Ant-plant specialisation influenced more by network types than by disturbance, elevation, or latitude

The ecological factors driving specialisation in species interaction networks along environmental gradients at large spatial scales are poorly understood. Although such drivers can have synergistic impacts, previous work has mainly assessed effects of network type and the abiotic environment separately. We conducted a meta-analysis of existing network data to assess the interactive effects and relative importance of these drivers of specialisation in ant-plant networks at global scales. We collated 74 ant plant networks from 1979–2023, categorised into four network types: plants that provide ants nesting sites (myrmecophytes); plants that provide only food sources (myrmecophiles); plants for which ants disperse seeds (myrmecochories); plants on which ants forage only (foraging). We explored how network specialisation varies between interaction types with elevation, latitude, and anthropogenic disturbance. We used a standard measure of network specialisation, tested whether standardising this against network null models influenced results, and measured phylogenetic network specialisation. We found that the unstandardised specialisation index was strongly affected by habitat disturbance, elevation and interaction type in a manner congruent with previous work, However, these effects are diluted after the index is standardised. These indicates that previous results may relate to species abundance and richness within the network rather than specialisation. This is supported by the existence of correlations between network species richness/weighted connectance and the unstandardized index. Phylogenetic network specialisation was greater for myrmecophytes than for other three network types. This probably relates to closer co-evolution between partners in myrmecophytic networks. Phylogenetic network specialisation also did not vary significantly with elevation, latitude or anthropogenic disturbance. Our results demonstrate that ant-plant network types, in this case relating to strength of mutualistic interaction, is more important is shaping network specialisation than geographical gradients.

Methods
We collated existing ant-plant interaction network datasets by searching for publications on the Web of Science (WOS) online database supplemented by direct requests to the authors of some datasets. Data collection activities were carried out throughout 2021, with no date limits imposed. We searched Web of Science database using the keywords "myrmecophyte" OR "extrafloral nectaries” (EFN) OR "fruiting bodies" (FB), OR "myrmecophily" OR "ant-plant interaction" OR "ant-plant foraging," OR "myrmecochory". We selected networks that consisted of at least three plant species and at least three ant species. We excluded networks that consisted of presence/absence data only, as many network descriptors are sensitive to binary data.