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

Tree dispersal strategies
portions of the monthly diet in Sainte Luce, being 23% of the total diet in January 2001
and 12% in July and August 2000 respectively (Donati 2002). The remaining three
species are rather marginal dietary items. Nevertheless, co-evolution in the strict sense
does not seem to occur here as it is mainly the large fruit size and weight that physically
excludes the other frugivores with smaller gape size. E. f. collaris is simply the last of the
remaining large-bodied lemur species in this littoral forest that can ingest these largesized
seeds, thus matching the situation of brown lemurs in some of Madagascar's dry
deciduous forests (Ganzhorn et al. 1999a). Many large-bodied frugivores have
disappeared in Madagascar recently and the extinction of at least 16 large lemur species
in the Holocene could have included some specialised seed dispersers (Godfrey et al.
1997). All these ‘specialist’ tree species depend critically on Eulemur fulvus collaris for
seed dispersal and recruitment. Even though these lemurs often drop the large seeds (up
to 30mm seed length) under the parent plant, occasionally seeds are swallowed and
defecated or dropped some distance away from the parent plant. Thus in terms of
conservation these relationships are of crucial importance to conserve the integrity of the
littoral forest.
Our attempt to test McKey's model was problematic for a variety of reasons. First, the
predictions are qualitative rather than quantitative in nature. ‘Investments’ are difficult to
specify and may not be the same at nutrient-poor as at nutrient-rich sites or at sites of
differing seasonality. Also, it is problematic to decide whether short but massive fruiting
might actually be less expensive for a tree than extended fruiting over longer periods of
time. Furthermore, the model was developed for bird-dispersed trees in the Neotropics
and its validity largely depends on the composition of the frugivore guild. With as few as
eight vertebrate seed dispersers in the littoral forest any ranking or subdivision into
specialists and generalists is likely to show too much variation to be detected statistically
in descriptive field studies. For the littoral forest of Madagascar, it might be risky for any
tree species to depend on only one of these few frugivores and therefore most tree
species seem to be characterised by a mixture of general traits from both the low and
high investment model. Even though in general there is little evidence for the McKey
model (but see Wheelwright 1986), the depauperate frugivore guild of Madagascar might
not be suitable to test this concept.
Of the three hypotheses to be tested, the distinction into dispersal syndromes was
the only one that could be supported by the present data. Fruits consumed and dispersed
by birds and mammals differ distinctly in fruit and seed size and weight, fruit shape and
seed number. Moreover, in a similar PCA analysis, Pizo (2002) found the same
importance of fruit size, fruit width, seed length (PCA1), fruit shape and seed number
(PCA2) which distinguished primate fruits from the bird and mixed fruits. Bird fruits tend to
be smaller and more elongated than primate fruits. Our results thus agree with studies on
fruit syndromes in other assemblages of plants and animals in different regions (Janson
1983; Knight and Siegfried 1983; Gautier-Hion et al 1985; Pizo 2002; Voigt et al. 2001).
The rather uniform results suggest that these syndromes are biologically meaningful.
However for biochemical traits no significant differences could found, which corresponds
to the findings of Pizo (2002) and Corlett (1996). Even though it has been shown that
mammals favour fruits rich in sugars while birds prefer fruits with high lipid and protein
content (Snow 1981; Fleming et al. 1987; Debussche and Isenmann 1989; Galetti 2000),
the present study only showed a slightly lower sugar and higher protein content in fruits
eaten by birds.
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