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

Chapter 3
1989; Galetti 1993; Saini et al. 1994; Pizo et al. 1995; Corlett 1998; Renton 2001). For
Madagascar Goodman et al. (1997a; 1997b) observed seed predation of Ficus seeds by
Coracopsis spp., while Böhning Gaese et al. (1999) described C. nigra as an occasional
seed disperser for Commiphora guillaumini.
According to the literature most Streptopelia spp. feed largely on dry seeds and fruits
and destroy most of the seeds they swallow in their muscular gizzards (Corlett 1998).
However, S. decaocto is said to regurgitate some large seeds (Corlett 1998). S. picturata
was most often seen feeding on the ground but no details on its feeding behaviour could
be obtained at our study site. Nevertheless this species is suspected to be a postdispersal
seed predator destroying most seeds, which is confirmed by Goodman et al.
(1997b). Obviously our list of food items sampled for this bird is largely underestimated
compared to their actual diet.
Before humans settled, the only seed predators in Sainte Luce were Coracopsis spp.,
S. picturata, and Eliurus webbi. The latter now has to contend also with a non-native
rodent species (Rattus rattus) even though there are no indications for food or habitat
competition between these two rodent species (Ramanamanjato and Ganzhorn 2001;
Ganzhorn 2003). Evidence of post-dispersal predation by rodents destroying the seeds of
fifty plant species was found, but Goodman and Sterling (1996) and Ganzhorn et al.
(1999a) suggested that native Malagasy rodents may store seeds in their burrows but no
evidence has been found so far for Eliurus webbi in the littoral forest. Goodman (1994)
describes a burrow of E. webbi in Andringitra containing 20 seeds of Cryptocarya sp. of
which only half were eaten. In Sainte Luce a few seeds were observed to escape total
destruction when germinating from the rodents’ food piles. This diet list can serve as a
first indication on the rodents’ food species in the littoral forest, taking into account that
completely digested seeds are missing here. Nevertheless by targeting seeds of selected
species, rodents can significantly alter forest composition (DuPuy 1996; Spehn and
Ganzhorn 2000).
Besides the described frugivores (Table 1), there are also other animal species in
Sainte Luce that occasionally feed on fruits. Bird species such as Coua caerulea
(Cuculidae) and Zosterops maderaspatana (Zosteropidae) (also observed by Goodman
et al. 1997a) could be seen feeding on fruits in Sainte Luce, as well as the terrestrial
mammal Tenrec ecaudatus (Tenrecidae) and the fruit bats Rousettus madagascariensis
and Eidolon dupreanum (Pteropodidae), but the latter species were very difficult to track.
Local people may also act as seed dispersers when collecting several fruit species (n=12,
App. I). As secondary seed dispersers Pheidole spp. (Myrmicinae) seem to be important
and they were observed to transport intact seeds of at least 20 species to their ground
nests, thus matching the important role of ants in other forests of Madagascar (Voigt et al.
2002; App. I).
As mentioned above, the dispersal quality of the frugivores differs substantially.
Frugivores are active at different times of the day and year, forage at different heights,
have distinct feeding behaviour, gut passage rates, and seed shadows. While birds may
defecate from nearby or far away perches either within primary forest or in the clearings,
flying foxes defecate during flight or under feeding or sleeping roosts, rodents
concentrate seeds at burrows or feeding sites and lemurs move seeds within the forest
fragment only. In the end, the combined action of a variety of fruit-eating vertebrates with
distinct seed shadows produces a very heterogeneous transport of seeds, which is very
important in the severely fragmented and degraded littoral forest to ensure regeneration
of most plant species within and outside forest fragments. So even though dietary overlap
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