Herbivory and plant density affect the consequences of apical damage

Background/Question/Methods

Plant strategies that minimize the consequences of insect herbivory have important implications for reproductive success.  However, the degree to which plants can reduce herbivore effects should depend on environmental context.  We examined to what extent conspecific plant density influenced the average amount of floral herbivore damage experienced by Cirsium canescens, and whether any detected differences influenced realized compensation through apical flower head damage.  Apical flower head damage should increase the number of flowering heads produced, and previous studies have shown plants can compensate, and potentially overcompensate, for apical head loss.  We quantified flower head damage and seed production on plants with damaged versus protected apical flower heads.  We determined the scale at which herbivory affected the likelihood of flower head damage by recording the density of flowering C. canescens individuals at four spatial scales.  We expected a positive relationship between ambient insect herbivory and conspecific plant density, which should result in increased compensation with low plant density. 

Results/Conclusions

We found that the amount of flower head damage decreased with surrounding conspecific density.  However, this effect was only detected at the largest scales.  Rather than increasing seed production through increase in the number of flowering heads, apical damage actually decreased whole plant seed production.  As a result, although insect herbivory decreased with plant density, ambient herbivory was still too high, overall, to realize plant compensation through apical damage.