Estimating density of fruit trees by simply dividing the number of
trees by the area of the island ignores the between-species differences
in the size distribution of adult trees and the intensity of production
activities, thus biasing the result. To avoid this,  have developed a productivity index:
Im = Fkm bk dk
Where Fm = the fruiting score of all sampled individuals in species 'k' during month 'm'.
bk = mean dbh (diameter at breast height) of any adult tree for species 'k'
dk = density (number per ha) of adult trees for species 'k'
Diversity of fruiting trees was estimated using the Shannon-Weaver
index and the frequency of each species in a block was compared to a
Poisson distribution as a check for the random distribution of the
species. Shannon-Weaver (based on a formula in ):
H = - pi (ln(pi))
where pi is the relative frequency of a species in a given sample.
The Shannon-Weaver index measures species richness. If only a few
species account for most of the biomass, then the Shannon-Weaver index
number is low. A reading of 2.0 would be a rich, diverse plant
community. S-W Index values (H) can range from 0 to ~4.6 using the
natural log (versus log10). A value near 0 would indicate that every
species in the sample is the same. Conversely, a value near 4.6 would
indicate that the numbers of individuals are evenly distributed between
all the species present.
Mather (1992b) 
compared several areas of Borneo and Peninsular Malaysia for
relationships with their density and distribution. Based on this, I
used Mathers's data for comparative purposes. By extrapolating data
from Mather's sites which had gibbons, an estimate for potential
numbers of gibbons/km2 on Mintin Island was calculated based on food tree and dipterocarp availability (Table 3),
to determine the potential carrying capacity. By extrapolating from the
other sites, an estimate for potential numbers of gibbons/km2 on
Mintin Island was calculated. This was to determine the potential
carrying capacity of the island based on the keystone species of figs .
Much of the data from other sites were based on one variable only (fig
density, dipterocarp density) and this was used to create a best-fit
for Mintin Island. Figs are keystone species due to their asynchronous
fruiting pattern [10-13]
i.e. all trees do not all fruit at the same time, thus there are always
some fig trees in fruit in the forest, making this genus an important
fall-back food for gibbons in times of food shortage (Cheyne,
unpublished data). Dipterocarps are important for gibbons for singing
locations, morning calling is from dipterocarp tress in 74% of singing
bouts (n = 1076; [14,15]. Dipterocarps are rarely primate feeding trees (except for leaves, , Cheyne, unpublished data) and their height is important for gibbons to broadcast their morning chorus and duet.
Direct observation of the canopy is the simplest and most convenient
method in open and low forest, but problems arise when the method is
employed in dense, tropical forest where emergent trees are often over
50 m high ().
This was not a problem on Mintin Island, where the forest is maturing
secondary vegetation with a maximum tree crown height of 23 m.
Based on Mather's  index, 17.1% of the trees on Mintin are in the 10 most important gibbon food genera (Table 2),
and the percentage of may be a valuable index for comparing potential
food productivity between sites. Results for Mintin, show varied levels
of productivity for the species/genera, perhaps because Mintin is an
island of recovering secondary forest and may have more varied and less
predictable productivity than pristine forest .
Adaptation to life in the forest presents many obstacles
for the gibbons and one of these should not be excessive food
competition through poor management of the islands resources. This
method does have limitations: the extrapolation only deals with data on
gibbon densities based on the percentage of trees in the important food
genera for gibbons and does not account for other fauna. E.g. Barito
Ulu does not have orang-utans or large groups of macaques, whereas
Mintin does have macaque groups. Thus, at Barito Ulu, gibbons may occur
at higher densities, because there is less competition from other
species. Other fauna e.g. macaques and hornbills will need figs when
other fruit sources become scarce, so, competition from these animals
may reduce the fruit available for gibbons and render my estimates too
high. The relationship between the four sites is not strong, as evident
by the result of the regression. To counter this I present a 95%
confidence interval range for the number of gibbons that could be
supported on Mintin i.e. the range at which the estimated mean value
from the regression analysis could lie.
Flower production was lowest during the dry season from June to
August 2003 and young leaf availability increased in the lead up to the
dry season. Fruit production was erratic, and would probably result in
the gibbons having to feed on trees not on Mather's list. This list
does contain 40 genera on which gibbons feed, thus flexible diet allows
them to adapt to food shortages.
A regression analysis was carried out on the available data for gibbon group size compared to the density of figs (R2 =
54.3 p = 0.015, n = 9). Figs comprised the main fruit source for two
months during this study (January-February 2003) as there was very low
availability of other sources of food e.g. non-fig fruit. As a result,
figs are likely to be an important fruit source for the other
frugivores on the island and there may be competition in times of
Dipterocarps are not common food sources for gibbons but they are
used for sleeping and singing. Density was calculated as the number of
trees/hectare. Mintin Island also supports a density of 1.8 dipterocarp
trees/ha, which are important for providing sleeping and singing
platforms for the gibbons (Table 3).
Mintin Island is placed into Table 4
to show the expected group size and number of groups that it could
support. The fig data appear to be very variable, hence the large
Mintin Island has a Shannon-Weaver H-value of 2.98 indicating that
the island hosts a rich and diverse plant and tree community. Pristine
forest is inevitably going to support a higher diversity than logged,
recovering forest but this indicates that the diversity of Mintin
Island is not quite species rich and diverse, and is probably capable
of supporting a similar diversity of fauna, albeit at lower densities.
Suitability of Mintin as a release site
Based on preliminary data (Table 5), Mintin Island should be able to support at least two pairs of gibbon with a total carrying capacity of 2.5–20 gibbons/km2 and group sizes of 1.5–3.5 individuals. The island should reach carrying capacity no later than the F2 generation,
thus the offspring will need to be translocated to a larger site. This
area has been identified as a 1500 ha forest near the Kalaweit Gibbon
Sanctuary in Central Kalimantan.
showed that there was a positive correlation between dipterocarp
abundance and the biomass of gibbons a forest can support. Dipterocarps
are an important structural component of the forest for gibbons as they
provide platforms for sleeping and singing. There are few large
dipterocarp trees left on Mintin, which, combined with the small area
of the island, means that Mintin probably cannot support more than two
groups, if dipterocarp density is indeed a limiting factor for the
Mintin gibbons. It is possible that the lack of dipterocarps was a
factor in explaining why the gibbons did not sing very often, though I
believe that the lack of singing is more likely due to the fact that
the pair was in very unfamiliar territory and that they separated
immediately after release. When the gibbons did start to sing, they
sang from dipterocarp trees (n = 6).
Most tropical, woody plants tend to produce new leaves and flowers
in bursts rather than continuously, and there are considerable seasonal
variation in the abundance of these food sources .
This variation affects the abundance of primary consumers (e.g.
frugivores and folivores) that will also have to modify their
behaviours accordingly. If the year 2003 was a bad fruit year it is
possible that there will be a bigger influx of animals to the island in
better years, thereby increasing food competition with the gibbons. On
the other hand, there may also be more food available if 2002/03 was
lower than normal for fruit production, thus mitigating the effects of
food competition. Clearly data are needed to determine exact levels of
competition between the gibbons and other wildlife for the island's
resources. This is a post-release issue and herein I discuss the need
for detailed pre-release habitat analysis.
The island also supports groups of macaques and proboscis monkeys.
Both these species are transient and were observed to swim from the
island to the mainland (10 sightings of macaques in two groups and 8
sightings of proboscis monkeys in one group). They were observed on the
island throughout the eight months that the gibbons were present,
though not every day; thus, the macaques and proboscis monkeys can be
considered as semi-permanent residents of Mintin Island. It is very
important to collect data on the resource use by the macaques (Macaca nemestrina and M. fascicularis)
and proboscis monkeys to effectively predict the resource availability
for gibbons in the same area. Hornbills, being frugivorous, could be a
potential competitor for gibbons given the limited production of fruit
across the year. Therefore, feeding requirements of hornbills for
limited fruit trees need to be identified and quantified for
determining long term survival of the released gibbon populations in
the proposed release site. Ideally, feeding rates can be collected on
the released gibbons and the wild macaques to obtain data on food
intake at different times of year and to observe how much overlap in
diet there is between the two species. These data have to be collected
post-release, and will be of great importance in the final phase of
release, i.e. gibbons reintroduced to contiguous forest. This study
does not include data on feeding rates and food intake, as the purpose
is to address the issue of habitat quality assessment pre- and
post-release. Feeding rates, food competition and food intake will form
part of the post-release monitoring of gibbons due for reintroduction