Ecological costs of plant resistance to herbivores in the currency of pollination

In this paper, we examine how ecological costs of resistance might be manif ested through plant relationships with pollinators. If defensive compounds are incorporated into floral structures or if they are sufficiently costly that fewer rewards are offered to pollinators, pollinators may discriminate against more defended plants. Here we consider whether directional selecti on for increased resistance to herbivores could be constrained by opposing selection through pollinator discrimination against more defended plants. W e used artificial selection to create two populations of Brassica rapa plan ts that had high and low myrosinase concentrations and, consequently, high and low resistance to flea beetle herbivores. We measured changes in floral characters of plants in both damaged and undamaged states from these popul ations with different resistances to flea beetle attack. We also measured p ollinator visitation to plants, including numbers of pollinators and measur es of visit quality (numbers of flowers visited and time spent per flower). Damage from herbivores resulted in reduced petal size, as did selection fo r high resistance to herbivores later in the plant lifetime. In addition, f loral display (number of open flowers) was also altered by an interaction b etween these two effects. Changes in floral traits translated into overall greater use of low-resistance, undamaged plants based on total amount of ti me pollinators spent foraging on plants. Total numbers of pollinators attra cted to plants did not differ among treatments; however, pollinators spent significantly more time per flower on plants from the low-resistance popula tion and tended to visit more flowers on these plants as well. Previous wor k by other investigators on the same pollinator taxa has shown that longer visit times are associated with greater male and female plant fitness. Beca use initial numbers of pollinators did not differ between selection regimes , palatability and/or amount of rewards offered by high- and low-resistance populations are likely to be responsible for these patterns. During period s of pollinator limitation, less defended plants may have a selective advan tage and pollinator preferences may mediate directional selection imposed b y herbivores. In addition, if pollinator preferences limit seed set in high ly defended plants, then lower seed set previously attributed to allocation costs of defense may also reflect greater pollinator limitation in these p lants relative to less defended plants.