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

Chapter 3b
species to persist (Ganzhorn et al. 2000) and therefore, it is critical to identify effective
seed dispersers in this ecosystem that might aid forest regeneration. Conversely we also
want to gain insight into seed predation and the traits that determine food selection of the
different granivorous species.
METHODS
Between November 1999 and February 2001, field data were collected by the first author
on the feeding ecology of Coracopsis nigra in the 377-ha littoral forest fragment of Sainte
Luce, called S9 (24º45'S 47º11'E), located in extreme southeast Madagascar. For a
detailed description of the study site, I refer to Bollen and Van Elsacker (2002a, Chapter
3a). Coracopsis species are naturally forest birds but also inhabit certain degraded areas
(Dowsett 2000). Both C. nigra and C. vasa were observed at our site, mostly in single and
occasionally in mixed species formations. However all feeding observations presented in
this paper are from C. nigra, which is present year-round and in high densities. C. vasa
was only observed on few occasions during austral summer (December 2000 - January
2001) and seems to migrate into the area sporadically.
Diets were assessed by direct feeding observations through tree watches (36hwatches)
and more casual observations while walking along transects. Feeding and
handling behaviour were described in detail in order to determine the role of C. nigra in
seed dispersal and/or predation. More indirect methods such as faecal analyses and
identifying fruit trap contents further contributed to the completion of the diet list (for
details on methodology, see Bollen et al., Chapter 2). Parrots’ bill marks on the rejected
fruit parts are easily recognizable and analyses of Coracopsis’ faecal samples enabled us
to evaluate the condition of seeds after gut passage.
All fruiting species encountered during the study site were characterised according to
the following variables; fruit type (berry, drupe, capsule, other), pulp type (juicy, fibrous,
none), seed protection (none, hard seed coat), seed number, fruit and seed length and
weight. The latter were measured with callipers and a ‘Kernbalans NM60’ scale with a
precision of 0.01mm and 0.01g respectively. Fruits were considered ripe when seeds
were fully developed, often coinciding with a change in colour, odour or texture.
Most of the variables measured have highly skewed distributions so the median value
is given instead of the mean. For the same reason non-parametric statistics were used,
such as Spearman rank correlation, Kruskal Wallis test and Contingency tables.
Statistical significance was accepted for α≤0.05 for all tests. All statistical tests were
carried out according to Siegel (1956) with the statistical software SAS for Windows.
RESULTS
C. nigra was recorded feeding on 40 plant species (36 genera, 25 families; Table 1), of
which 39 are endemic and one, Psidium guajava, is exotic. Three plant species,
Symphonia sp., Rhopalocarpus coriaceus and Eugenia sp. are exploited for their flowers
only, while both the flowers and fruits of Polyscias sp. are eaten. All other plant species
listed in Table 1 are visited for their fruits or seeds. Summarizing, 68% of the food
species served as a seed source, 22% are eaten for seed and pulp and 10% for flowers
(Table 1).
The range of plant species consumed by C. nigra includes berries (31%) and drupes
(47%) as well as capsules and other fruit types (22%). Most consumed fruits have a juicy
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