Can Predation Pressure Help Explain the Curious Evolution of Ballistic Seed Dispersal?

Ballistic seed dispersal (ballochory) involves the autonomous explosive release of seeds from adult plants. The unconventional mechanics of this strategy have understandably drawn considerable scientific attention. The explosive release of seeds is achieved by a variety of physical mechanisms but broadly involves the rapid coiling or shattering of seed pods to transfer kinetic energy to seeds, facilitated largely by either the evaporation or absorption of water in seed pod tissues. There has been a bias toward researching physiological and physical aspects of ballistic plants, with the evolutionary ecology being comparatively neglected. Although ballochory is represented in 23 plant families, it has never become common. This fact should invite curiosity regarding the selective pressures that encourage its evolution. Previous research has been unable to correlate ballochory with plant traits such as morphology, generation time or habitat preferences, and so we take an alternative approach in considering the evolutionary advantages that can provide insight on the shared set of circumstances that favour the evolution of this strategy. We review the known selective advantages that ballistic dispersal can confer to plants and promote a hypothesis that ballochory may be particularly selected for in instances of concentrated predation pressure on parental canopies. For plants in static and patchy landscapes, such a strategy could balance a trade-off between escaping concentrated natural enemies while maximising the probability of transport to suitable habitat. We account for its rarity by considering the major opportunity cost that may only be justified when other seed dispersal mechanisms are limited. Moving forward, we suggest experimental manipulations to test this hypothesis and promote a research agenda in the field of ballistic seed dispersal that illuminates its intriguing evolution.