Ecology of Manana Island, Hawaii — Tomich et al. 
365 
10-15 years after establishment of rabbits. This 
great number of animals almost certainly was 
generated and sustained by an original reserve 
of woody shrubs and creepers, and other plants. 
Such species as Scaevola taccada and Ipomoea 
pes-caprae that are common on similar offshore 
islets (e.g., Mokulua) reasonably could be ex- 
pected to have occurred on Manana. Their ab- 
sence from the more than 3 5 -year record of 
plants on Manana may well be a result of their 
extermination and subsequent exclusion by rab- 
bits. Parallel conditions of depletion have been 
observed on Laysan and Lisianski (Watson, 
1961). Perhaps on Manana the pattern of 
destruction was modified and prolonged by a 
heavy bag of rabbits taken periodically for 
meat, annual seasonal recovery of the vegeta- 
tion, and a series of unusually wet years. What- 
ever the details, the result is what we see today, 
a reasonable equilibrium between a low to 
moderately dense rabbit population and vegeta- 
tion composed predominantly of hardy intro- 
duced grasses and forbs. 
Rainfall at Makapuu Point, Oahu, 2.4 km 
from Manana, averages 597 mm per year (U.S. 
Weather Bureau, 1964), close to the 635 mm 
estimated for the open ocean in this region 
(Blumenstock, 1961), and is therefore useful 
as a reference to precipitation on Manana. Of 
the annual rainfall 77% falls between October 
and March, and so the April-to-September 
period is typically arid. Watson (1961) has 
suggested that on islands with rainfall regimes 
such as this, periodic (normally annual) 
droughts result in a drastic reduction of the 
rabbit population, which enables the vegetation 
to, "recover sufficiently to survive.” The present 
annual cycle seems to be as follows. During the 
dry season (summer) the vegetation dies back, 
and the rabbit population declines considerably. 
Seeds have been produced and are present on 
the ground. When the rainy season begins 
(early winter), the seeds germinate, seedlings 
become established, and regrowth of perennials 
occurs before the rabbit population increases. 
In the winter an adequate food supply is avail- 
able, but onset of the next dry season results in 
a return to conditions of food shortage and 
decline in numbers of rabbits. Drought, ac- 
companied by recession of the vegetation, may 
be the strongest limiting factor for this popula- 
tion. However, the European rabbit does not 
need free water to drink and by remaining deep 
in its burrows in the heat of the day it mini- 
mizes transpiration loss. In pen experiments on 
water deprivation, rigid selection occurs, but a 
few rabbits survive (Hayward, 1961). The 
strain on Manana may have faced drastic 
drought conditions several times in the long 
period of its adaptation and may be capable 
of behavior resembling estivation. 
In the three wet seasons of October to March, 
from 1961 through 1964, rainfall was normal 
or above normal. There were no winter 
droughts. These conditions support the likeli- 
hood that there was a regular November- to- 
April breeding season during these three years 
as derived from age classification of 12 rabbits 
collected in 1964. The driest period was June 
through September 1963 when only 38 mm of 
rain fell (half of the normal for these months), 
and the wettest was in March through May 
1963 when 866 mm fell (five times normal). 
Because of the rapid percolation of water 
through volcanic soils and the high rate of 
evaporation at the latitude of Hawaii, repeated 
moderate rains are ideal for normal growth of 
most plants on Manana. Distribution of rain- 
fall in time, therefore, may be more important 
than total precipitation for any year. 
In addition to their effective control of the 
vegetation by grazing, rabbits are capable of 
contributing to seed dispersal. Several animals 
had plant reproductive structures entangled in 
their pelage. Thorough combing of one skin 
yielded many spikelets with subtending bristles 
from Setaria verticillata, some spikelets of 
Cenchrus echinatus, and one spikelet of Chloris 
infiata. Rabbits must disseminate plants from 
one part of the island to another, but, because 
of the small area, other means of transport also 
could account for the fairly uniform distribution 
of plant species. 
Because our study has been limited to a 
review of the scanty history of conditions on 
Manana, brief sampling of the present flora 
and fauna, and interpretations of the few data 
available to us, we are not in a position to 
make a final judgment on what management 
practices should be adopted. It is apparent im- 
mediately, however, that Manana does support 
a great many nesting oceanic birds each year 
in the presence of rabbits. We concur with 
Richardson and Fisher (1950) that bird-rabbit 
