Impacts of Lepidium latifolium on food web structure of Suisun song sparrows in a brackish marsh

Background/Question/Methods

Over the last two decades, human-induced habitat changes, such as urbanization and introduced species, have degraded 90% of California’s coastal wetlands. Lepidium latifolium (perennial pepperweed) is a pervasive invader of California wetlands, potentially altering ecological and community dynamics. This project assesses L. latifolium’s impact on food web structure for Suisun song sparrows (Melospiza melodia maxillaries) in the brackish marsh of Rush Ranch Open Space Preserve, a component of the San Francisco Bay-Delta Estuary. Food web structure was assessed through analysis of invertebrate communities (soil infauna and canopy insects) and stable isotope analysis of bird blood and feathers. Invertebrate samples were taken in all four seasons within a randomly selected location in the marsh. Suisun song sparrow feather samples were taken in the winter and summer of 2011. Prior to L. latifolium bloom and during peak bloom, sparrow blood was sampled to examine trophic shifts associated with L. latifolium emergence. ANOVAs were used to examine shifts in invertebrate abundance and diversity based on L. latifolium phenology. PRIMER statistical software was utilized to examine changes in invertebrate community similarity, as well as similarity in isotopic signatures for bird blood and feathers. Mixing models will be utilized to examine food web structure.  

Results/Conclusions

The infauna community at our site does not vary with presence of L. latifolium, but is structured by season (species richness; ANOVA, F=6.28, p=0.001) with habitat zone significantly impacting abundance in the winter (ANOVA, F=5.38, p=0.017). The mobile canopy insect community follows a similar pattern of change across season, but preliminary results suggest this community is primarily structured by the emergence of L. latifolium in the summer. Summer samples taken within L. latifolium are much more similar to one another than those taken in natural marsh vegetation (ANOSIM, Global R=0.276, p=0.032). L. latifolium’s unique isotopic signature is detectable in some invertebrates (ex. amphipods, ANOSIM, Global R=0.117, p=0.026). There is an isotopic shift in bird blood between winter (pre-L. latifolium) and summer (L. latifolium bloom) (ANOSIM, Global R=0.186, p=0.014), but it is currently unclear if this shift is due to seasonal changes in diet or shifts in vegetative and invertebrate communities as influenced by the presence of L. latifolium. We are currently employing mixing models to examine this question.  We predict integration of L. latifolium into the food web of Suisun song sparrows, implying L. latifolium eradication should be paired with native plant restoration to avoid impacts on sensitive marsh vertebrates.