



