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).