Ant-plant specialisation depends on network type, but not disturbance,
elevation, or latitude.
Abstract
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 (H2’), tested whether standardising this against network
null models influenced results (H2’ z-score), and measured phylogenetic
network specialisation (dsi*). We found that H2’ was strongly affected
by habitat disturbance, elevation and interaction type in a manner
congruent with previous work, However, these effects disappeared once
H2’ was standardised (H2’ z-score). The disappearance of these effects
indicates that previous results may relate to variation in network
structure rather than specialisation. This is supported by the existence
of correlations between network species richness/weighted connectance
and H2’. Phylogenetic network specialisation (dsi*) was greater for
myrmecophytes than for other three network types. This probably relates
to closer co-evolution between partners in myrmecophytic network.
Phylogenetic network specialisation 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 the main driver of network
specialisation, and that previously reported impacts of latitude,
elevation and anthropogenic habitat disturbance are likely to have been
mediated mediated via correlations with network size.