Although herbivory often has negative effects on plants, these effects may be mitigated by tolerance, or the degree to which a plant maintains fitness despite damage. A plant’s ability to tolerate damage may vary with resource availability. In addition, the costs of tolerance may change in resource limited environments. There are relatively few tests of changes in costs of tolerance among environments, but more often costs are higher in more stressful environments (e.g., nutrient limitation, water stress, etc.). In this study, we examined the effects of intraspecific competition on both tolerance to herbivory and costs of such tolerance. We used the perennial herb Solanum carolinense, which interacts with multiple herbivores and has shown both resistance to herbivory and tolerance of damage. We selected genotypes with high and low tolerance to damage in the field, and grew them in the greenhouse at either high (4 neighbors/pot) or low (0 neighbors/pot) density. After five weeks of growth, we bagged Leptinotarsa juncta larvae on half of the plants from each density-genotype combination and allowed them to feed for up to five days, or until 50% of leaf area was removed. At the end of the experiment, we estimated fitness through both sexual reproduction (number of flowers produced) and asexual reproduction (total plant size, measured at total leaf length and correlated with production of new stems via rhizomes).  

Results/Conclusions

We found that density and damage both reduced overall plant fitness, whether measured through sexual or asexual reproduction. Genotypes varied in how their fitness was affected by density, although these effects were not statistically significant. The effects of density on plant tolerance were more complex. Although tolerance did not differ among density treatments for both asexual and sexual measures of fitness, there was a trend towards higher tolerance in the high-density treatment. Because S. carolinense is a long-lived perennial, these effects could become more pronounced if fitness were measured over multiple years. In addition, costs of tolerance were detected only for plants growing at high density, and only for sexual reproduction. Thus, costs of tolerance appear to be greater for plants growing under more stressful conditions.