Ecology of Marcus Island — SAKAGAMI 
95 
staff member of the weather station, a marine 
strider seems to occur in the lagoon. 
The area receiving direct human influences 
possesses no more than a well-known assem- 
blage of domestic species. The number of species 
is far less than that found in similar environ- 
ments on continents, but, reflecting the diversity 
of environmental conditions, it is fairly large 
in comparison with other habitats, in spite of 
the small space. A rat, two roaches, some domes- 
tic flies as omnivorous scavengers, two granary 
beetles, and domestic silver fish, Ctenolepisma, 
were the chief members in or around the weather 
station and accompanying buildings. Skinks, 
land crabs, and the two ants invaded all build- 
ings. Gecko and Heteropoda lived there as 
residential predators, although they were found 
in the A zone, too. Earthen nests of Sceliphron 
were abundant on ceilings, walls, and other 
parts of buildings. In a ruined cottage standing 
near the northern point of the island, a fairly 
large compound nest containing 62 cells was 
observed attached to a broken chimney (Fig. 
17). In other nests, the number of cells counted 
was as follows: 1 cell alone (1 instance), 2 
cells (3 instances), 3 (4) , 6 ( 3 ) , 7 ( 1 ) , 8 ( 1 ) , 
12 (3), 14 (1), 30 (1), 57 (1). 
Numerous dead insects were observed in win- 
dow screens of the dining room, etc., due to 
treatment with DDT. Examination of these ac- 
cumulations showed an overwhelming abun- 
dance of Atherigona, although the main species 
found within the dining room during our stay 
were Musca, Lucilia, and Sarcophaga. 
INDIVIDUAL 
NUMBER 
(Sexes not 
separately 
SPECIES counted) 
Atherigona excisa 1,505 
Drosophila melanogaster 43 
Lucilia sericata 22 
Aneurina sp 22 
Dolichopodidae gen. sp 18 
Parasarcophaga misera 16 
Agromyzidae gen. sp. 13 
Sphaeroceridae gen. sp 10 
Musca domestica 4 
Tribolium castaneum 3 
Ephydridae gen. sp 2 
Gardiastethus fulvescens 1 
Lasius niger ssp 1 
A micro-moth 1 
All of the moths listed in Table 3, as well 
as Oligotoma and Eobia, were attracted to lamps. 
Because of its oedemogenic secretion, Eobia is 
the only insect species injurious to human be- 
ings. No fleas, mosquitoes, or blackflies occur 
on the island. 
After this brief sketch of the different hab- 
itats, a mystifying fact must be mentioned: a 
dragonfly, Diplacodes bipunctata, occurred on 
the island, even in the absence of fresh water. 
The adults of this species appeared a consider- 
able time after our visit. I observed only a 
single specimen, at a passway penetrating the 
E. triangle, but a staff member of the weather 
station repeatedly confirmed the appearance of 
numerous dragonflies, and later he kindly sent 
me the specimen which was identified. If this 
species multiplies on the island, then not only 
must fresh water be available somewhere but 
also a number of aquatic organisms to be preyed 
upon by its nymphs. In the absence of any 
evidence of fresh water, the only other ex- 
planation must be the seasonal migration of this 
relatively delicate species across thousands of 
kilometers of ocean — although this is an ex- 
planation that I myself find hard to believe. 
FURTHER ECOLOGICAL NOTES 
Ecological interactions among various organ- 
isms in a given area, however few there may 
be, are always difficult to demonstrate clearly. 
But, the uncomplicated environment and simple 
biota of Marcus permit schematizing it as in 
Figure 18. Even if the schema is still far from 
complete in many points, the principal courses 
of biotic energy-flow in the island are obvious 
and may be classified into two major groups 
with respect to the energy sources: those start- 
ing from green plants, and those from the 
products of human activities. The two groups 
are relatively independent of each other, al- 
though, as discussed later, many elements con- 
stituting the former group were brought to the 
island by various human activities. As a glance 
at the figure will show, the extreme disharmony 
between the food chains and the occurrence of 
numerous unoccupied niches is impressive. The 
extraordinary abundance of a few dominant 
species depends, without doubt, on this too 
