Hummingbird-pollinated flowers

This system involves the communities of nectar-inhabiting fungi and bacteria that develop in flowers of a hummingbird-pollinated shrub, Mimulus aurantiacus, at the Jasper Ridge Biological Preserve, located about 5 km from Stanford's main campus. This system is ideally suited for studying community assembly for several reasons. First, floral nectar is initially sterile, and microbial species undergo primary succession as various species colonize the initially sterile habitat. Short microbial generation times mean that populations grow quickly in nectar, enabling well-replicated observations of community assembly over many generations. Second, hummingbirds as the microbes' main dispersal vectors enable detailed quantification of immigration history. We are developing methods to ascertain the timings of potential immigration events by monitoring bird visits to flowers using motion-activated cameras (see this Stanford Report article for more detail). Third, because their life is essentially inseparable from the pollinators in terms of immigration to flowers and nectar utilization in flowers, nectar-inhabiting microbes can affect the function of flowers for plant reproduction, potentially resulting in close links between community structure and function.

We have so far found that the microbial species are distributed non-randomly within the Preserve, indicative of dispersal limitation driven by non-random foraging by hummingbirds. We have also found that priority effects are widespread in these microbial communities, with early-arriving species inhibiting or enhancing growth of late-arriving species. The magnitude of priority effects is highly variable, with phylogenetically closer relatives affecting one another more strongly. We have been able to explain much of this variation based on niche differences among species and environmental variability in temperature. We have also found that microbial species differ in their effects on pollination and plant reproduction.

We are using this system to study the effect of historical contingency on the feedback between community structure and function. As mentioned above, historically induced differences in community structure can influence the flower's functional role by affecting pollinator visits. In turn, changes in pollinator visits can alter microbial immigration history and thus community structure. But how strong is this feedback? How much does it depend on environmental conditions? At what spatial and temporal scales does the feedback happen? These are some of the questions we will investigate over the next several years.

An additional emphasis of this project is integration of research, education and outreach.