Ecology of Marcus Island— Sakagami 
93 
both species number and ecological endemicity. 
This is natural because this habitat occupies 
more than half of the island and is biologically 
the most productive and stable zone. It must be 
mentioned, however, that A has a relatively poor 
fauna, depending on its very simple vegetation, 
as in C' 2 . 
3. D' is obviously the poorest habitat because 
of its aphytic conditions; this conclusion, of 
course, pertains only to our observations upon 
the macroscopic animals. Bio-economically, this 
habitat really is the front of the marine littoral 
ecosystem extending into the land. On the other 
hand, the relatively rich number of species found 
in B and C, in spite of their small size, is ap- 
parently due to their ecotonal character. 
4. The poor differentiation of Q and Bi 
(compare the two serial orders in Table 4) may 
be understood if these strata are considered as 
a mere extension of an ecological gradient, of 
which the peak lies in Ai. The structure of the 
floor fauna varies, therefore, at first when the 
plant cover almost disappears in D. C'i has also 
a few characteristic species corresponding to its 
littoral nature. 
DESCRIPTION OF EACH HABITAT 
The several habitats distinguished above must 
not be considered to be like cages or walled 
areas which confine various inhabitants within 
them. They are merely devices of a coordinated 
system for the clear understanding of the ecolog- 
ical make-up of the island. Eventually, certain 
species pass freely from one habitat, or from 
one stratum, to another. Before describing each 
habitat and its inhabitants, brief notes will be 
given concerning these mobile species. 
The rat, Rattus rattus ssp., is the only mam- 
mal inhabiting the island. Formerly, the staff 
of the weather station kept cats which con- 
trolled a considerable number of rats. In the 
absence of any intensive controls, the rats are 
now fairly abundant and their activities were 
traced everywhere in the island. 
The skink, Crypto blepharus, and the land crab, 
Geograpsusp were also seen everywhere, except 
° As most recorded genera are represented by a sin- 
gle species, only generic names will be given in the 
following descriptions. 
Ba and D' in the case of the former species, 
and except B 2 , Ba, C 2 , C' 2 , and D' in the latter 
one. Both can climb up Messerschmidia and 
Pisonia to fairly high twigs. They even appear 
in the upper stories of buildings: crabs were 
often observed when they were crawling up 
vertical walls nearly to the ceiling. It is certain 
that these animals, one as a predator and the 
other as a scavenger, play important roles in 
the bio-economy of the island. 
Two ants, Lasius and Tapinoma, may be added 
to the list of widely roaming species. They were 
observed utilizing the runners of Ipomoea to 
invade even into area C' 2 » where other animals 
were scarcely seen. Although it is a relatively 
sedentary creature, a cricket, Landreva, was col- 
lected in almost every floor stratum except Q 
and D'. Its songs could be heard in the daytime, 
but they were more impressive at night, dom- 
inating this tiny bit of land in the midst of the 
immense ocean. 
Setting these mobile species aside, some char- 
acteristic features of each habitat will be out- 
lined. 
Zone A is the largest, richest, and most stable 
habitat in the island. This is also the only area 
where the formation of humus is relatively con- 
spicuous. Consequently, because of the lodging 
it affords various cryptic animals ( roaches, land- 
isopods, myriapods, etc., under stones, Oxydema 
in decayed wood), Ai has the richest fauna in 
the island. A 2 consisted of Ipomoea foliage 
alone. Sphinx, Prodenia, and Coccus were the 
major pests of the vigorous creeper. Prodenia, 
especially, was locally very abundant, and con- 
siderable damage was observed, as is shown in 
Figures 6 and 7. 
Locusta and Utetheisa, both feeding on Mes- 
serschmidia, surprised us by their spectacular 
abundance. The adults of JJtetheisa are active 
irrespective of diurnal rhythm. In daytime, they 
were seen everywhere in the A zone, feebly 
fluttering from one tree to another. At night 
they swarmed abundantly around lamps. The 
first instar larvae live concealed within the young 
sprouts (Fig. 14); older ones feed on exposed 
leaf surfaces, and pupae are seen near the tips 
of leaves, in a thin hammock spun by themselves 
(Fig. 13). 
