PS 11-141 - Litter removal increases overall plant community diversity by increasing native species abundance but not diversity

Abstract: In California, where invasion of exotic annual grasses is widespread, multiple studies have found that litter accumulation is the primary mechanism by which exotic grasses reduce the performance of native forb species. This suggests that litter removal could be a powerful restoration tool; however, few studies have examined the effects of litter removal on community-level patterns of biodiversity. Litter removal could shift community structure by either allowing native species once inhibited by litter to establish or by increasing the abundance of native species already co-occurring with exotic annual grasses. Alternatively, litter removal could cause no change in native species abundance but instead could increase the abundance of exotic forbs. In a heavily invaded Southern California coastal sage scrub system, we asked: What is the effect of litter removal on species composition, overall species diversity, and native species diversity?

To address this question, we established 80 1.5 x 3 m plots using a paired design, where one plot receives litter removal via raking, while the other is left as an unmanipulated control (40 pairs total). Raking was conducted in the fall before the onset of the rainy season and all recumbent litter was removed from the plots. Plant species composition data was collected in the plots at peak biomass using point-intercept methods. We analyzed species richness and Shannon diversity of the overall community and native species only using linear models. We also tested for differences in species composition using PERMANOVA.

We found that litter removal increased the abundance of native forbs but did not increase the abundance of exotic forbs. The abundance of native species was low even in treatment plots indicating that exotic grasses remained dominant with litter removal. Litter removal did not increase overall species richness but significantly increased overall Shannon diversity and significantly altered overall community composition. However, litter removal did not significantly alter native species richness, Shannon diversity, or composition. Taken together, our results suggest that in our system, litter removal alters community composition primarily by increasing native species abundance, not native species diversity. Overall, our results add to a number of studies suggesting that plant litter is an important driver of plant community structure in herbaceous systems, and suggest that litter removal can increase overall plant community diversity by increasing the abundance of native species already present in the community.