Variation in herbivore impacts on tall thistle reproduction: Roles of soil resources and herbivore diversity

Background/Question/Methods

Large spatial and temporal variation in herbivore impacts on host plant fitness are common. Key conceptual models hypothesize that plant resources may explain this variation in herbivore-host plant interactions.  Further, there is increasing evidence that impacts of individual herbivore species on plant fitness depend on the diversity of the herbivore community in which the focal herbivore is embedded. To address such interactive effects, we established a nitrogen addition X herbivore exclusion experiment that manipulates access by insects, aboveground mammals and belowground mammals in restored prairie at Wichita State’s Ninnescah Reserve near Viola KS.  In the context of this larger experiment, here we ask 1) does nitrogen availability limit seed production of adult tall thistles (Cirsium altissimum)?, 2) does herbivory by insects and aboveground mammals on adult tall thistles limit seed production? and 3) are there interaction effects between herbivore taxonomic groups or between herbivory and nitrogen availability on tall thistle fecundity?  In 2010, we censused and collected flower heads from naturally-occurring adult tall thistles across all combinations of nitrogen addition, insect exclusion, aboveground mammal exclusion and belowground mammal exclusion in our experiment. 

Results/Conclusions

Nitrogen addition increased the proportion of flower heads per plant damaged by herbivores (F_1,47 = 8.11, p = 0.0065), but did not impact the proportion of lateral branch meristems per plant damaged.  Seed production per plant also increased with nitrogen addition (F_1,61 = 17.82, p < 0.001).  Reduced insect herbivory increased seed production (F_1,61 = 7.39, p = 0.0085), but the effect was greater if aboveground mammals were also excluded (F_1,61 = 5.24, p = 0.0256).  We found a significant interaction effect of insect herbivore reduction and nitrogen addition on mean weight of seeds that were determined to be viable by visual inspection (F_1,60 = 5.63, p = 0.021).  Seed weight was greater where insects were excluded, but only if nitrogen was added.  These results from the first year of our experiment suggest that effects of individual herbivore groups on plant lifetime seed production can depend upon the taxonomic diversity of the herbivore community as well as resource availability.