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

Frugivore guild
analyses and analyses of fruit trap contents (78x1m² traps under 29 tree species) (Bollen
and Van Elsacker 2002a, Chapter3a; Bollen et al., Chapter 2). For Pteropus rufus faecal
droppings were collected weekly under the roost site year-round. Dietary data on the
rodents resulted mainly from identifying gnawing marks on seed remains collected at
feeding sites. Since Eulemur fulvus collaris was studied intensively in parallel to the
present study (Baldi 2002; Donati 2002; Morelli 2002), fruits eaten by this species may be
more thoroughly sampled than other species. The con-generic species Cheirogaleus
medius and C. major could not always be distinguished during observations and are
treated as Cheirogaleus spp. in the analyses. The same applies to rodent species,
Eliurus webbi and Rattus rattus (introduced), as not all gnawing marks could be attributed
to a single species. Details of the diets of the various species are given in Appendix I. For
the frugivorous bird species, diets are treated separately, but for feeding selection
Alectroenas madagascariensis, Treron australis, and Hypsipetes madagascariensis were
combined, as they are the only seed dispersing birds at our site.
Dietary overlap was calculated among pairs of frugivores using Sørensen’s similarity
index (Krebs 1989). This index generates a value ranging from 0 to 1, with 0 representing
no overlap and 1 representing complete overlap. The dataset comprises accounts of fruit
species consumed by different frugivore consumers along with the impact they have on
the seeds. Based on this, frugivores can be classified into the following categories: seed
dispersers or fruit consumers (D), neutral or pulp consumers (N) and seed predators (P)
according to Gauthier-Hion et al. (1985) and Debussche and Isenmann (1992). The first
group disperses intact seeds by endozoochory through droppings or synzoochory
through regurgitation, while the second group eats fruit pulp but drops the seeds under
the parent plant. The last group eats and destroys the seeds. It is difficult to assign a
certain frugivore to one category only, as one species may have different impacts on the
same and on different plant species. The stage of ripeness of the consumed fruits was
scored as well. To differentiate between unripe and ripe fruits changes in size, colour and
consistency were looked at.
Data analyses
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. Chisquare
analyses were conducted to compare discrete fruit traits in the diet with those in
the overall dataset, whereas Mann Whitney U tests were carried out to control for feeding
preferences when comparing continuous traits between food and non-food items.
Afterwards sequential Bonferroni corrections were performed on the significance levels
(Rice 1989). To understand which factors influence the diet of the different frugivores
separately a logistic generalised model was applied in which morphological and
biochemical variables were included as fixed factors. Dietary data were used as binomial
response variables (0=non-food, 1=food item) in a generalised mixed linear model with
logit link (glimmix procedure in SAS 8.1.) with forward procedure retaining significant
variables. As Cheirogaleus spp. and Microcebus rufus go into torpor in austral winter,
comparison of their diet and total dataset available were restricted to the fruits that were
present during their active period. 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.
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