increase the vulnerability of oceanic islands to plant invasions
Christoph Kueffer, Lilian Kronauer and Peter J. Edwards
C. Kueffer (email@example.com), L. Kronauer and P. J. Edwards, Inst. of Integrative Biology, ETH Zurich, CHÁ8092 Zurich, Switzerland.
PlantÁanimal mutualisms such as seed dispersal can play an important role in enabling some species to become invasive.
For example, an introduced species could become invasive because birds prefer its fruits to those of native plants. To
investigate this possibility, we compared various measures of fruit quality of 22 of the most common native and invasive
woody species on the oceanic island Mahe´ (Seychelles, Indian Ocean).
Individual measures of food quality tended to vary much more amongst invasive species than amongst native species;
thus, whereas for particular traits the fruits of some invasive species had higher values than any native species, others had
relatively low values. However, invasive species consistently produced fruits with a lower water content, resulting in a
higher relative yield (i.e. dry pulp weight to total wet fruit weight ratio), and a higher energy content. The fruits of the
most abundant invasive tree Cinnamomum verum (Lauraceae) were of particularly high nutritional quality, with
individual berries containing 3.5 times more protein and 55 times more lipid than the median values of the native species.
We suggest that our results may reflect a general tendency for island plants to produce fruits of low energy content,
perhaps reflecting reduced competition for dispersal agents on isolated islands. In addition, we argue that inconsistent
results on the relevance of fruit quality for plant invasions reported in the literature may be resolved by comparing the
widths of trait spectra for native and alien floras rather than average values. Gaps in the native fruit trait spectrum may
provide opportunities for particular invasive species, and weaken the resistance of regional floras to invasions. Such empty
niche opportunities may occur for several reasons, including generally broader trait spectra in globally assembled alien
than regional native floras (especially on oceanic islands), or the loss of native species due to human activities. More
generally, a focus on trait variation rather than average trends may help to advance generalisation in invasion biology.
Invasions of alien plant species pose major threats to
biodiversity and ecosystem functioning (Millennium Eco-
system Assessment 2003), particularly on oceanic islands
(Denslow 2003). Increasingly, the importance of plantÁ
animal mutualisms such as pollination and seed dispersal
in plant invasions has been recognized (Richardson et al.
2000). However, although many of the most problematic
invasive plants are frugivore-dispersed (Buckley et al. 2006)
Á especially among tropical woody species (Binggeli 1996,
Kueffer et al. 2004) Á there have been few studies of how
frugivores influence plant invasions (Gosper et al. 2005,
Buckley et al. 2006).
It has been suggested that some invasive plants are
successful because native frugivores prefer their fruits to
those of native plants (Vila and D’Antonio 1998, Gosper
et al. 2005, Buckley et al. 2006), and they therefore gain an
advantage in dispersal. Effective dispersal by frugivores
could facilitate an invasion by increasing the rate of spread Á
even enabling propagules to reach remote areas Á and by
weakening the resistance of habitats to invasion through
increased propagule pressure at a site (Von Holle and
Simberloff 2005). Furthermore, the presence of invasive
plants could attract dispersers away from native plants,
thereby negatively affecting the mutualisms between native
plants and their frugivores (Traveset and Richardson 2006).
If such effects are common, fruit traits may help in
predicting the invasiveness of alien plants (Buckley et al.
2006). It is therefore important to understand whether
fruiting traits of invasive plants Á at the level of both the
single fruit, e.g. size and nutritional content, and the whole
plant, e.g. phenology and size of the fruit crop Á tend to
make them more attractive to birds than native species.
Several studies have shown that invasive plants often
produce larger fruit crops or have longer fruiting periods
than native species, but few have considered the traits of the
fruits themselves, and no clear and consistent picture has
emerged of differences between native and invasive species
(Buckley et al. 2006). In particular, we are unaware of any
study comparing the fruit characteristics of a set of common
native and invasive plants in one region.
We compared the fruit characteristics of bird-dispersed
native and invasive woody plants on the oceanic island
Mahe´ in the Seychelles (Indian Ocean) to test the
hypothesis that native plants of oceanic islands produce
fruits of lower nutritional value than introduced aliens. It
has been said that isolated islands are particularly vulnerable
Oikos 118: 1327Á1334, 2009
2009 The Authors. Journal compilation # 2009 Oikos
competitive than mainland species (Denslow 2003). This
idea has so far mainly been tested for competition for
growth-related resources (Denslow 2003, Schumacher et al.
2008, 2009), and we extended it to include competition for
seed dispersers. There are a number of reasons why oceanic
island plants may be less competitive than mainland species.
In communities that owe their origin to long-distance
dispersal, particular niches are commonly filled by taxo-
nomic groups that are not necessarily well suited to this role
(disharmony, e.g. woodiness of typically herbaceous gen-
era), and ecologically very plastic generalists typically
dominate habitats across broad environmental gradients
(e.g. Metrosideros polymorpha in Hawaii, or Northea hornei
in Seychelles) (Carlquist 1965, Whittaker 1998, Denslow
2003). Thus, more specialized mainland species may be
stronger competitors under particular environmental con-
ditions. And in the case of competition for seed dispersers,
high dispersability may be less important for plant fit-
ness on islands than on the mainland (Carlquist 1965,
The island of Mahe´ offered a convenient study system
for our purposes because the diversity of both plants and
dispersers is small, and the dispersal systems are highly
generalized (Stoddart 1984, Kueffer 2006). Thus, about 20
native and 10 invasive woody plants dominate these forests,
and the majority of these produce small fruits (less than
about 15 mm in diameter) that are dispersed by three
common frugivorous birds (Stoddart 1984, Kueffer 2006) Á
an endemic bulbul Hypsipetes crassirostris, an endemic fruit
pigeon Alectroenas pulcherrima, and an introduced Mynah
Acridotheres tristis. In addition to birds, one endemic fruit
bat species Pteropus seychellensis is also an important seed
To test the hypothesis that invasive species produce fruits
of higher nutritional quality than native species, we
compared various measures of fruit quality of 22 common
native and invasive woody species.
island of the Republic of Seychelles (48S, 558E, total area
, 0Á900 m a.s.l.). The island has a humid tropical
depending on altitude. Although there is no pronounced
seasonality in rainfall, the period from June through
September is generally drier (mean monthly rainfall: 80Á
150 mm at sea level) than from November through
February (300Á450 mm) (Schumacher et al. 2008). The
forest vegetation of Mahe´ was heavily affected by human
activities until the 1970s, and is mostly secondary and
dominated by alien trees, especially Cinnamomum verum
(Kueffer and Vos 2004).
The original frugivore fauna of the inner Seychelles
islands consisted mainly of various lizards (Mabuya and
Phelsuma spp.), a giant tortoise (now extinct), an endemic
fruit bat Pteropus seychellensis seychellensis, and six bird
species. The latter included two mainly insectivorous white-
eyes Zosterops modestus, (of which only ca 300 individuals
are left on one small island); Z. semiflava (extinct), two fruit
predatory parrots Coracopsis nigra barklyi (ca 300 indivi-
duals left on one island); Psittacula eupatria wardi (extinct),
a bulbul Hypsipetes crassirostris (length: 24Á25 cm), and a
fruit pigeon Alectroenas pulcherrima (a genus related to the
PtilinopusÁDucula fruit pigeons, length: 23Á25 cm). Today,
an introduced mynah Acridotheres tristis (length: 25Á26 cm)
is also an important seed disperser. The largest fruits known
to be swallowed by the common native bird species are of
10Á15 mm diameter (depending on firmness) for Hypsipetes
and 20 mm diameter for Alectroena (Kueffer 2006).
The plant species used in the study included 14 native and 8
invasive woody species (Table 1); the species range from
shrubs to large trees and include members of 12 plant
families. The native plants selected are all common in
inland habitats in the Seychelles (Friedmann 1994), and
include 11 species endemic to the Seychelles, and three
(Aphloia theiformis, Canthium bibracteatum and Dracaena
reflexa) that also occur on other Indian Ocean islands. Most
of the invasive species are common in semi-natural to
natural mid-altitude habitats in the Seychelles, though
Litsea glutinosa is mainly restricted to lowland areas (Kueffer
and Vos 2004). Two of the invasive (Chrysobalanus icaco,
Syzygium jambos) and one native species (Syzygium wrightii)
have larger fruits that are only dispersed by fruit bats,
although they are eaten by birds (Kueffer 2006). Fruits of
Psidium cattleianum are dispersed in parts by birds and as
whole fruits by fruit bats. The species studied include most
of the widespread fleshy-fruited native and invasive woody
species of inland habitats in Seychelles, the main omissions
being the invasive Lantana camara, and the natives Deckenia
nobilis, Ludia mauritiana, Pouteria obovata, and two Ficus
species (Fleischmann 1997, Kueffer and Vos 2004, Kueffer
Samples for chemical analysis were collected on the
fruits were taken from several individuals and two to four
sites per species in secondary forests and inselberg (rocky
outcrop) vegetation, except for those of Litsea glutinosa,
which were collected from gardens, and those of Phoenico-
phorium borsigianum, which were collected from the
botanic garden in Victoria. Each species sample consisted
of 25 to several 100 ripe fruits, or 10Á15 fruits in the case of
Gastonia crassa, Memecylon caeruleum and Syzygium
Pulp and seeds were separated and weighed. They were
then dried at 40Á508C for four days and weighed again to
determine the water content and dry mass per fruit. One
pooled sample per species was stored in plastic bags with
silica gel before being analysed at the Swiss Federal Res.
Inst. for Farm Animals and Dairy Farming (ALP) in
Posieux, Switzerland. For the chemical analyses, the dried
pulp was ground in a laboratory mill with a one millimetre
sieve. A constant dry matter content was determined with a
following minimum quantities of dry fruit were used for the
various analyses: 5 g for lipid determinations, 5 g for sugar
and fibre and 0.5 g for protein. There was insufficient fruit
material of some species to perform all these analyses (Table 1).
Crude fibre as determined by the Weende method (von
Lengerken 2004) represents the content of organic struc-
tural material such as cellulose or lignin that is not soluble
in a weak acid or alkali solution (similar to the conditions in
animal digestion). The plant material was dissolved in
sulphuric acid (1.25% H
solution, 1 h at 958C) and
potassium hydroxide base (1.25% KOH solution, 1 h at
958C). The insoluble residue was then washed with water
and acetone, dried (1 h at 1308C), weighed and ignited at
5308C for 1 h. The loss in weight on ignition was identified
as crude fibre.
To determine total sugar content, the sample was shaken
with 80% ethyl alcohol at 808C for 1 h. After filtration,
total sugar content was quantified colorimetrically based on
a reaction with 1 g 3,5-dihydroxyltoluol (Orcin) in 1 l 70%
sulphuric acid (H
Crude protein was determined using the DUMAS
method (von Lengerken 2004). Nitrogen content was
determined with a nitrogen analyzer after complete oxida-
tion of the plant material at ca 11008C. From these results,
crude protein content was calculated as total nitrogen
content multiplied by 6.25.
Crude total lipid content was determined with a Soxhlet
system. The plant material was hydrolysed in boiling 10%
HCl solution for 1 h (Berntrop method), and then extracted
with petrol ether at 40Á608C in the Soxhlet system.
Calculations and statistical analyses
Mean fruit characteristics per species were used in the data
analyses. The energy content of the pulp was calculated
based on the following conversion factors: 5.2 kcal g
(protein), 9.3 kcal g
(lipid), and 4.0 kcal g
were calculated: relative yield (ratio of dry pulp to total wet
fruit weight including seeds), seed burden (seed to total wet
fruit weight ratio), overall profitability (nutritional contents
per total wet fruit weight), and single fruit profitability
(nutritional contents per single fruit). The differences in
fruit characteristics between native and invasive species were
tested with a WilcoxonÁMannÁWhitney-test. All statistical
analyses were performed with R ver. 2.7.0 (R Development
Core Team 2008).
The fruits of invasive species had a lower average water
a 50% higher relative yield (p 00.01), i.e. ratio of dry pulp
to total wet fruit weight including seeds (Fig. 1, Table 2).
However, the seed burden, i.e. the seed to total fruit weight
ratio, did not differ between the two groups (p 00.2, Fig. 1).
There were no significant differences between native and
invasive fruits in the average values of fibre, protein, lipid
and sugar contents (p]0.5). However, the highest values of
these traits Á which ranged from around twice the median
value of native species for fibre, protein and sugar, to 30
Table 1. The characteristics of the studied woody native and invasive plant species and the measured parameters. The wet fruit weight is
given in mg and the fruit diameter in mm. For asymmetric fruits the minimum diameter is given. A small tree is B10 m. Nomenclature and
maximal stem height was taken from Friedmann (1994).
of invasive species (Fig. 1). The average energy value of
invasive fruits (calculated from the protein, sugar and lipid
contents) was 1.6 times higher than that of native fruits, and
the overall profitability (i.e. energy content per total wet
fruit weight) was 2.4 times higher, both of these differences
being significant (p 00.02 and 0.01, respectively; Fig. 1).
The variances in the trait values were mostly lower for
native species than for invasive species (Fig. 1), with the
differences being especially marked for water and lipid
contents (ratios of variances of 0.18 and 0.08, respectively).
Across all species, the protein content of pulp was
positively correlated with the lipid content (r 00.6, p 0
0.01) and negatively with the sugar content (r 0(0.7, p 0
0.002). Lipid and sugar contents were negatively correlated
(r 0(0.5, p 00.05). The water content was negatively
correlated with the lipid and energy contents (r 0(0.6,
A trend for low energy content among native woody
plants on an oceanic island?
We found that native species on the tropical oceanic island
of Mahe´ tended to produce fruits of lower energy content
than invasive species (Fig. 1, Table 2). The reason for the
difference was the higher water (resulting in a lower relative
yield) and lower energy contents per dry pulp of fruits of
native species. These trends were also supported in the two
intra-generic comparisons (Memecylon eleagni vs M. caer-
uleum, Syzygium wrightii vs S. jambos). Among the invasive
species studied, Litsea glutinosa is restricted to some lowland
areas, but exclusion of this species from the analysis did not
substantially alter the results.
The mean water content of the invasive fruits (77.2%)
was similar to values reported for bird-dispersed fleshy fruits
Figure 1. The water content of the pulp (%), relative yield (dry pulp weight: total wet fruit weight,%), seed burden (wet seed: total fruit
weight,%), and the chemical composition in mg or kcal (energy content) per g dry pulp of 14 native (N) and 8 invasive (I) woody species.
The overall proﬁtability (OP) is calculated based on the relative yield and the content per dry pulp as the energy content per g of total wet
fruit weight. The boxÁwhisker-plots indicate the median (line), mean (ﬁlled circle), ﬁrst and third quartiles (box), and the range of the
data, with outliers indicated by open circles and deﬁned as being more than 1.5 times the interquartile range above/below the ﬁrst/third
fleshy fruits of the native flora of Hong Kong (78%, Corlett
1996), and alien plants in Hong Kong, Australia and New
Zealand (76Á78%, n 012Á15 species per region, Williams
and Karl 1996, Corlett 2005, Gosper et al. 2006), while the
native species had a considerably higher water content
(86%). Methodological differences make it difficult to
compare our data on energy with those from other studies,
but Jordano (1995) reported an inverse relationship
between water and energy contents for the global angios-
perm flora, and the same trend was also evident in our
study. Although a high water content can contribute to fruit
quality during the dry season in areas with a seasonal
climate (Herrera 1982), such an effect is unlikely to be
relevant in the Seychelles, with its humid tropical climate.
In view of these comparisons, we conclude that fruits of
native species tend to be poor in energy, rather than those of
the invasive species being particularly energy-rich. Indeed, it
may be hypothesised that, compared to mainland species,
plants on oceanic islands have evolved under conditions of
less severe competition for seed dispersal agents, and
therefore do not invest as much in attracting frugivores
(Kueffer 2006). This could be partly because of the lower
plant and frugivore species richness on islands, and partly
because high dispersability is less important for plant fitness
(Carlquist 1965, Whittaker 1998). We are unaware of any
comparative data that could be used to support this
hypothesis apart from a single observation that fruits of
the island endemic Arbutus canariensis have a substantially
lower energy content than those of the continental congener
A. unedo (A. G. Fernandez de Castro pers. comm.). More
research is needed to test the hypothesis that fruits from
island plants have generally a lower energy content than
those of continental species.
Higher variation of fruit traits among invasive than
A second pattern in our data was that the variation of fruit
traits was generally considerably higher among invasive than
native species (Fig. 1). In particular, the highest contents of
sugar, protein and lipids were consistently found among the
invasive species. The most common invasive tree in the
Seychelles, Cinnamomum verum, had particularly high
protein and lipid contents, so that the single fruit profit-
ability (i.e. protein or lipid intake per fruit) was 3.5 times
higher than the median values of native species for protein,
and 55 times higher for energy. Given that searching and
feeding on fruits are costly in terms of both time and
energy, these differences may significantly affect fruit
choices by birds.
In the case of the native species, the scarcity of fruits with
high contents of particular nutrients may reflect a paucity
of specialised fruits in the flora of a small oceanic island.
Fruits that are particularly rich in either lipids, protein or
particular types of sugars are often interpreted as being
adapted to dispersal by specialist frugivores (Snow 1981,
Corlett 1998, Witmer and Van Soest 1998, Jordano 2000,
Levey and Martinez del Rio 2001). In particular, relatively
large fruits of high nutritional quality are typically dispersed
by specialised frugivorous birds such as fruit pigeons (Snow
1981, Corlett 1998, Meehan et al. 2002). However, few if
any plants on the Seychelles depend only upon the endemic
fruit pigeon Alectroenas pulcherrima for fruit dispersal
(Kueffer 2006); and, except for the palms, no native species
belong to families producing specialised lipid- or protein-
rich fruits (e.g. Burseraceae, Lauraceae, Meliaceae, Myris-
ticaceae, Rutaceae, Solanaceae, Jordano 1995, Corlett 1996,
1998). Rather, the Seychelles fruit pigeon may depend on a
more opportunistic diet composed mainly of smaller fruits
Table 2. Water content of the pulp (%), relative yield (%), and nutritional composition in mg or kcal (energy content) per g dry pulp of the
studied native and invasive woody species from the Seychelles.
Water content Relative yield
to our data, some of the smaller fruits do, indeed, have
relatively high lipid and/or protein contents, e.g. Phoenico-
phorium borsigianum and Erythroxylum seychellarum; the
same may also apply to some of the species that we did not
study, such as Trema orientalis and Rapanea spp. (Snow
1981). Further research is needed to determine if and why
specialised fruits are missing from small oceanic islands.
There are other possible explanations for the lower
variation of fruit traits among native species that also
deserve to be mentioned. For example, human activities
could have reduced the abundance of species with lipid or
protein-rich fruits disproportionately (e.g. the now rare
Trema orientalis or Rapanea spp.), though it is not clear why
this should have happened. Or trait variation in the invasive
flora may be broader than that in the local flora, simply
because the invasive species have been sampled’ from a
much wider spectrum of biogeographic and ecological
Do fruit traits help to predict the invasiveness of alien
Effective dispersal is often important for an alien species to
become invasive. Debussche and Isenmann (1990) sug-
gested that competition for dispersal by a diverse native
community of fleshy-fruited plants may have hindered the
invasion of fleshy-fruited alien species in the Mediterranean.
In contrast, several studies have shown that invasive species
may profit from preferential dispersal of their fruits by the
local frugivore community (Buckley et al. 2006).
However, apart from the importance of producing a
fruit of a comparable size to some native fruits (Richardson
et al. 2000, Renne et al. 2002, Gosper et al. 2005), rather
little is known about which fruit traits contribute to the
invasiveness of an alien species (Buckley et al. 2006).
Previous studies comparing pairs of native and invasive or
alien species have produced contradictory results, so that no
general conclusions can be drawn about the importance of
fruit quality: in some cases, fruits of invasive or alien species
were found to be larger (Sallabanks 1993, Corlett 2005) or
to have a lower seed load (Corlett 2005), a higher lipid
content (Gosper et al. 2006) or a higher energy content
(Vila and D’Antonio 1998), but in other cases the native
species produced fruits with a higher energy (White and
Stiles 1992, Drummond 2005) or mineral (calcium, iron
and sodium) content (Nelson et al. 2000), or there were no
differences in fruit quality (Gosper 2004). Our study
appears to be the first to compare the fruit characteristics
of a broad set of common native and invasive plants in a
given region, but our results seem to add further to this
incoherent pattern. Invasive species in Seychelles produce
fruits with a wide range of properties: some have particu-
larly high nutrient contents, especially protein or lipids, but
others are of lower quality than most or all native species
invasive species in Seychelles that produce fruits with
particularly high contents of some nutrients profit from
an efficient dispersal. Following heavy deforestation at
around 1800, Cinnamomum verum (true cinnamon), a
species that produces fruits of particularly high protein and
lipid contents, rapidly colonized large areas (Sauer 1967,
Stoddart 1984). Today C. verum is by far the most
abundant species on Mahe´, accounting for 80% of the
canopy trees in most inland habitats. In a food preference
experiment in which captive birds were offered the fruits of
several native and alien species, Seychelles bulbuls Hypsipetes
crassirostris Á the most common frugivorous bird Á preferred
fruits of C. verum to those of all other species except the
endemic Dillenia ferruginea (Kueffer 2006). Clidemia hirta,
which produces fruits with 1.6 times more sugar and
protein than the median of the native species, has recently
spread rapidly on Mahe´; within four years of it being first
recorded in the late 1990s, it had spread into natural areas
throughout the island (Kueffer and Zemp 2004).
The relevance of variation rather than general trends
for comparing traits of native and invasive floras
We suggest that the inconsistent conclusions about the
relevance of fruit quality for plant invasions reported in the
literature may be resolved by comparing the width of trait
spectra between native and alien floras rather than average
values. General differences in fruit traits between the means
for native and invasive species groups may not be expected
for several reasons. First, only some invasive species may
depend on efficient dispersal or on high fruit quality for
efficient dispersal, while others rely on some other
advantage for reproduction or dispersal (e.g. fruiting
phenology, asexual reproduction). For instance, the species
that produced the fruits with the lowest energy content
among the invasive species, Syzygium jambos, has a patchy
distribution in the form of dense, monospecific clumps,
indicating low dispersability (Kueffer pers. obs.). Second,
because of tradeoffs in fruit specialisation, fruits tend to
have either high sugar or lipid and protein contents (Snow
1981, Corlett 1998, Witmer and Van Soest 1998, Jordano
2000, Levey and Martinez del Rio 2001). In fact, in our
dataset sugar content was negatively correlated with lipid
and protein contents. Thus, invasive species may profit
from opposing specialisations, which has recently also been
emphasised for growth-related traits (Daehler 2003, Schu-
macher et al. 2008, 2009). Thus, rather than comparing
native and invasive floras in terms of the average values of
particular traits, it may be more informative to compare
their ranges of variation, so as to identify gaps in the native
trait spectrum that could be exploited by an alien species
(Moles et al. 2008).
This study has shown that gaps in the native fruit quality
spectrum, especially the lack of lipid-rich fruits, may
provide opportunities for invasive species with such fruits,
and weaken the resistance of an oceanic island flora to
invasions. On continents, empty niche opportunities for
lipid-rich fruits may also occur, for example when anthro-
pogenic disturbances affect plants with specialised fruits
more than those with generalised fruits. Or, as discussed
above, a sampling effect may generally lead to broader trait
spectra within invasive than native floras. Several other cases
have been reported of invasive species with particularly
lipid-rich fruits becoming problematic because of efficient
bird-assisted seed dispersal (e.g. Cinnamomum camphora,
2004, Vos 2004, Gosper et al. 2006, Neilan et al. 2006).
We conclude that a focus upon trait variation rather than
on average differences between native and invasive floras
may help to resolve inconsistencies in conclusions about the
relevance of fruit quality for plant invasions. Even if there is
no general tendency for lower nutritional quality among
native species, empty niche opportunities might make
oceanic islands more vulnerable to invasions by some alien
species with particular fruit traits. More generally, a focus
on trait variation rather than general trends may help to
overcome inconclusive results on the invasiveness of alien
plants, and advance generalisation in invasion biology.
Acknowledgements Á The constant support of the Seychelles
Ministry of Environment and Natural Resources, and particularly
the Forestry and Conservation sections, was crucial for the success
of the project. We thank Walter Glauser and the Swiss Federal
Institute for Farm Animals and Dairy Farming for the chemical
analyses of the fruit material, and Ge´rard Rocamora, Perley
Constance, Rodney Fanchette, James Mougal and Eva Schuma-
cher for their assistance with the data collection in Seychelles. The
paper proﬁted substantially from the comments on earlier versions
by Christopher Kaiser, Katy Beaver and Dennis Hansen. Funding
was provided by a research grant from the Swiss Federal Institute
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