Liana-tree network structure
We found anti-nested and modular structure in the three liana-tree
interaction networks in Asenanyo Forest Reserve. This trend has also
been reported by Addo-Fordjour et al. (2021, 2016), and to some extent
by Magrach et al. (2016) whose liana-tree networks showed anti-nested
structure (see supplementary data). Nevertheless, our study is at
variance with that of Sfair et al. (2010) which recorded nested
structure in three distinct vegetation formations in Brazil, and also
differs from the networks of Sfair et al. (2015) which did not show
modularity. In the suhuma Forest Reserve, all the three networks were
not nested but modular. Though the two nestedness patterns shown by the
networks in the Asenanyo and Suhuma Forest Reserves refer to non-nested
structure, that of the former depicts non-random assembly of species
whereas the latter indicates random assembly of species. We argue that a
clear distinction should be made between the two types of non-nestedness
in network studies so that the distribution pattern of each of them
would be fully understood. The presence of non-significant nestedness in
the Suhuma Forest Reserve may be due to differences in liana species
ability to colonise host trees and/or the use of defense strategies of
hosts to avoid lianas (Addo-Fordjour et al., 2016; Genini et al., 2012).
As a recap, a nested structure is formed when there are interactions
involving generalists and generalists, and specialists and generalists,
but no interaction of specialists and specialists (Landi et al., 2018).
Staniczenko et al. (2013) showed that for a nested quantitative network,
interactions of generalist-generalist species are strongest, followed by
those of generalist-specialist species, with no specialist-specialist
interactions (or when present with much weaker interactions). Thus, for
a nested structure to occur in a quantitative network like ours, there
should be a good number of specialist and generalist species undergoing
interactions. However, in our networks, we observed only a few
generalists of lianas and trees that interacted, but with many
specialist species interacting among themselves. This situation
increased the likelihood of specialist-specialist interactions at the
expense of generalist-generalist and generalist-specialist interactions,
resulting in absence of nested structure in the various networks. A
similar trend was observed in mycorrhizal networks (Jacquemyn et al.,
2015). The specialist-specialist interactions in our networks may
account for the non-asymmetry and weak asymmetry exhibited by the
networks. This finding shows that our networks tended to be more
symmetric in their interactions, a trend which causes non-significant
nestedness and significant modularity in ecological networks (Guimarães
et al., 2007). Overall, the findings on liana-tree network structure
reported in the current and previous studies show that there is no
universal pattern in the structure of liana-tree interactions. The
patterns obtained may be dependent on the network complexity, and
species traits and abundance, which are known to influence the
organisation of liana-tree interactions (Sfair et al. 2018, 2010). The
existence of high modular structure in the various networks may increase
their stability and robustness by limiting diffusion of perturbations
through network (Thébault & Fontaine, 2010). This may explain why the
patterns of network structure in edge site was consistent with those in
interior and deep-interior sites, irrespective of disturbance at edge
site. The modular structure of our networks may help conserve the
networks of species interaction, which in turn, may lead to the
conservation and maintenance of ecosystem functioning.
Though the nature of liana-tree interaction is still a subject of
debate, it tends to be antagonistic, in view of the fact that lianas are
not only structural parasites of trees (Tang et al., 2012), but also
compete intensely with trees for resources (Sfair et al., 2018). Species
of antagonistic networks often evolve high specialisation in order to
survive the antagonism of the interactions (Maliet et al., 2020). Our
results revealed strong species and network specialisation in the forest
sites, which demonstrates the existence of strong liana-host specificity
across the various networks in the two forest. Host specificity and
network specialisation have been reported to cause non-nestedness and
modularity in networks (Cordeiro et al., 2020; Dallas & Cornelius,
2015; Wardhaugh et al., 2015; Maliet et al., 2020). Given this
information, the non-nested and modular structure observed in our
networks may be driven by the specialisation of the networks and host
specificity of the liana species. The specialisation in the liana-tree
networks may be related to co-evolution in lineages of lianas and trees
in the networks (Sfair et al., 2015). The possibility of co-evolution of
lianas and trees in our networks is supported by Ponisio et al. (2019,
2017) who showed that ecological communities that co-evolve become more
anti-nested and modular over time. Montoya et al. (2015) found out that
functional group diversity increases with modularity in complex
networks, and that functional groups form modules in communities. In
this regard, the presence of high number of modules per network in the
forest sites may reflect the existence of different liana functional
groups that interact with tree communities in the forests. Such networks
with high level of modularity may possess increased resistance to
disturbance (Olesen et al., 2007; Saunders & Rader, 2019). Differences
in colonisation rates in fish parasites were found as a cause of
anti-nested structure in such networks (Poulin & Guégan, 2000). In each
of the networks, different liana species showed varying degree of
specialisation, while others exhibited generalisation. This phenomenon
suggests that the rate of colonisation would differ markedly among the
species, with highly specialised species having lower rate of
colonisation, while species with low specialisation, or generalisation
exhibit higher colonisation rate. In this regard, like the parasite-fish
network (Poulin & Guégan, 2000), the anti-nested structure in our
networks could have partly been occasioned by variation in colonisation
rates of the liana species. Generally, our study adds to the number of
studies that have demonstrated the existence of non-nestedness and
modularity in liana-tree networks (e.g., Addo-Fordjour et al., 2021;
Magrach et al., 2016).