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

Chapter 2
On a community level the low number of frugivores seems to have a profound impact
on the composition and relative contribution of functional groups to regional ecosystems.
Neotropical sites have very speciose frugivorous guilds with more than 50 bird species
(Wheelwright 1986; Galetti and Pizo 1996). In contrast Madagascar has very few
frugivores and specifically very few frugivorous bird species (Fleming et al. 1987), which
seems to be reflected in the striking low number of ‘actual’ bird fruits. These
circumstances obviously narrow down the options for tree species to specialise on certain
bird species in Sainte Luce. In a comparison of fruits in deciduous forests of Madagascar
and South Africa, Bleher and Böhning-Gaese (2001) and Voigt et al. (2001) showed that
the latter has much more bird-dispersed fruits, whereas in Madagascar more mammal
fruits exist. This is consistent with our findings from the humid evergreen littoral forest.
The lack of bird fruits is opposed to the findings in India (Ganesh and Davidar 2000),
Hong Kong (Corlett 1996), La Selva in Costa Rica (Levey et al. 1993) and Malawi
(Dowsett-Lemaire 1988). At all these sites the majority of fruits are dispersed by birds or
by both birds and mammals. In our dataset, there are only slightly more mixed fruits and
the majority are mammal fruits, again indicating low dependence for most tree species on
birds but high dependence on lemurs and flying foxes for seed dispersal.
Even though dispersal strategies may include some specifically selected
morphological traits known as syndromes, general traits make up the bulk of the floral
diversity in the littoral forest. In particular the nutritional reward for the animals does not
seem to be taxa related in Sainte Luce, as is also the case in Corlett (1996) and Pizo
(2002). Many tree species attract their seed dispersers by more or less generalist fruit
traits. However we have to be cautious as Zamora (2000) stressed that in communitywide
studies ‘the noise often overwhelms the pattern’ and thus the diffuse co-adaptations
we found may be the result of the complexity of interactions, few strong ones but many
weak ones. Thus we cannot exclude the possibility that certain strong interactions were
overlooked in this study, even though the impact of the species poor frugivore guild
seems to be determining in this ecosystem.
In summary, it seems that in the littoral forest of Madagascar the combination of life
history traits of tree species have not been shaped under the constraints imposed by
vertebrate seed dispersers upon the trees as co-evolution and the low-high investment
model of McKey (1975) state. A classification based on taxonomic affiliation of seed
dispersers does provide a more clear pattern supporting the idea that animals eat what is
available and what they can swallow and digest, mainly based on size characteristics.
The observed links between traits of fruits and seeds and their consumers may be more a
consequence of the morphological and physiological heritage and constraints of the
consumers but not the result of co-evolution. The lack of tight co-evolutionary interactions
makes sense in Madagascar, as the community of vertebrate frugivores is so species
poor that there might have been few options for co-evolution. It could have been too
dangerous for a tree species to rely on a single animal species for seed dispersal. Or
simply, the species poor community of frugivores in Madagascar might not have had a
large enough impact to produce specific tree traits. Nevertheless, some large seeds can
only be dispersed by E. f. collaris. These tree species are likely to suffer from the
extinction of the larger frugivorous lemur species.
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