Butterfly and nectar community structure among a prairie-oak oak woodland gradient in the Willamette Valley, Oregon

Background/Question/Methods

In the Willamette Valley, prairie-oak butterfly species are rapidly declining as a result of loss and degradation of prairie-oak ecosystems upon which these obligate species rely. Butterflies are important for ecosystem integrity because they provide services such as pollination, prey species, and decomposition. The objectives of my research are to examine butterfly and nectar species richness and abundance across three different stand types of prairie, oak savanna, and oak woodlands. We also characterized butterfly and nectar species composition to environmental variables. We established a minimum of 20 circular variable plots (8 m²) along line-transects at 12 study sites and collected data on butterfly and nectar source abundance.  To test that butterfly and nectar community structure differ among stand types a multi-response permutation procedure was performed. We used nonmetric multidimensional scaling for an ordination of butterfly and nectar sources with stand types, and a joint plot overlay of the environmental variables in order to assess community correlations with environmental variables. We used a Mantel Test to evaluate the congruency between butterfly and nectar distance matrices.  

Results/Conclusions

Both Butterfly and nectar source density and species richness was highest at prairie sites and lowest in the oak woodland sites. Preliminary results show that stand types differ in butterfly community structure but differences were small. The three strongest environmental variables related to butterfly community structure were canopy cover, ground cover, and litter.  Stand types also differ significantly in nectar community structure. The three strongest variables related to nectar community structure were percent ground cover, litter, and canopy cover. Nectar and butterfly species communities were significantly (p=0.02) positively correlated (r=0.49).  These finding may help to guide long-term management efforts for private landowners and federal agencies for the restoration or conservation of remnant prairies, oak savannas, and oak woodlands in the Willamette Valley.