Fly-fungus and fungus-grass symbioses: spatial variability and interaction of three non-native species in western Oregon

An important question in ecology is how spatial variation among mutualists affects population growth. We examined the spatial variability of interaction in a “pollinator” mutualism involving a fungus and fly in western Oregon. Epichloë typhina (Ascomycetes: Clavicipitaceae) is an endophyte infecting Dactylis glomerata, orchardgrass. The fungus is heterothallic; it must outcross gametes to produce ascospores, propagules which can infect new hosts. A fly, Botanophila lobata (Diptera:Anthomyiidae), "pollinates" the fungus during an oviposition behavior in which the fly outcrosses gametes by defecating fungal spermatia. The fly, fungus and grass are all non-native to western Oregon. The objectives of this study were to: 1) characterize the spatial variability of both fly and fungus within cultivated orchardgrass fields, and 2) examine the response of the fly to spatial pattern of the fungus. During the 2008 growing season, counts and GPS locations of the fly, fungus and grass were recorded in ten cultivated orchardgrass fields. Four fields were surveyed again in 2009 to compare results across seasons. We found that although fly presence was quite variable between and within sites, fertilization rate was nearly 100 percent at all locations. Both the fly and fungus showed variable aggregation patterns between sites. Additionally, the fly exhibited positive density dependence in response to the fungus. The applicability of mutualism as a description for fly-fungus association within the context of western Oregon is discussed.