Plantations of
According to the “enemy release hypothesis”, the invasiveness of some
alien species may be explained by the scarcity of natural enemies in the
introduced range, such as pathogens, parasites and predators (Keane and
Crawley, 2002; Mitchell and Power, 2003; Colautti et al., 2004). A high
propagule pressure originating from plantations (Rouget and Richardson, 2003;
Rejmánek et al., 2005), associated with few seed losses by post-dispersal
predation, may help to explain the higher recruitment of
In southeast Australia, where
In order to investigate seed predation, it is common practice to use seed removal experiments (e.g. Holmes, 1990; Wandrag et al., 2013; Montesinos et al., 2012). In these experiments, a known amount of seeds are made available to animals, and the number of seeds removed is registered systematically. Many studies distinguish among types of animals, particularly vertebrates and invertebrates, using animal-exclusion treatments (e.g. Alba-Lynn and Henk, 2010; Auld and Denham, 1999), because vertebrates and invertebrates may use different foraging areas (Hulme, 1997; Alba-Lynn and Henk, 2010), and the fate of the seeds may depend on the type of harvesting animals (Fedriani et al., 2004; Alba-Lynn and Henk, 2010; Holmes, 1990). Some studies also expose the studied seeds to different habitats and micro-habitats, targeting different animal populations that may have different foraging habits and food preferences (Meiss et al., 2010; Barberá et al., 2006; Ordóñez and Retana, 2004).
The present study is based on a seed removal experiment, conducted in central
Portugal, designed to answer a core research question related to the
invasiveness of a widespread timber species: do animals play a relevant role
in the fate of post-dispersed
Seeds from
The experiment was undertaken in a peri-urban area of Coimbra, in the central
west of Portugal (40
Site characterisation was made through a visual estimate of different
variables within a 5 m radius from the centroid of each sampling unit (10 in
each site). In each of these areas, we identified all plant species with
cover greater than 5 %
(including canopy cover). The first site was an
Five seeds of each study species, totaling 15 seeds, were placed inside
feeders – plastic Petri dishes with no lid (10 cm diameter and 1 cm deep).
We designed three types of feeders in order to select the type of animals
that had access to the seeds. The invertebrate feeder was designed to prevent
the access to the seeds by vertebrate animals. It was a Petri dish, placed on
the ground, surrounded by a wire mesh cage (mesh size 1 cm
All feeders, including the control feeders, were emptied every day and
replenished between 09:00 and 11:00 UTC
Fresh seeds were used every day in order to assure that all seeds had equal
chances of being removed by animals. Seeds were selected randomly, with no
size criteria. All
A few days after the seed removal experiment, under similar meteorological
conditions, we tried to identify some of the animal species using the seeds.
This task was done afterwards to avoid further disturbance of the feeding
stations during the experiment. In order to identify invertebrates,
particularly ants, known to be the main seed-harvesters of the studied
species, we placed one total access feeder with five of each seed species at
eight locations showing the highest activity by invertebrates: four locations
in the
In our attempt to identify vertebrates, we focused on rodents, because we had
found frequent evidence of rodent activity inside or next to the feeders
(faeces; seed remains resulting from nibbling). In order to obtain proof of
rodents using the seeds we used camera traps. Camera traps were placed only
in the
In order to explore the spatial variation throughout the study area regarding
the activity of different seed predators and seed preferences, we performed a
principal component analysis (PCA). The values of the PCA matrix were the
proportions of seeds used (missing; eaten; elaiosome) in relation to the
total number of available seeds of each species (9 days
We modelled the effect of explanatory variables: feeder type (three levels:
invertebrate, vertebrate and total access feeders), seed species (three
levels:
In the second stage, we used a similar procedure to model the proportion of seeds used separately for each seed species, but using feeder type, type of animal action on the seeds (missing, eaten, elaiosome detached) and the interaction between the two factors, as explanatory variables. Therefore, four models were produced (one general and one for each studied species) and the corresponding multiple comparisons.
We undertook additional analysis aimed at assessing the potential of seed
depletion of each seed species by recording only the occasions (observations)
when any seed was used (missing, eaten, elaiosome detached), i.e. occasions
when we know that animals found the seeds and could reveal their preferences.
Therefore, for each feeder type and seed species, we computed the frequencies
of occasions (animal visits) where no (0), some (1–4) and all (5) seeds were
either eaten or missing. Elaiosome detachment on
Only 27 out of 4050 seeds (0.7 %) were missing from the control feeders
and only two extra seeds from
Average daily number of seeds used (missing, eaten, elaiosome
detached) per feeder (
The first two components of the PCA explained 83 % of the variance. The
analysis of the biplot allowed distinguishing three clusters of feeding
stations, characterised according to both the rate of seed use by animals and
the predominant animal group (vertebrates; invertebrates), regardless of the
seed species (Fig. 2). One first cluster of 13 stations (green cluster) shows
areas with reduced or negligible seed predation (
Biplot resulting from the PCA. Dashed
circles enclose clusters of feeding stations according to the following
patterns: negligible seed predation (green circle); predominance of
invertebrates (orange); predominance of vertebrates (blue). Letters a, c and
e represent the feeding stations located respectively in the
Location of the feeding stations according to shared patterns regarding the predominant seed predators and the level of seed predation.
The general GLMM explained 27 % of variance. The coefficients of the
fixed model terms, feeder type, seed species and their interaction were all
significant (model description in Supplement S2). Pairwise comparisons showed
that seeds from
Proportion of seeds used (missing, eaten, elaiosome detached) according to the seed species and feeder type. Different letters above the bars indicate significant differences (post hoc pairwise Tukey tests).
Proportion of seeds used according to the seed species, feeder type and the type of action on the seed (missing, eaten, elaiosome detached). Different letters above the bars indicate significant differences (post hoc pairwise Tukey tests).
Number of animal visits (any seed used on a given day: missing, eaten, elaiosome detached) where no (0), some (1–4) or all (5) seeds were
missing or eaten, according to the feeder type and seed species. Eu:
The GLMM for
The GLMM for
The GLMM for
The frequencies of occasions (animal visits) where no (0), some (1–4) or all
(5) seeds were eaten or missing were significantly related with the seed
species (
Four ant species were observed using the seeds in different locations.
Individuals from
The camera traps recorded small rodents (
One of the key findings of this study is the experimental demonstration of
predation (and removal) of post-dispersed
Invertebrates, particularly ants, had less importance in this study, which
was unexpected for several reasons. First, in the native range, it is well
documented that seeds of both
Contrary to our expectations, invertebrates, particularly ants, did not
prefer
In contrast, vertebrates were the main seed predators, removing a considerable
amount of
The video footage obtained (Supplement S3) and the crushed seed testa confirm
that small rodents predated
In the invertebrate feeders, a few
The spatial variation found in the study area regarding the predominant seed
predators and the rates of seed use may imply different seed fates depending
on the location where they are shed: (a) predation by vertebrates;
(b) predation by invertebrates with occasional dispersal events; (c) escaping
predation and integration into soil seed banks. Therefore, the spatial
variation of seed predation and dispersal by animals may shape the
distribution of many plant species (Andersen, 1982; Whelan et al., 1991;
Hulme, 1998, 1997; Alba-Lynn and Henk, 2010). Several factors beyond the
scope of this study may explain this spatial variation, such as habitat and
micro-habitat traits (Alba-Lynn and Henk, 2010; Manson and Stiles, 1998;
Whelan et al., 1991), predator risk for seed-eating animals (Sivy et
al., 2011), season and phenology of plant species (Bastida et al., 2009),
seed abundance (Holmes, 1990; O'Dowd and Gill, 1984) and alternative food
sources. Locations with reduced or negligible seed predation may be prime
areas for seedling establishment, because seeds have greater chances of
becoming embedded in the soil or litter, and are less likely to be detected
and predated in the future (Andersen and Ashton, 1985; Hulme, 1994; Vander
Wall, 1994). Seeds of
Our findings encourage further exploration of the role of seed-harvesting
animals in the invasiveness of eucalypt species in the introduced range.
Probably, throughout the introduced range, animals predate and disperse
post-dispersed seeds of other widespread eucalypt species (e.g. Harwood,
2011; Rejmánek and Richardson, 2013), with implications for the
demography and the dynamics of the populations. Further research is needed to
evaluate the suitability for seedling establishment and survival of the areas
where seeds largely escape predation. Also, the seasonal variation of seed
predation, observed in the native range (Ashton, 1979; Andersen and Ashton,
1985), is likely to occur in the introduced range, and is worthy of
examination since seed dehiscence occurs throughout the year
(Calviño-Cancela and Rubido-Bará, 2013; Cremer, 1965). It may also be
appropriate to investigate the role of other animals in the fate of
post-dispersed eucalypt seeds (e.g. birds). The abundance and the scattered
dispersion of areas of reduced seed predation, together with site traits, may
therefore help to explain the heterogeneous recruitment patterns of
Seeds of
The database can be accessed online using the following
link:
The supplement related to this article is available online at:
ED, CF, JSS and HM designed the experiment. ED and CF collected the field data. JSS and ED analysed the data. ED prepared the manuscript with contributions from all authors. The order of authors reflects the level of contribution to the present manuscript.
The authors declare that they have no conflict of interest.
This research was funded by Fundação para a Ciência e a Tecnologia (FCT) as part of the “WildGum” project (FCT PTDC/AGR-FOR/2471/2012). ED was supported by a doctoral grant (PB/BD/113936/2015). We thank Filipe Carvalho for helping with the identification of the ants. We thank Santa Casa de Misericórdia de Coimbra for authorising the installation of 10 feeding stations on their property. We thank Kevin Moull for the English revision. Edited by: Martijn Bezemer Reviewed by: Paulo Henrique Silva and three anonymous referees