Plant-microbe specificity varies as a function of elevation
Specialized associations between interacting species are a fundamental determinant of the diversity and distribution of both partners. How specialization of guilds of organisms varies along environmental gradients underpins popular theories of biogeography and macroecology, whereas the degree of specialization of a species is typically considered fixed. However, the extent to which environmental context impacts specialization dynamics is seldom examined empirically. In this study, we examine how specialization within a bipartite network consisting of three co-occurring plant species and their foliar fungal endophyte symbionts changes along a 1 000-meter elevation gradient where host species were held constant. The gradient, along the slope of Mauna Loa shield volcano, represents the entire elevational range of two of the three plants. Network and plant specialization values displayed a parabolic relationship with elevation, and were highest at middle elevations, whereas bipartite associations were least specific at low and high elevations. Shannons diversity of fungal endophytes negatively correlated with specificity, and was highest at the ends of the transects. Although plant host was a strong determinant of fungal community composition within sites, fungal species turnover was high among sites and plant host predicted a weak, though significant proportion of compositional variance. There was no evidence of spatial or elevational patterning in fungal community compositon. Our work demonstrates that specificity can be a plastic trait which is influenced by the environment and centrality of the host within its natural range.