Interaction type influences ecological network structure more than local abiotic conditions: evidence from endophytic and endolichenic fungi at a continental scale

Abstract

Understanding the factors that shape community assembly remains one of the most enduring and important questions in modern ecology. Network theory can reveal rules of community assembly within and across study systems and suggest novel hypotheses regarding the formation and stability of communities. However, such studies generally face the challenge of disentangling the relative influence of factors such as interaction type and environmental conditions on shaping communities and associated networks. Endophytic and endolichenic symbioses, characterized by microbial species that occur within healthy plants and lichen thalli, represent some of the most ubiquitous interactions in nature. Fungi that engage in these symbioses are hyperdiverse, often horizontally transmitted, and functionally beneficial in many cases, and they represent the diversification of multiple phylogenetic groups. We evaluated six measures of ecological network structure for >4100 isolates of endophytic and endolichenic fungi collected systematically from five sites across North America. Our comparison of these co-occurring interactions in biomes ranging from tundra to subtropical forest showed that the type of interactions (i.e., endophytic vs. endolichenic) had a much more pronounced influence on network structure than did environmental conditions. In particular, endophytic networks were less nested, less connected, and more modular than endolichenic networks in all sites. The consistency of the network structure within each interaction type, independent of site, is encouraging for current efforts devoted to gathering metadata on ecological network structure at a global scale. We discuss several mechanisms potentially responsible for such patterns and draw attention to knowledge gaps in our understanding of networks for diverse interaction types.

Endolichenic fungi: the lesser known fungal associates of lichens

“…This gains importance since such a low specificity in the selection of photobiont by the mycobiont in forming a lichen thallus widens the ecological amplitude enabling the lichen to colonise extreme environments (Muggia et al 2014) and the diversity of endolichenic fungi is influenced by climate, host lineage and geographic isolation (U’Ren et al 2012). In a recent study, Chagnon et al (2016) conclude that endolichenic fungi are not strictly host specific and behave as generalists when compared with endophytes. Some endophytic fungi such as Colletotrichum, Pestalotiopsis, Phomopsis and Xylaria (Suryanarayanan et al 2011; Govinda Rajulu et al 2013; Sudhakara Reddy et al 2016) also exhibit loose host affiliation and infect taxonomically unrelated plants.…”

“…Recently, the use of molecular method has revealed the occurrence of 11 new species of lichen-forming fungi (Leavitt et al 2016). Chagnon et al (2016) argue that both culture-based and culture-independent studies have to be made to get a more complete picture of the diversity of endolichenic fungi. Additionally, lichen-enriched growth media could be used to isolate fastidious endolichenic fungi which fail to grow on normal growth media (Biosca et al 2016).…”

Citizen science data reveal ecological, historical and evolutionary factors shaping interactions between woody hosts and wood‐inhabiting fungi

“…Some recent network studies have broadened this perspective by including endophytic and endolichenic fungi (e.g. Zhang & Yao, ; Chagnon et al ., ; Vincent et al ., ), but still many of the interactions between fungi and other organisms remain unexplored from a network perspective. This is true also for saprotrophic fungi, although they are the main agents of plant litter decomposition in terrestrial ecosystems world‐wide (Boddy et al ., ).…”

Research priorities for harnessing plant microbiomes in sustainable agriculture

“…Network analysis can be particularly useful for identifying linked microbes and “hub” microbes, akin to keystone species, that interact strongly with many other microbes and thereby strongly impact both the structure and function of the community [71]. Comparisons of microbiomes across habitats [72,73], the addition or removal of a single biotic component [71,74], dilution experiments [75], and pioneering work manipulating culturable bacteria [62,71] are beginning to provide insight into how biotic interactions influence microbiome assembly and persistence in plants. Ultimately, testing synthetic communities under a variety of abiotic and biotic conditions will improve our ability to predict how these inocula will perform across diverse and variable farm environments (Fig 1C, Fig 2A).…”