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\begin{document}

\title{The disruption of a keystone interaction erodes pollination and seed
dispersal networks}



\author[1]{Agustin Vitali}%
\author[1]{Yamila Sasal}%
\author[2]{Diego Vázquez}%
\author[3]{Mariano Rodriguez-Cabal}%
\affil[1]{INIBIOMA}%
\affil[2]{Instituto Argentino de Investigaciones de las Zonas Áridas, CONICET, CC 507, 5500}%
\affil[3]{INIBIOMA, CONICET}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Understanding the impacts of global change on mutualistic networks is a
major challenge in modern ecology. The gain or loss of particular
species and the disruption of key interactions are both consequences and
drivers of global change that can lead to the disassembly of complete
mutualistic webs. We explored whether the disruption of a
hummingbird-mistletoe-marsupial keystone interaction by the invasion of
non-native species can have cascading effects on both pollination and
seed dispersal networks in the temperate forest of Patagonia. We found
that the disruption of the keystone interaction resulted in diverse
indirect effects that led to less complex pollination and seed dispersal
networks. Similarly, the disruption of the keystone interaction
decreased the functional redundancy among generalist plants and
pollinators. Our results demonstrate how the disruption of a keystone
interaction can have cascading effects through the community, reducing
the complexity and stability of ecological networks.%
\end{abstract}%



\sloppy


\textbf{Introduction}

A central attribute of ecological communities is that only few species
and interactions may be essential to maintain community structure and
ecosystem processes, even though many species might coexist and interact
(Ellison \emph{et al.} 2005; Valiente-Banuet \emph{et al.} 2015). The
loss of essential species and the disruption of essential interactions
may have cascading effects in the community and increasing species
extinction rates (Gilbert 1980; Koh \emph{et al.} 2004; Dunn \emph{et
al.} 2009; Rodriguez-Cabal \emph{et al.} 2013). Thus, studying the loss
of native species and gains of non-native invasive species is crucial to
understanding the structure and functionality of communities in a
changing world.

In particular, mutualistic interactions such as pollination and seed
dispersal are essential in supporting terrestrial ecosystems because
most of the plant reproduction success relies on these interactions
(Bond 1994; Ashman \emph{et al.} 2004). Ecological networks provide a
systematic way of representing, characterizing and comparing the
complexity of ecological communities, in which species are represented
by nodes and interactions by links between nodes (Pimm 1982; van
Veen\emph{et al.} 2008). In the last two decades, the study of
mutualistic networks has been critical to understand the structure and
functionality of communities (Bascompte \emph{et al.} 2003; Bascompte \&
Jordano 2007; Rohr \emph{et al.} 2014). However, although several
studies have focused on the impacts of human-caused environmental change
on mutualistic interactions (Traveset \& Richardson 2006;
Rodriguez-Cabal \emph{et al.}2007; Tylianakis \emph{et al.} 2008;
Traveset \& Richardson 2014), few studies have focused on how such
change triggers the disassembly of mutualistic webs (Fortuna \emph{et
al.} 2013; Rader \emph{et al.} 2014). Moreover, the indirect effects on
interaction webs are not well understood because indirect effects are
complex to study and because perturbations can be attenuated by
functional redundancy (Lawton \& Brown 1993; Terborgh \& Feeley 2010).

The temperate forest of Patagonia is known for its highly endemic flora
and high proportion of plant species requiring animal mutualists for
pollination and seed dispersal (Aizen \& Ezcurra 1998). Almost 60\% of
plant species require at least one animal species for pollination
(Riveros 1991) and nearly 50\% of plants produce fleshy fruits
suggesting animal seed dispersal (Armesto \emph{et al.} 1987). In the
northern portion of this temperate forest occurs a unique mutualistic
interaction involving a hummingbird (\emph{Sephanoides sephaniodes} ), a
marsupial (\emph{Dromiciops gliroides} ), a mistletoe (\emph{Tristerix
corymbosus} ), and its main host (\emph{Aristotelia chilensis} ), a
common understory shrub. The nectar produced by the mistletoe is the
primary food resource for the hummingbird during winter (Smith-Ram\selectlanguage{ngerman}írez
1993; Aizen \emph{et al.} 2002). In spring and summer the hummingbird
pollinates almost 20\% of the endemic woody genera in this region (Aizen
\& Ezcurra 1998; Aizen 2003). The marsupial provides an efficient seed
dispersal service by dispersing at least 16 fleshy-fruited species,
including large fruits (\textgreater{}11 mm) not dispersed by the native
birds (Amico \emph{et al.} 2009), and is the only seed disperser of the
mistletoe (Amico \emph{et al.} 2011), which thus depends fully on the
marsupial for its persistence. Previous studies found that herbivory
on\emph{A. chilensis} by non-native ungulates leads to the disassembly
of the hummingbird-mistletoe-marsupial interaction
(Rodriguez-Cabal\emph{et al.} 2013). Owing to the high degree of
asymmetry of plant-animal mutualisms in the temperate forest of
Patagonia, where most of the plants depend only on a few mutualistic
partners (Armesto\emph{et al.} 1987; Riveros 1991; Aizen \& Ezcurra
1998), the disruption of this interaction could trigger a cascade of
linked extinction events throughout the community.

In this study, we evaluate whether the disruption of a
hummingbird-mistletoe-marsupial interaction by non-native ungulates can
have cascading effects on pollination and seed dispersal networks in the
temperate forest of Patagonia, Argentina. Specifically, we address the
following questions: (1) Is the hummingbird-mistletoe-marsupial
mutualism a keystone interaction? We expect that the complexity and
functional redundancy among generalist species of pollination and seed
dispersal networks will be greater in sites with this interaction than
in sites without the interaction. We also expect that the ecological
importance of the species involved in this proposed keystone interaction
will be greater in sites with this interaction, occupying a high number
of positions in the networks. (2) Does the disruption of this proposed
keystone interaction produce cascading effects on the community? We
expect that the complexity and functional redundancy among generalist
species of pollination and seed dispersal networks will be lower in
sites invaded by non-native ungulates than in intact forest sites.

\textbf{Material and methods}

\textbf{Study Area and Natural History}

The study was conducted in Nahuel Huapi National Park and Llao Llao
Municipal Reserve in northwestern Patagonia, Argentina. The climate in
this region has a dry season in spring-summer and a humid season in
autumn-winter, with 1800 mm of average annual precipitation and 9°C of
average annual temperature. The forest is dominated by the evergreen
southern beech (\emph{Nothofagus dombeyi)} and cordilleran cypress
(\emph{Austrocedrus chilensis} ), with an understory layer dominated by
the shrub \emph{Aristotelia chilensis} and bamboo \emph{Chusquea
culeou}(Mermoz \& Martín 1986). Currently the non-native ungulates red
deer (\emph{Cervus elaphus} ), dama deer (\emph{Dama dama} ), and
domestic cattle (\emph{Bos taurus} ) are the most abundant ungulates in
these forests (Jaksic 1998; Jaksic \emph{et al.} 2002; Merino \emph{et
al.}2009) and occupy 56\% of the area of Nahuel Huapi National Park
(Lauría-Sorge \& Romero 1999).

In this study we took advantage of the contagious distribution of the
mistletoe populations (García \emph{et al.} 2009) that allows us to
compare sites with high and low mistletoe density. We selected six 1-ha
native forest sites, separated by more than 2 km. Four sites were
located in intact forest lacking introduced ungulates: two sites with
high density of mistletoes (\textgreater{}400 reproductive individuals
per hectare) and the presence of \emph{A. chilensis} , and two sites
with low density of mistletoes (\textless{}20 reproductive individuals
per hectare) and the presence of \emph{A. chilensis} . Two additional
sites were located in forests that have been invaded by non-native
ungulates. We considered invaded sites those with records of non-native
ungulates over the last 100 years. Due to the herbivory pressure by
non-native ungulates, the abundance of the mistletoe and its host
\emph{A. chilensis} is low and the hummingbird-mistletoe-marsupial
interaction is ecologically extinct (Rodriguez-Cabal \emph{et al.}
2013). Each site was chosen haphazardly from a pool of possible sites.
Fieldwork and data collection were carried out during two consecutive
austral springs and summers (2017-2018 and 2018-2019).

\textbf{Pollination networks}

During the flowering season (October--February) we identified all plant
species pollinated by animals in each site and estimated the rate of
flower visits. To this end, we conducted 10 min censuses per plant where
we recorded the identity, number of pollinator visits, and number of
flowers per individual or branch visited (when the sample plant has a
large number of flowers). A visit was recorded when the visitor touched
a reproductive structure of the flower. Pollinator species that could
not be identified in the field were collected and identified in the
laboratory. We performed the censuses from 11:00 to 17:00 h in sunny,
non-windy days. We carried out at least 12 censuses per plant species.
We conducted 2277 observation periods for all sites and plant species in
2017-2018, and 2269 observation periods in 2018-2019. These numbers
varied across species and sites because species differed in their
abundance, duration of flowering period and flower production (See Table
S1 in Supporting Information).

\textbf{Seed dispersal networks}

During the fruiting season (January--April) we identified all
fleshy-fruited plant species in each site. To quantify seed dispersal by
birds, we observed one hour per plant and recorded the number of fruits
consumed by birds. We discarded the records of seed predators and pulp
pickers. Censuses were performed from 6:30 to 10:30 h. In addition, to
evaluate seed dispersal by the endemic marsupial, we collected and
analyzed their feces. The feces were collected with a mesh on the floor
of Tomahawk traps (10 cm × 10 cm × 30 cm). We placed 17 Tomahawk traps
at a height of 1-2 m above the ground on shrubs or trees. Traps were
arranged in a star grid and separated from each other by 15 m to cover
an effective area of one hectare. We controlled the traps daily at dawn
during four consecutive days in February and March. To determine the
number of fruits dispersed by \emph{D. gliroides} , we divided the
number of seeds of each plant species found in the sample (feces) by the
average number of seeds per fruit of the same species. Moreover, to
accumulate more observation hours per plant species we used two infrared
camera traps (Bushnell trophy cam) per site, programmed to record during
day and night. Cameras were placed focusing branches bearing fruits of
different plant species and relocated periodically to obtain records of
all species present in the site. From the videos, we documented the same
variables recorded with direct observations, considering a dispersal
event when we visually recorded the consumption of fruit by the seed
disperser. Overall, 132 observation periods were conducted for all sites
and plant species in 2017--2018, and 158 observation periods in
2018--2019. These numbers varied across species and sites because
species differed in their abundance, duration of fruiting period and
fruit production (Table S2).

\textbf{Data analysis}

For each site we built one pollinator and one seed dispersal network per
season, resulting in a total of six plant-pollinator and six plant-seed
disperser networks (i.e. each site-year combination has a corresponding
network). The plant-seed disperser network in 2017-2018 in one of the
sites without the keystone interaction was excluded from the analysis
because several plant species produced no fruits.

Our analysis focused on network ``motifs'', which may be regarded as the
building blocks of networks, and consist in sub-networks composed of a
small number of species exhibiting particular patterns of interactions
(Milo \emph{et al.} 2002). Motifs are valuable tools to assess the
structure and ecological importance of species in networks because they
have two structural levels of organization. In one structural level, a
motif represents a unique pattern of interactions among a subset of
species within a community (Simmons \emph{et al.}2018), while in the
other level a motif is composed by two or more unique positions that can
be occupied by different species simultaneously (Fig. S1). Each of these
positions represents a different ecological pattern with direct and
indirect interactions (Stouffer\emph{et al.} 2012; Baker \emph{et al.}
2015; Cirtwill \& Stouffer 2015; Simmons \emph{et al.} 2019). Therefore,
the frequency of positions that a particular species occupies defines
its ecological importance in the community (Simmons \emph{et al.} 2019).
Even if different species occupy the same position, the motif still
conserves its ecological function. For example, in the motif integrated
by two plant species and a pollinator species, plant species could be
``A and B'' or ``C and D'' but the motif would still indicate
competition or facilitation between the two plant species. The advantage
of motifs is that they are significantly more sensitive to changes in
network structure than the network indices commonly used (i.e. degree
distribution, nestedness) (Simmons \emph{et al.} 2019). In addition, the
meso-scale level motif analysis incorporates indirect interactions
undetected by macro-scale network indices (i.e. nestedness, connectance)
and lost at species-level indices such as species strength (Simmons
\emph{et al.} 2018).

Is the hummingbird-mistletoe-marsupial mutualism a keystone interaction?

To test if the hummingbird-mistletoe-marsupial mutualism is a keystone
interaction we compared network complexity, functional redundancy among
generalist species, and the ecological importance of keystone
interaction members between intact forest sites with and without the
keystone interaction.

\emph{Network Complexity -} Network structure is influenced by the
number of motifs and their frequency (Simmons \emph{et al.} 2018).
Networks composed by a greater number of different motifs are more
complex structures because they harbor more different direct and
indirect ecological interactions (Milo \emph{et al.} 2002;
Simmons\emph{et al.} 2018). Likewise, network complexity increases with
the frequency of the largest motifs (those composed of six species; Fig.
S1). To estimate network complexity, network structure was described
using the frequencies of motifs containing between two and six species,
resulting in 44 possible motif combinations. Motif frequencies were
calculated for each pollination and seed dispersal network using the
``bmotif'' R package (Simmons \emph{et al.} 2018). We then normalized
motif counts to control for network size. We calculated motif frequency
using the method ``normalize\_sum'', which expresses counts as the
proportion of motifs in the network and considers whether species are
more involved in smaller or larger motifs (Simmons \emph{et al.} 2018).

To assess dissimilarity in motif frequencies between intact forest sites
with or without the hummingbird-mistletoe-marsupial mutualism
interaction, here after keystone treatments, we used a non-parametric
permutational multivariate analysis of variance (PERMANOVA). This method
allows comparing dissimilarity among and within groups using a pseudo
F-statistic (Anderson 2001). We used Bray-Curtis dissimilarity to
quantify differences among network structures because it is a robust
measure of dissimilarity for multiple ecological properties, including
motifs (Anderson 2001; Baker \emph{et al.} 2015; Simmons \emph{et
al.}2019). We conducted a PERMANOVA test with keystone treatments as
fixed factor and considered networks from different years built in the
same site as replicates. We performed PERMANOVA with the adonis2
function of the vegan package (Oksanen \emph{et al.} 2012) of R (R Core
Team), and using 9999 permutations to generate the null distribution of
test statics. Finally, we validated the results of PERMANOVA test by
estimating dispersion in the data using the betadisp function of vegan
package and testing with one-way ANOVA whether dispersion varied between
treatments using lme4 package (Bates \emph{et al.} 2015). We found no
significant differences between dispersion values, which indicates that
PERMANOVA results are not caused by heterogeneous dispersion of the
data.

\emph{Functional redundancy among generalist species} - In mutualistic
networks, nestedness is associated with stability and robustness of
species extinction (Memmot \emph{et al.} 2004; Burgos \emph{et al.}2007;
Thébault \& Fontaine 2010). In nested networks, a core of generalist
plants and animals interacts with each other while specialist species
interact only with generalist species, enhancing robustness to species
extinction. In addition, functional redundancy among generalist species
can increase stability by replacing the position of other generalist
after its extinction and reducing the cascading effect on other species
(Memmott \emph{et al.} 2004; Kaiser-Bunbury \emph{et al.}2010).

We estimated the functional redundancy of generalist species and
compared between keystone treatments to assess community stability on
pollination and seed dispersal networks. First, for each network we
calculated the number of species that occupy generalist positions in
motifs using bmotif package in R. When we disassemble a network in
subnetworks, we obtain 148 different species positions across the 44
motifs (Fig. S1). For pollinator and seed disperser species, we selected
the positions number 16 and 148, which represent generalist species, as
these positions have the highest number of direct interactions with
plants (four and five respectively; Fig. S2). For plants, we selected as
generalist positions the number 17 and 47, with four and five
interactions respectively (Fig. S2). Functional redundancy among
generalist species will be higher when more species occupy these
positions. Second, we built regression models for each generalist
position (16, 17, 48, and 148) with number of species as the response
variable to check differences of functional redundancy between keystone
treatments. We selected keystone treatments as fixed factor and
considered networks from different years built in the same site as
replicates to test this difference. We used the Poisson distribution in
our GLMs with a log link function because the response variable is
positive and consists in count data (Zuur \emph{et al.} 2009).
Regression models were conducted using the lme4 package of R.

\emph{Ecological importance of the keystone interaction members} - As a
consequence of the constrained relationship among
hummingbird-mistletoe-marsupial, the reduction in the abundance or
alteration of behavior of any of them could have the potential to
disrupt this keystone interaction. Therefore, we expect that the
ecological importance of these species change between intact forests
sites with and without the keystone interaction. Because motif positions
represent different direct and indirect effects and have different
ecological meanings (Stouffer \emph{et al.} 2012; Baker \emph{et
al.}2015; Cirtwill \& Stouffer 2015); hence, the greater the variety of
positions a species occupies, the greater its ecological importance.

To assess the difference in the ecological importance of the mistletoe,
hummingbird, and marsupial, we first calculated the number of positions
occupied by these species using bmotif package in R. The mistletoe and
hummingbird importance were calculated from pollination networks, while
the importance of the marsupial was calculated from seed dispersal
networks. We normalized the data using the method ``sum'' to control the
tendency that nodes (e.g. species) with more interactions will occupy
more positions than nodes with fewer interactions. This method expresses
position counts as the proportion of total occurrences a node occurs at
any position. Additionally, to check the difference between keystone
treatments we built regression models for each species with number of
positions as response variable and keystone treatments as fixed factor.
We considered networks from different years built in the same site as
replicates to test for this difference. We used the negative binomial
family distribution because we found overdispersion in the data and this
distribution is a good alternative to deal with it in count data
(Zuur\emph{et al.} 2009). Regression models were conducted using the R
package lme4.

Does the disruption of the keystone interaction by non-native ungulates
produce cascading effects on the community?

To test if non-native ungulates produce cascading effects on the
community by disrupting the keystone interaction we compared network
complexity, functional redundancy among generalist species, and the
ecological importance of keystone interaction members between intact
forest sites with the keystone interaction and sites invaded by
non-native ungulates. Network complexity, functional redundancy among
generalist species, and the ecological importance of keystone
interaction members were estimated using the same methodology explained
above.

\textbf{Results}

Overall, we recorded 5151 pollination individual interaction events and
2487 seed dispersal events (Fig. 1). For pollination networks, most of
these interactions (56\%) were recorded in intact forest sites with the
keystone interaction, while 29\% of interactions were recorded in sites
invaded by non-native ungulates and only 15\% in intact forest sites
without the keystone interaction. Similarly, most seed dispersal events
(50\%) were recorded also in sites with the keystone interaction,
followed by 27\% in invaded sites and 23\% in sites without the keystone
interaction.

\emph{\textbf{Is the hummingbird-mistletoe-marsupial mutualism a
keystone interaction?}}

\emph{Network complexity} - We found different complexity for both
pollination (\emph{F} \textsubscript{1,7} = 7.021, \emph{P} = 0.031) and
seed dispersal (\emph{F} \textsubscript{1,6} = 4.794, \emph{P} = 0.028)
networks between intact forest sites with and without the keystone
interaction. We found that in sites with the keystone interaction both
pollination and seed dispersal networks were supported by 35\% and 47\%
more different motifs respectively, and hence by greater direct and
indirect ecological interactions, than networks in sites without this
interaction (Fig. 2a and 2c). In addition, both pollination and seed
dispersal networks had 29\% and 12\% greater frequency of largest motifs
integrated by six species in sites with the keystone interaction than in
sites without it (Fig. 2b and 2d), which suggests a greater proportion
of complex interactions supporting these networks.

\emph{Functional redundancy among generalist species} - The number of
pollinator and plant species that occupied generalist positions was over
three times higher in intact forest sites with the keystone interaction
than in forests without it (Fig. 3a; Table S3). Moreover, in sites
without the keystone interaction no pollinator species occupied the most
generalist position and they barely occupied the second one. This
analysis was not possible in seed dispersal networks because of the
reduced number of frugivorous species.

\emph{Ecological importance of the keystone interaction members} - We
found differences in the ecological importance of the mistletoe
(\emph{Z} \textsubscript{1,7} = 3.452, \emph{P} \textless{} 0.001) and
the hummingbird (\emph{Z} \textsubscript{1,7} = 2.579, \emph{P} =
\textless{} 0.001) but not on the ecological importance of the marsupial
(\emph{Z} \textsubscript{1,7} = 0.941, \emph{P} = 0.146) between intact
forest sites with and without the keystone interaction (Fig. 3b). In
sites with the keystone interaction, the mistletoe and the hummingbird
occupied 10x and 29x more positions than in sites without this
interaction. These results suggest a greater ecological importance of
these species in pollination network because they exert more direct and
indirect ecological interactions.

Does the disruption of the keystone interaction by non-native ungulates
produce cascading effects on the community?

\emph{Network complexity -} We found differences on network complexity
in pollination networks (\emph{F} \textsubscript{1,7} = 5.032, \emph{P}
= 0.029) but only marginal in seed dispersal networks (\emph{F}
\textsubscript{1,7} = 3.849, \emph{P} = 0.087) between intact forest
sites with the keystone interaction and sites invaded by non-native
ungulates. The complexity on both pollination and seed dispersal
networks was lower in invaded sites than in sites with the keystone
interaction, exhibiting 23\% and 59\% fewer motifs respectively, and
hence, fewer direct and indirect ecological interactions supporting the
community (Fig. 4a and 4c). In addition, both pollination and seed
dispersal networks exhibit 12\% and 11\% fewer frequency of largest
motifs integrated by six species in invaded sites than in sites with the
keystone interaction (Fig 4b and 4d).

\emph{Functional redundancy among generalist species} - The number of
pollinator and plant species that occupied generalist positions was
twice as high in intact forest sites with the keystone interaction than
in invaded sites (Fig. 5a; Table S3). Moreover, in presence of the
keystone interaction a great number of pollinator species occupied the
most generalist position providing more network stability. On the other
hand, the low functional redundancy among generalist species found in
invaded sites suggests low stability in pollinator networks. This
analysis was not possible in seed dispersal networks because of the
reduced number of frugivorous species.

\emph{Ecological importance of the keystone interaction members} - We
found differences in the ecological importance of the members of the
keystone interaction between intact forest sites with the keystone
interaction and invaded sites (Fig. 5b). In sites with the keystone
interaction, the number of positions occupied by the mistletoe was at
least three times higher than in invaded sites (\emph{Z}
\textsubscript{1,7} = -2.998, \emph{P} = 0.002), suggesting a greater
ecological importance of the mistletoe in the pollination network. In
addition, we caught no individuals of \emph{D. gliroides} in any field
season in the invaded sites, which indicates that the marsupial may be
ecologically extinct in these sites (Rodriguez-Cabal \& Branch 2011).
Consequently, we were unable to assess statistically the ecological
importance of the marsupial between treatments. Furthermore, we found no
differences in the ecological importance of the hummingbird between
treatments (\emph{Z} \textsubscript{1,7} = -0.715, \emph{P} = 0.475).

\textbf{Discussion}

Our results indicate that the hummingbird-mistletoe-marsupial mutualism
has the potential to promote the complexity and stability of pollination
and seed dispersal networks in the Patagonian temperate forest due the
ecological importance of its members. Moreover, our results show that
the disruption of this keystone interaction by non-native ungulates has
cascading effects on the rest of the community by eroding pollination
and seed dispersal networks in invaded sites.

\emph{\textbf{Is the hummingbird-mistletoe-marsupial mutualism a
keystone interaction?}}

Complexity of both pollination and seed dispersal networks were
different between intact forest sites with and without the keystone
interaction. The diversity of motifs and the frequency of largest motifs
suggest that networks are more complex in the presence of the
hummingbird-mistletoe-marsupial interaction, with a wider range of
direct and indirect interactions (Milo \emph{et al.} 2002;
Simmons\emph{et al.} 2018). The high diversity of interaction types
found may increase persistence and stability of communities due to the
influence of direct and indirect interactions on the structure and
organization of communities (Wootton 1994; Mougi \& Kondoh 2012;
Aschehoug \& Callaway 2015). In addition to affect persistence and
stability of communities, the diversity of interactions may influence
ecological processes on a larger time scale. Ecological interactions
play an essential role on driving the selection of traits, mainly in
multiple partner mutualism such as pollination and seed dispersal
interactions (Strauss \& Irwin 2004; Guimarães Jr \emph{et al.} 2011).
Under normal disturbance conditions, the high number of direct and
indirect pathways in sites with the hummingbird-mistletoe-marsupial
interaction may allow to reshape continuously and slowly the direction
of trait selection, promoting the emergence of species traits (Guimarães
Jr \emph{et al.}2011; Guimarães Jr \emph{et al.} 2017). However, this
high number of pathways also may slow down the response of species when
rapid environmental changes occur, increasing its vulnerability to
strong disturbances (Guimarães Jr \emph{et al.} 2017).

The higher number of plant and pollinator species at generalist
positions suggests greater network stability in intact forest sites with
the keystone interaction than in sites without it. Several studies have
demonstrated that the loss of generalist species produces stronger
cascade effects and faster network collapse than the loss of specialized
ones (Memmot \emph{et al.} 2004; Kaiser-Bunbury \emph{et al.} 2010).
However, the impact of generalist species loss may be attenuated by
functional redundancy among species when they perform similar roles in
the community (Lawton \& Brown 1993). The great number of pollinator and
plant species that occupied positions with a high number of direct
interactions indicates low network susceptibility to generalist species
extinction by partial functional redundancy among them. Thus, the
hummingbird-mistletoe-marsupial mutualism can be considered as a
keystone interaction that provides a strong robustness to species
extinction on pollination networks by promoting the presence of multiple
generalist species that support specialized ones (Okuyama \& Holland
2008; Thébault \& Fontaine 2010).

When the keystone interaction is present, the mistletoe and hummingbird
are key members of the community by occupying high number of positions
and exerting great direct and indirect interactions. The increase in
their ecological importance may be induced by the spatial distribution
of the mistletoe and the co-evolutionary history of both species. Since
mistletoe population has a contagious distribution (García \emph{et
al.}2009), the high mistletoe abundance in may increase its probability
of interacting directly and indirectly with other species (Vázquez
\emph{et al.} 2009). Moreover, the mistletoe-hummingbird interaction
become specialist in winter because mistletoe flowers are the only
nectar resource available for the hummingbird, promoting its permanence
in patches with high mistletoe abundance (Smith-Ramírez 1993). In
contrast with the mistletoe and hummingbird, the presence of the
keystone interaction did not affect the dispersal role of the marsupial.
Its ample diet of insects, bird eggs, insects and fleshy fruits
(Fontúrbel\emph{et al.} 2012) may allow the marsupial to sustain a
stable population without consuming large amount of mistletoe fruits in
sites with low mistletoe density. Indeed, the presence of the marsupial
and mistletoe in these sites may be a potential source of the mistletoe
contagious distribution and become sites with high density of the
keystone interaction in the future due to the full dependence of the
mistletoe on the marsupial for its seed dispersal (García \emph{et
al.}2009; Amico \emph{et al.} 2011)

Does the disruption of the keystone interaction by non-native ungulates
produce cascading effects on the community?

Non-native ungulates produce cascading effects on pollination networks
by disrupting the keystone interaction but produce only marginal impacts
on seed dispersal networks. Non-native ungulates caused the ecological
extinction of the marsupial and reduced the ecological importance of the
mistletoe, preventing its recruitment and leading currently its
population to extinction. Pollination networks are much simpler and less
stable in invaded sites than in intact forest sites with the keystone
interaction, presumably because these sites support fewer direct and
indirect interactions and fewer generalist plants and pollinators. The
low interaction diversity in invaded sites concentrates the energy flow
in few pathways, reducing the persistence and stability of pollination
networks (Mougi \& Kondoh 2012). Similarly, the simplification of
ecological networks has been demonstrated for others human disturbances
(Albrecht \emph{et al.} 2007; Tylianakis \emph{et al.} 2007;
Galiana\emph{et al.} 2014). Moreover, the low number of plants and
pollinators that occupied positions with high number of direct
interactions makes the network susceptible to generalist species
extinction when non-native ungulates are present.

On the other hand, we found no cascading effects on seed dispersal
network despite the high vulnerability of this forest to the loss of
mutualistic interactions. The impact of non-native ungulates on seed
dispersal networks may be attenuated by the strong presence of the most
generalist bird seed disperser of this forest, \emph{Elaenia
albiceps}(Amico \& Aizen 2005). This migratory bird is abundant in
summer, overlapping with fruit ripening and playing a critical role
forest regeneration (Bravo \emph{et al.} 2015).

\textbf{Conclusion}

One or a few species and their direct and indirect interactions can be
critical to maintain the structure of communities and ecosystem
processes. Thus, identifying which species or interaction to conserve is
critical to prevent cascading effects in the community. Here, we have
demonstrated the role of a hummingbird-mistletoe-marsupial mutualism as
a keystone interaction in Patagonian temperate forest and the cascading
impacts of the disruption of this interaction by non-native ungulates.
We analyzed interaction motifs as the building blocks of pollination and
seed dispersal networks, and found that the
hummingbird-mistletoe-marsupial mutualism increases complexity on
pollination and seed dispersal networks, supporting a high diversity and
frequency of complex motifs and improving the persistence of the
community. In addition, the keystone interaction can increase the
robustness of pollination networks to generalist species extinction due
the high number of plant and pollinator species at generalist motif
positions. Moreover, we found that non-native ungulates disrupt this
keystone interaction, affecting the ecological importance of the
mistletoe and marsupial and reducing the complexity and stability of
pollination networks and, marginally, of seed dispersal networks.
Recognizing the keystone interactions is essential to achieve the
conservation of communities due its role to maintain the structure and
functionality of them, especially in the current scenario of global
biodiversity loss and spread of invasive species. Moreover, incorporate
indirect effects in ecological studies is necessary to increase the
realism of global changes impacts on communities, widely underestimated
when direct effects are only considered.

\textbf{Acknowledgements}

We thank staff of Nahuel Huapi National Park, D. Mujica and C. Chehebar,
staff of Parque Municipal Llao-Llao and Dirección de Áreas Protegidas,
M.S. Millerón for logistic support and permission to carry out
fieldwork. We also thank A.P. Duarte, A. Fernández, A. Santone, B.R.
Delgado, B. Lovazzano, E. Valfosca, I. Villa, J.G. Calzada, J. Gastaudo,
K. Buteler, L.M. Valfosca, M.E. Valfosca and P. San Pedro for valuable
assistance in the field. Special thanks go to Ezequiel Rodriguez-Cabal
who made the drawings. This research was supported with grants from
``Agencia Nacional de Promoción Científica y Tecnológica'' of Argentina
(PICT 2014-2484) to MARC and from ``The Rufford Foundation'' to AV (ID
26510-1).

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\textbf{Figures}

\textbf{Figure 1.} \textbf{Sites with the keystone interaction recorded
more individual interactions in pollination and seed dispersal networks}
. Treatments from top to bottom: intact forest sites with the keystone
interaction (\textbf{a-b} ), intact forest sites without the keystone
interaction (\textbf{c-d} ), and sites invaded by non-native ungulates
(\textbf{e-f} ). Left and right panels correspond to pollination and
seed dispersal networks, respectively. In each panel, boxes represent
species of plants (green), pollinators (red), and seed dispersers
(blue). Lines represent ecological interactions between species. Box
width is proportional to number of interactions.

\textbf{Figure 2.} \textbf{The complexity of pollination and seed
dispersal networks was greater in intact forest sites with the keystone
interaction} . Number and relative frequency of motifs related to their
complexity (number of nodes-species- per motif) for both pollination
(\textbf{a} and \textbf{b} ) and seed dispersal (\textbf{c}
and\textbf{d} ) networks. Bar color represents treatment: intact forest
sites with (violet) and without (white) the keystone interaction. Bars
represent means +- standard error.

\textbf{Figure 3. The functional redundancy among generalist species and
the ecological importance of the} \textbf{mistletoe, hummingbird and
marsupial} \textbf{were greater in intact forest sites with the keystone
interaction than in sites without it} . \textbf{a)} Number of pollinator
and plant species occupying generalist positions in pollination
networks. Number within the circles represents the generalist position
occupied by the species: the positions number 46 and 17 indicate four
direct interactions, and the positions number 148 and 47 indicate five
direct interactions. Circle color represents the trophic group occupying
the generalist position: pollinators (red) and plants
(green).\textbf{b)} Ecological importance -number of motif positions
occupied in the network- of the keystone mutualism members. From left to
right: mistletoe, hummingbird, and marsupial. Letters mean significant
difference between treatments. Bars represent means +- standard error.

\textbf{Figure 4.} \textbf{Complexity of pollination and seed dispersal
networks was greater in intact forest sites with the keystone
interaction than in sites invaded by non-native ungulates.} Number and
relative frequency of motifs related to their complexity (number of
nodes -species- per motif) for both pollination (\textbf{a}
and\textbf{b} ) and seed dispersal (\textbf{c} and \textbf{d} )
networks. Bar color represents treatment: intact forest sites with the
keystone interaction (violet) and sites invaded by non-native ungulates
(orange). Bar represent means +- standard error.

\textbf{Figure 5.} \textbf{The functional redundancy among generalist
species and the ecological importance of the} \textbf{mistletoe,
hummingbird, and marsupial} \textbf{were greater in intact forest sites
with the keystone interaction than in sites invaded by non-native
ungulates.} \textbf{a)} Number of pollinator and plant species occupying
generalist positions in pollination networks. Number within the circles
represents the generalist position occupied by the species: the
positions number 46 and 17 indicate four direct interactions, and the
positions number 148 and 47 indicate five direct interactions. Circle
color represents the trophic group occupying the generalist position:
pollinators (red) and plants (green). \textbf{b)} Ecological importance
-number of motif positions occupied in the network- of the keystone
mutualism members. From left to right: mistletoe, hummingbird, and
marsupial. Letters mean significant difference between treatments. Bars
represent means +- standard error.

\textbf{FIG. 1}\selectlanguage{english}
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