Bee host-associated gut microbes of the Sky Islands
Microbial communities are ubiquitous globally and, like macroorganisms, exhibit spatial diversity patterns. The drivers of these patterns are not well understood, particularly for host-associated microbes, which vary in the strength of their associations from strong to weak. Strongly host associated microbes are adapted to the host and primarily transmitted during interactions between hosts, while weakly host associated microbes occur more incidentally in the host and are primarily acquired through interacting with the environment. These ecologically-distinct types of microbial association likely play a role in shaping demonstrated spatial diversity patterns, yet are rarely considered separately in microbial biogeography studies. Furthermore, few studies of microbial biogeography examine patterns in multiple co-occurring host genera. Here we show that strong and weak microbial associates vary in the drivers of their spatial diversity patterns both within and between host genera. Using Bayesian structural equation modeling, we found negative relationships between strong associate community phylogenetic distance and both host diversity and diet breadth, but for weak associates, a negative relationship with body size in Bombus hosts. In Melissodes host, we found a negative relationship between strong associate phylogenetic distance and host abundance. Decomposing site-level interaction networks revealed that strong associate communities are spatially structured by rewiring, while weak associate communities are structured by host-driven turnover. Our results indicate that host-associated microbes with different association strengths do not follow the same diversity patterns, underscoring the need for new theory for a mechanistic understanding of host-associated microbial biogeography.