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

Chapter 3
(1997) already pointed out that frugivores in Madagascar might have adapted to lean
periods by having broad diets (parrots and lemurs) and moving considerable distances in
search of food (P. rufus and fruit pigeons). For post-dispersal granivores food availability
is less problematic as seeds are available much longer than fruits.
Thus in general few traits consistently determine food selection of the thirteen
consumer species in the littoral forest. There may be several reasons for this. First, most
frugivores in Sainte Luce seem to be flexible to eat what is available. Secondly, the large
dietary overlap among frugivores at our site indicates diffuse mutual relationships
between plant and consumer species, which is similar to most other study sites (Terborgh
1983; Gautier-Hion et al. 1985; Dowsett-Lemaire 1988; Debussche and Isenman 1989;
Kitamura et al. 2002; Carlo et al. 2003). In this respect, Fleming (1979) pointed out that in
the Paleotropics dietary overlap is generally higher than in the Neotropics. This is
probably linked to the higher spatio-temporal patchiness of fruit resources here that
favours dietary generalisation, higher inter-specific dietary overlap, and fewer coexisting
species if food levels become critically low. This may indeed be true for the littoral forest
where high inter and intra-annual differences can be found in ripe fruit availability (Bollen
and Donati, Chapter 1). This might represent one of the reasons why Madagascar's guild
of frugivores is depauperate (Fleming et al. 1987; Langrand 1990; Ganzhorn et al. 1999b;
Wright 1999) and has higher levels of dietary overlap than found at other sites.
Diet of frugivores and their role in seed dispersal and/or predation
E. f. collaris has a highly frugivorous diet at Sainte Luce (74.0% ripe fruits, 5.4% unripe
fruits) (Donati 2002), which corresponds with findings from other studies on Eulemur
species (Overdorff 1993a,b; Rigamonti 1993; Curtis 1997; Birkinshaw 1999, 2001). E. f.
collaris can be considered a sequential specialist, feeding on a wide array of endemic
fruit species (111 species) but with only two to three dominant fruit species each month
(Donati 2002). Their relatively large home range (up to 100ha) and extensive day range
lengths (1500-3500m) (Donati 2002) indicate that long distance seed dispersal within a
fragment is likely. Furthermore, being the largest frugivores they represent a large
proportion of the frugivore biomass in this ecosystem. They eat high amounts of fruit
throughout the year and are the only ones that are able to swallow larger seeds. All these
aspects suggest that indeed E. f. collaris is one of the most important seed dispersers in
this ecosystem. The only limiting factor is that this species is reluctant to cross the
grassland between fragments and thus rarely disperses seeds across the boundaries of
the fragment. This important role in seed dispersal has been found for other Eulemur and
lemur species (Ralisoamalala 1996; Scharfe and Schlund 1996; Dew and Wright 1998;
Ganzhorn et al. 1999a; Birkinshaw 1999, 2001; Britt 2000).
The smaller nocturnal lemurs seem to have a less diverse fruit diet and can be
considered more omnivorous (Martin 1973; Petter et al. 1977; Hladik et al. 1980;
Tattersall 1982), even though several studies in different forest types found that a high
proportion of fruit is included in the diet of Microcebus rufus (Wright and Martin 1995;
Atsalis 1999). Fietz and Ganzhorn (1999) recorded 25 fruit species in the diet of
Cheirogaleus medius in the western dry deciduous forest of Kirindy (CFPF). Atsalis
(1999) scored 24 fruit species of M. rufus in the mid-altitude humid forests of
Ranomafana. Compared with these numbers our list of food items likely represents the
bulk of their diet in the littoral forest. These lemurs are smaller in body size, eat less,
occupy limited ranges (1-4ha, Fietz 1999; Atsalis 2000), and have a rather limited gape
size. Furthermore in Sainte Luce as in other Malagasy forests (Fietz and Ganzhorn 1999;
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