Constraints on plant signals and rewards to multiple mutualists?

Many plants invest substantial resources in signaling to and rewarding two kinds of 'interguild' mutualists, pollinators and seed dispersers. The signals and rewards are expressed via traits of flowers and fruits. Pollinators and seed dispersers could act in synergistic or antagonistic ways to influence selection on these traits. Here, we address the issue of whether plant species might be constrained in signaling to and rewarding multiple mutualists that provide different types of benefits to plants. Specifically, does investment in one type of mutualist limit investment in another? We examined the correlation between flower size and fruit size for 472 plant species spanning three regional floras. Our analyses made the assumption that structure size is related to plant investment in signals and/or rewards. We expect that a constraint due to interguild mutualisms would be evidenced by a negative correlation between flower and fruit size. Instead, we found significantly positive relationships between flower size and fruit size in all three regional floras. These relationships remained robust after correcting for plant evolutionary history using phylogenetically independent contrasts. These patterns may reflect synergies in selection by pollinators and seed dispersers, genetically-based or resource-based constraints on investment in reproductive tissues, and/or an underlying trade-off in structure size versus number.