Print PagePriority effects, in which the outcome of interspecific interactions depends on the order of species arrival, are increasingly recognized as an important process affecting predictability, stochasticity and historical contingency in community assembly. However, little is known about how priority effects vary in strength among species and what factors explain this variation. Using fungi colonizing floral nectar, we conducted a laboratory experiment to investigate variation in the strength of priority effects among species. The experiment involved most of the yeast species found in the floral nectar of a hummingbird-pollinated shrub at the Jasper Ridge Biological Preserve in northern California. At the preserve, bagging of flowers reduces the frequency of yeast detection in nectar, indicating pollinators are the main dispersal agent for yeasts. Individual flowers last about 7 to 8 days, and yeast abundance in nectar appears to reach carrying capacity when flowers are 3 to 4 days old. Using all possible pairwise combinations of yeast species, we introduced 200 cells of a species to 9 microliters of filtered fresh nectar and, 2 days later, introduced another species to the same nectar, and vice versa. We measured the abundance of each of the two species 3 days after the second species introduction.
Results/Conclusions
We found that priority effects were widespread, with early-arriving species almost completely excluding late-arriving species in many of the species pairs examined. Effects were stronger between phylogenetically more closely related species, presumably because they were more similar in resource requirements. It has been previously shown that floral nectar represents a strong habitat filter, resulting in a predictable subset of fungal species that are commonly found in nectar. Yet, within this subset of potential colonists, the strong priority effects revealed by our experiment suggest a high potential for historical contingency in community assembly. Historical contingency makes community assembly appear stochastic when the order of species arrival is variable but unknown. Because the foraging behavior of hummingbirds and other pollinators is affected by nectar chemistry, which can in turn be altered by nectar yeasts, priority effects occurring within nectar may influence not just the species composition of the yeasts, but also the reproductive function of the flowers. Our results suggest that consideration of phylogenetic relatedness may help to predict which of the species in a given region make historical contingency important to the assembly and function of ecological communities.
