PARENT SESSION
Oral Session #28: Invasions - Resistance and facilitation.
Presiding: D. Holway
Tuesday, August 6. 8:00 AM to 11:30 AM. Gila Meeting Room, TCC.

Species interactions in altered landscapes: herbivory and facilitation mediate plant invasion.

Cushman, J. Hall^*,1, Lortie, Christopher¹, Christian, Caroline², ¹ Sonoma State University, Rohnert Park, CA, (cushman@sonoma.edu)² University of California - Davis

ABSTRACT- Non-native species are increasingly recognized as prominent features of most, if not all, ecological landscapes throughout the world. These invaders must interact with an array of native and non-native species in their new environments, and such interactions may play critical roles in either promoting or hindering the invasion process. Here, we summarize results from comparative and experimental studies that explore the importance of two factors - herbivory and facilitation - in mediating the growth and fecundity of an increasingly invasive African grass, Ehrharta calycina (veld grass), in coastal dune systems of northern California. Using exclosure experiments, we show that a native herbivore - black-tailed hares (Lepus californicus) - dramatically reduces the growth and fecundity of this exotic grass. However, comparative data and neighbor-removal experiments reveal that these adverse effects can be greatly reduced when Ehrharta associates with neighboring plant taxa, especially the fellow exotic Ammophila arenaria (European beach grass). Further, Ehrharta is found with beneficial neighbors more frequently than expected due to chance and less frequently found in open, more exposed dune habitat. We hypothesize that Ehrharta gains protection from hare herbivory by associating with its neighbors either because they are less apparent to herbivores or less accessible. In summary, our results highlight the roles that species interactions can play in mediating the success of invasive plants.

KEY WORDS: invasive plants, herbivory, facilitation, sand dunes