Exotic annual species have invaded many Mediterranean climate areas around the globe, and have been observed to germinate earlier than native species in response to seasonal rains. Early-season growth can enable exotic species to pre-empt space and resources, competitively suppressing later-germinating native species (a seasonal priority effect). This raises an important question: why do native species delay germination despite the competitive advantage associated with earlier activity? One potential explanation is that individuals germinating earlier in the growing season are more apparent to generalist herbivores, as there is often little herbaceous vegetation suitable for consumption prior to the arrival of seasonal rains. Furthermore, the cost of such herbivory may be greatest early in the growing season, due to the lower capacity of seedlings to defend against and tolerate attack. This suggests a potential trade-off between seasonal phenology and susceptibility to herbivory. To evaluate this hypothesis, we conducted a field experiment at the University of California, San Diego in which we planted 14 focal species (seven each native and exotic) in monoculture and polyculture, monitored germination and growth, and experimentally excluded generalist herbivores both early and later in the growing season.
Results/Conclusions
Consistent with past studies, exotic species germinated earlier than native species. We found that herbivory had the greatest negative impact on biomass early in the growing season. Regardless of species origin, earlier germinating species achieved greater biomass by the end of the experiment, but were more negatively impacted by herbivory, particularly in the early part of the growing season. In polyculture, this greater impact of early-season herbivory on early-active species led to a reduction in the competitive advantage of exotic species grown in polyculture, and improved the performance of later-germinating natives. These results suggest that a trade-off between early growth and susceptibility to herbivores could be an important force in community assembly in herbaceous-dominated ecosystems. However, we are unable to conclude that these patterns were observed solely due to differences in apparency, and no other mechanism (such as a trade-off between growth and defense). These results also show how herbivore exclusion favors early-active exotic species in this system, with important implications for management in many areas invaded by exotic annual species.