Plant defense theories predict that allocation to antiherbivore resistance should impose costs on plants, which may manifest themselves as trade-offs between plant defense and growth or reproduction (fitness costs), trade-offs between multiple defenses, and/or trade-offs between different resistance mechanisms (e.g., defense and tolerance). I have examined the above types of trade-offs by conducting meta-analyses of published studies examining associations between different types of resistance traits.

The first meta-analysis examined fitness costs of plant resistance based on 70 studies on 55 species of wild plants published during 1966-1999 and reporting 137 defense-growth or defense-reproduction correlations. The results of the analysis showed that production of antiherbivore defenses imposes a fitness cost on plants, as indicated by significant negative genetic and phenotypic correlations between defense and fitness measures. The greatest phenotypic costs were observed in the presence of herbivores, at high levels of resource availability, and for defenses associated with alkaloids or phenolics. The detected patterns of variation in fitness costs of defense imply that indirect costs which arise through interactions between plants and external factors in their environment (opportunity and ecological costs) may be more important than direct costs due to trade-offs in the allocation of common limited resources between defense, growth and reproduction within an individual plant. I also found that the magnitude of the reported fitness costs decreased over years.

The second meta-analysis examined genetic correlations between different types of defenses based on 31 studies conducted on 22 different plant species and published during 1976-2002. Meta-analysis provided little evidence of trade-offs among antiherbivore defenses. Signficant negative genetic correlations were found only between constitutive and induced defenses whereas chemical defenses belonging to the same group of compounds were positively correlated. It appears that plants may possess several defensive strategies without paying significant trade-offs because these strategies may incur low costs, interact synergistically, and be effective against different herbivores and/or on different spatial and temporal scales.

Third meta-analysis examined genetic correlations between plant defense and tolerance based on 31 studies conducted on 16 different plant species and published during 1980-2003. The analysis revealed no overall trade-off in plants between tolerance and defense to herbivores. The relationship between tolerance and defense was stronger and more positive in crop species as compared to non-crop species. The only significant negative correlation found between tolerance and specific defensive traits was very small. It is thus unlikely that trade-offs between defense and tolerance significantly constrain the evolution of different resistance strategies in plants.

The results of meta-analyses suggest that the magnitude of direct costs of defense and their importance in evolution of plant defenses has probably been overestimated and that more attention in future studies should be paid to indirect (ecological) costs of defense and positive interactions between defensive mechanisms.