INTRODUCTION
Mutualistic interactions often involve multiple, non-related species in
a community (Bascompte and Jordano 2007b), potentially affecting the
ecology and evolution of entire assemblages. However, mutualistic
interactions are not distributed homogeneously across the assemblage of
potentially interacting species (Vázquez et al. 2005). Rather, some
species may interact with many potential partners available in the
community, while others interact with only a few (Bascompte et al. 2003,
Fagundes et al. 2017, Plowman et al. 2017, Guimarães 2020). This
heterogeneous distribution of interactions across species can be
influenced by the traits of the individuals and populations involved in
the mutualism and their response to the local biotic and abiotic
environment (e.g.Vázquez et al. 2005, Miller 2007, Albrecht et al. 2010,
Dáttilo et al. 2013b, Maia et al. 2019). Whenever habitat conditions
influence these mutualistic traits, it can indirectly affect the
patterns of interaction in mutualistic networks and, consequently, the
mechanisms shaping natural communities and determining species responses
to environmental conditions (see Tylianakis and Morris 2017). Therefore,
understand how interactions are distributed in response to environmental
conditions can help us to understand the consequences to species
interactions over global climatic changes.
Defensive mutualistic interactions between ants and plants with
extrafloral nectaries (EFNs) are an example of mutualistic interaction
in which ant and plant traits can be influenced by environmental factors
(Kersch & Fonseca 2005, Heil 2008, Pringle et al. 2013). Ant
competitiveness determines their visitation patterns to plants with EFNs
and their quality as bodyguards (Leal and Peixoto 2017, Leal et al.
2022). Dominant ants display a series of aggressive behaviors that
ensures them a highly competitive ability (Stuble et al. 2017). This
aggressiveness also makes dominant ants better mutualistic partners for
plants with EFNs because they are more likely to behave aggressively
towards herbivores, unlike subordinate ant species who do not typically
exhibit aggression (Buckley and Gullan 1991, Xu and Chen 2010,
Flores-Flores et al. 2018, but see Melati & Leal 2018). Like the ants,
plant species also vary in their attractiveness to ant bodyguards, with
those producing more concentrated and abundant nectar being more
appealing to their partners (Blüthgen and Fiedler 2004a, c,
Flores-Flores et al. 2018). Therefore, dominant ant species are likely
to monopolize the most attractive plant species in the community,
displacing subordinate species to less attractive plants (Blüthgen and
Fiedler 2004c). At a broad scale, this kind of assortative pairing can
predictably shape the patterns of ant-plant interaction along spatial
gradients (Dáttilo et al. 2013b), especially along those in which the
competitive pressure among ant species is also variable (Leal and
Peixoto 2017, da Silva et al. 2019, Lasmar et al. 2021). In this case,
ant-plant interactions should become less generalized and form groups of
interacting species as the stronger competitive pressure among ant
species, with dominant and subordinate ants interacting with a more
dissimilar group of plants in the community.
A meta-analysis found that, at a macroecological scale, plants with EFNs
benefit more from ant attendance as the environment dries out, due to an
increased probability of plant attendance by dominant ant species (Leal
and Peixoto 2017). This is because the extrafloral nectar, a water
source for ants (Heil 2011, 2015), becomes more valuable to ants when
water is scarce, making plants with EFNs more attractive to ants
(Ruffner and Clark 1986, Contreras et al. 2013, Leal and Peixoto 2017).
However, it neglects that, even in drier habitats, plant species would
not be equally valuable to ants. Then, dominant ant species might
increase the monopolization of the best plant partners available in
drier habitats, increasing the competitive exclusion of subordinate ants
from the more valuable plant species. Consequently, it is possible that
at a macroecological scale ant-plant interactions become more
specialized and grouped as scarcer the water availability – something
that can have relevant implications for the eco-evolutionary dynamic of
this mutualism at a broad scale. For instance, increasing the
contribution of direct effects on trait matching over indirect effects
on interactions, accelerating the coevolutionary dynamics on interacting
species with the group of interactors.
Here, we investigated how emergent patterns of interactions between ants
and plants with EFNs (ant-plant interactions hereafter) at the community
level vary over environmental water gradients and evaluated the role of
ant competition for plant species secreting extrafloral nectar on the
emergence of such patterns. For that, we hypothesized that (i) ant-plant
interactions become more specialized and forming sub-groups of
interaction partners as drier the environment and that (ii) these
effects will be driven by a decline in the overlap of dominant and
subordinate ant species in the use of these plants over the
environmental water gradients. To evaluate our hypotheses, we used two
different approaches based on the ecological network theory (Bascompte
et al. 2003, Jordano et al. 2003, Bascompte and Jordano 2007b). In the
first approach, we evaluated how the structure of ant-plant ecological
networks and plant usage by all ant species varied along a
macroecological gradient of water availability (first hypothesis). In
the second approach, we evaluated the water availability effect on the
patterns of plant usage specifically by dominant and subordinate ant
species (second hypothesis).
2. METHODS