Fruit-frugivore interactions in a Malagasy littoral forest - Universiteit ...

General conclusion
as in other sites (Gautier-Hion et al. 1985; Dowsett-Lemaire 1988; Herrera 1995, Corlett
2002; Pizo 2002) without requiring the close co-variation of fruit traits with their dispersers
as predicted by the tested models.
To evaluate the relative contribution of the different animal species in relation to seed
dispersal and predation, the core part of this study concentrated on plant-animal
interactions on a species-specific as well as on a community level. These mutual
relationships determine the dynamics of the littoral forest ecosystem. Fruit and seed size
appear to be the most determining physical traits in food selection of all consumer
species. While birds show a strong preference for specifically coloured fruits (red, purple
and black), nocturnal lemurs show clear preference for soft and juicy fruit pulp and thinhusked
fruits irrespective of colour. Arillate or soft and juicy fruits are also favoured by
flying foxes. Nutritionally, birds prefer lipid-rich fruits whereas certain mammals (Eulemur
fulvus collaris, Pteropus rufus) avoid those, which may be linked to their differential
capacity to digest and assimilate lipids. Mouse and dwarf lemurs select fruits with high
sugar content. This allows them to prepare and store fat reserves before going into torpor
(see also Bonnaire and Simmen 1994, Fietz and Ganzhorn 1999). These findings are the
only indications for diet selection of all frugivores, while for the vast majority of fruit traits,
both biochemical and morphological, the frugivores consume whatever is available. This
weak selection pressure represents another reason for the lack of strong mutual
relationships among fruit traits and dispersers.
Compared to other study sites worldwide (Fleming 1979; Gautier-Hion et al. 1985;
Kitamura et al. 2002) dietary overlap among frugivores seems to be rather high in Sainte
Luce. However, without considering the proportional use of different food items this
overlap may be overestimated. Dietary overlap among frugivores may be strongly
influenced by phenology, increasing when fruit is abundant and decreasing when it is
scarce (Overdorff 1993; Johnson 2002). Phenological data from Sainte Luce show that
fruiting is highly seasonal and that lean periods differ substantially inter-annually. The
overall low fruit productivity and high unpredictability of food resources in Sainte Luce
and other Malagasy forests (Overdorff 1996; Goodman and Ganzhorn 1997; Wright
1999) may be the at the base of low feeding selection pressure and thus relatively high
dietary overlap. This also corresponds with the theory of Fleming (1979) that high spatiotemporal
patchiness in the Paleotropics leads to much higher dietary overlap and at the
same time to the co-existence of fewer frugivore species as opposed to the
Neotropics. In this respect, Terborgh (1986) says that periods of fruit scarcity are crucial
to set the carrying capacity of tropical forests for their frugivore community. During
bottlenecks frugivores have to switch to other food items (such as young leaves, flowers
or insects) to compensate for the lack of fruit and to avoid inter-specific competition. In
Sainte Luce and other Malagasy humid forests phenophases are highly inter-correlated in
time, which means that alternative diet items are not available either during lean periods.
As a result many lemur species remain highly frugivorous even when fruits are scarce but
concentrate on a few important food species (Overdorff 1993; Vasey 2000; Donati 2002;
Johnson 2002). Some of these plant species may be potential keystone species and
Ficus species are known to often play this role in the tropics for numerous tropical
frugivores (Terborgh 1986a; Johnson 2002 but see Gautier-Hion and Michaloud 1989).
However Goodman and Ganzhorn (1997) mention that Madagascar in general has a
reduced Ficus diversity. This is also the case for Sainte Luce, where other plant species
are likely to fulfil this role (Syzigium sp.2, Dypsis prestoniana).
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