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

GENERAL CONCLUSION
General conclusion
The main purpose of this study was to gain insight into overall fruit-frugivore interactions
in the littoral forest of Sainte Luce. It represents the first community-wide approach to
primary seed dispersal in a Malagasy forest type from the perspective of both the tree
and the consumer species.
Three hypotheses concerning evidence of co-evolution between life history traits of
plants, their diaspores and animal consumers were tested by studying the frugivorous
vertebrates and the dispersal strategies of 34 tree species. Phenological, morphological
and biochemical fruit traits from these species were measured to look for co-variation with
their seed dispersers. No evidence was found for species-specific co-evolution in this
study. The lack of tight co-evolutionary relationships was suggested as well by many
other studies (Gautier-Hion et al. 1985; Herrera 1986; Fisher and Chapman 1993;
Erikson and Ehrlen 1998). Five large-seeded tree species however did seem to depend
critically on the largest lemur, Eulemur fulvus collaris, for seed dispersal and recruitment.
This strong dependence however does not represent a case of co-evolution in the strict
sense but can be interpreted as an indirect consequence of the extinction of the larger
frugivorous birds and lemurs, which would also have been capable of dispersing these
large fruits. Nevertheless, in terms of conservation these fruit-frugivore interactions are of
crucial importance to conserve the integrity of the littoral forest.
The low-high investment model (McKey 1975) subdivides tree species into
specialists and generalists but again my results do no support this model. McKey’s model
was originally developed for bird-dispersed trees in the Neotropics and its validity seems
to depend largely on the site-specific composition of the frugivore guild (see Wheelwright
et al. 1984). It seems that the species-poor guild of frugivores in Madagascar did not lead
to specialised dispersal strategies. Most tree species seem to be characterised by rather
‘generalist’ fruit traits allowing them to attract as many seed dispersers as possible. This
way the risk of relying on only one frugivore species is avoided, which may be dangerous
in an ecosystem with few frugivores. Furthermore, the low species diversity of avian
frugivores resulted in significantly few bird fruits compared to other sites.
Of all three hypotheses, the concept of dispersal syndromes (Van der Pijl 1969)
was supported most clearly, as there were indeed indications that certain morphological
traits correspond to taxonomic groups of dispersers. I found that diaspores dispersed by
birds, mammals or both groups (called mixed fruits) differ in their fruit and seed size, fruit
shape and seed number, but not in biochemical composition. These results agree with
studies on fruit syndromes in other tropical regions with distinct assemblages of plants
and animals (Janson 1983; Knight and Siegfriend 1983; Gautier-Hion et al. 1985, Corlett
1996; Pizo 2002). Nevertheless, dispersal syndromes can only partly clarify the variability
displayed in tree dispersal strategies at Sainte Luce, while the influence of abiotic factors
on fruit traits may serve as an additional explanation along with the phylogenetic heritage
of plant taxa. In this respect, Hampe (2003) mentions the importance of temperature,
water availability and day length in relation to seed growth, fruit size and sugar or lipid
content respectively. As such, there may be similar meaningful correlations that I
overlooked. Overall, efficient plant-disperser interactions do exist in Sainte Luce as well
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