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

Chapter 5
(Ganzhorn et al. 1999a). Madagascar in general, already has a depauperate avian
frugivore community (Langrand 1990; Goodman et al. 1997a) and lacks larger frugivores
such as ruminants, ungulates and elephants. Moreover one third of lemur species
became extinct years ago (Godfrey et al. 1997). Thus emphasizing even more the
importance of the remaining seed dispersers here. Large frugivores, such as E. f. collaris,
are often most vulnerable to habitat fragmentation which is conform findings at other sites
(Johns and Skorupa 1987; Kannan and James 1999). The vulnerability of E. f. collaris is
reflected by the fact that this species is only present in the two largest and most intact
fragments, S9 and S17, but absent in all other, smaller fragments in Sainte Luce
(Ganzhorn et al. 2000). E. f. collaris is in particular important for seed dispersal of
numerous plant species and it is the only frugivore present here that is able to swallow
and thus disperse larger seeds (up to 16.5mm diameter, Table 1). Therefore, local
extinction of this species could ultimately lead to the lack of regeneration of different
dependant plant species. Other specialist frugivores, such as Pteropus rufus, Treron
australis and Alectroenas madagascariensis, are very vulnerable as well, in particular
when important food sources are selectively logged (Table 1). Frugivorous birds and
flying foxes are the most important mobile seed dispersers bringing seeds into grasslands
and early succession vegetation. The simple structure of these habitats pose less of a
barrier to them than it does for arboreal lemurs. Genetic exchange and long distance
dispersal between and among fragments are less likely to occur, if populations of mobile
flying dispersers decrease or vanish.
Our studies showed that no evidence for co-evolution could be found, nor were there
strong indications for syndromes that attract taxonomic groups by certain morphological
and/or nutritional traits (Bollen et al., Chapter 2, 3, 4; Bollen et al. in press). Instead we
found that there is great dietary overlap among frugivore species and that dispersal is
achieved through redundant systems. Most frugivores seem to eat according to what is
available, given the limitation of fruit and seed size and certain feeding preferences.
Animal seed dispersers are vital for the regeneration of littoral forests. Especially since
losses of ecologically interdependent species will be permanent to an even larger extent
here as forest fragments are very isolated. Due to co-dependency of frugivores and tree
species, the hunt on frugivores leads to suboptimal or insufficient dispersal and
recruitment success of certain plant species (Chapman and Onderdonk 1998; McConkey
and Drake 2002) while deforestation, forest degradation and fragmentation has a drastic
impact on the food chain of the frugivorous fauna.
PROTECTION OF THE LITTORAL FOREST
Forest
The remaining intact forest fragments, which act as reservoirs from where indigenous
floral and faunal species can colonize new habitats are in urgent need for protection.
Even though the littoral forest only represents 5% of all forest cover in the south-eastern
region (QIT Madagascar Minerals 2001), the importance of its conservation cannot be
emphasized enough. At present, Sainte Luce contains the most intact littoral forest
fragments on sand, which differ strongly in floristic diversity from the one on laterite and
from the inland montane forest (Rabevohitra et al. 1996; Dumetz 1999). Ideally
conservation zones should be large enough to encompass the micro-heterogeneity in
order to avoid their effective elimination. In this respect, the forest fragments S9 and S17
have the highest conservation priority, representing the two largest and most intact forest
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