Selection by a predispersal seed predator constrains the evolution of avian seed dispersal in pines

Abstract

SummaryPrevious studies have demonstrated that wind dispersal is an effective mode of seed dispersal for pines (Pinus, Pinaceae) with seeds weighing <90 mg, but not for larger‐seeded (≥90 mg) pines. Consequently, most large‐seeded pines rely on birds in the family Corvidae for seed dispersal, but some do not, and most of their seeds fall near the parent tree. Why seeds of these pines are not dispersed by corvids, and have not evolved traits that facilitate seed dispersal by corvids, is enigmatic.One factor that may constrain the evolution of seed dispersal by corvids in pines, or in other plants that rely on birds for seed dispersal, is predispersal seed predation. The most important predispersal seed predators of pines are often tree squirrels in the generaTamiasciurusandSciurus, which have repeatedly been shown to drive the evolution of seed defences in conifers.We first use published data showing how selection on cone traits of two bird‐dispersed pines by tree squirrels (Tamiasciurus) favouring increased seed defences, conflicts with selection by a corvid (Nucifraga columbianaWilson) for cone traits that facilitate seed dispersal, to make predictions about changes in cone and seed structure of large‐seeded pines that should evolve in response to selection by either tree squirrels or corvids.The cone and seed structures from several other large‐seeded pines in regions with and without pine squirrels were consistent with these predicted changes. Consequently, large‐seeded pines that co‐occur withTamiasciurusor other tree squirrels are well defended against both squirrels and corvids, and instead probably rely on other animals, such as ground‐foraging rodents, that disperse fallen seeds (secondary seed dispersal). Only where tree squirrels are uncommon or absent are conifers likely to evolve traits that enhance seed harvest by corvids in large‐seeded pines.