Seedlings often suffer considerably higher mortality than any other part of the plant life cycle. Although many agents are responsible, herbivore attack is usually the most important cause of seedling death. However, inherent variation in seedling defense can dictate patterns of herbivore selection. Similarly there is significant species-specific variation in how seedlings tolerate tissue damage. When coupled with competitive interactions between regenerating seedlings, species-specific variation in seedling resistance and tolerance exerts major effects on plant species coexistence. However herbivore pressure is not constant. Populations of invertebrate and vertebrate herbivores exhibit significant seasonal and year-to-year variation, and with this variation, the potential for major temporal shifts in herbivore pressure on newly emerging seedlings.
Results/Conclusions In addition to developing a conceptual framework showing how temporal variation in seedling herbivory can be used to explain plant species coexistence, we present data collected in the chalk grasslands of southern England, that examined variation in species-specific seedling response to herbivory in congeneric species pairs. Our results show how the congener most susceptible to herbivore attack at the seedling stage is also the one that exhibits generally higher seedling growth rates. In a competition experiment between two sympatric Trifolium species, we also demonstrate how the balance of competition can be strongly influenced by the intensity of herbivory experienced during the regeneration phase. Thus we provide a mechanistic basis for a relationship between temporal variation in seedling herbivore populations, and differential sensitivity to inter-specific seedling competition and susceptibility to herbivore attack and thus a temporal model for plant species coexistence.
