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

Chapter 2
the successful establishment of seedlings (e.g. Janzen 1970; Connell 1971; Howe and
Smallwood 1982; Terborgh et al. 2001). Therefore attracting frugivores is crucial for a
plant in order to ensure reproduction.
Several hypotheses have been proposed to explain the evolution of zoochory. First,
under the assumption that the evolution of life history traits of plants, their diaspores and
their consumers are mutually dependent, the most restrictive hypothesis assumes very
tight co-evolutionary relationships between one single fruit and frugivore species. So far
no evidence has been found in support of this hypothesis (Howe and Smallwood 1982;
Herrera 1984; Howe 1984; Gautier-Hion et al. 1985; Fisher and Chapman 1993;
Chapman 1995; Erikkson and Ehrlen 1998; Lambert and Garber 1998). Secondly, from
the plants’ point of view McKey (1975) postulated different patterns of resource
investment in plants that rely on multiple versus specialised seed dispersers. According
to this model low investment plants (generalists) invest little in single fruits but display
large fruit crops during a short fruiting period to attract a large variety of opportunistic
frugivores. In contrast, the high investment plants (specialists) have fruit pulp with higher
nutrient content, a more limited fruit production and extended fruiting seasons that attract
few specialists (Howe 1979). Again not much support could be found for this idea so far
(Dowsett-Lemaire 1988; Wenny 2000; Wütherlich et al. 2001; but see Wheelwright 1986).
Thirdly the hypothesis of dispersal syndromes postulates broad morphological
adaptations of fruit traits associated with different consumer taxa, mostly distinguishing
diaspores dispersed by birds, mammals or both groups (mixed fruits). This model
emphasizes the taxonomy and phylogenetic heritage of dispersers with their associated
sensory capacities (Van der Pijl 1969; Janson 1983; Knight and Siegfried 1983; Gautier-
Hion et al. 1985; Martinez del Rio 1994; Corlett 1996; Kalko et al. 1996; Korine et al.
2000; Pizo 2002; Voigt 2001).
The forests of Madagascar provide good opportunities to test the above hypotheses.
There is a high level of floral and faunal endemism in Madagascar with 96% endemics
among the tree species (Schatz 2001), more than 50% of its birds (Langrand 1990) and
90% of its mammals (Goodman et al. in press). This results in communities with an
evolutionary history, which is largely independent from the communities for which the
hypotheses have been developed and tested and therefore allows independent tests for
these hypotheses. For this, we selected 34 different tree species of the evergreen littoral
forest of south-eastern Madagascar in order to answer the following questions;
1. Which frugivores feed on the different fruiting tree species and what is their impact on
the seeds?
2. Is there evidence for tight species-specific co-evolution?
3. Can tree species be categorized as low or high investment species according the
McKey’s hypothesis?
4. Are there trees which rely on certain taxonomic groups for dispersal and if so: do fruits
with different dispersal strategies vary significantly in their morphological and biochemical
attributes?
METHODS
Study site and the frugivorous guild
The littoral forest of Sainte Luce (24º45'S 47º11'E) is located in south-eastern
Madagascar, 50km north of Fort-Dauphin. The first author collected data between
November 1999 and February 2001 in a 377-ha forest fragment, called ‘S9’. Average
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