Seeds are born into a hostile and unfriendly world. Animals are eager to devour them while pathogens are keen to infect them. Adding insult to injury, some birds swallow seeds whole and then poop them out far from home. But for some plants, this is exactly the point. In an elegant new study published in Ecology Letters by Evan Fricke and an international team of colleagues, the authors show how getting eaten can be a seed's best route to salvation.
Attracted to the fleshy fruits that encase seeds, birds consume fruits and then disperse the gut-processed seeds to new environments. In the early 1970s, ecologists Daniel Janzen and Joseph Connell independently proposed that this benefited seeds, not only because it helped them colonize new habitats but also because it allowed them to escape the species-specific pathogens and predators that were found near their parents. Birds, according to this idea, transport seeds to locations that are free from harm. However, in consuming seeds, birds do far more than disperse them. Seeds are modified during gut passage and it is perhaps these chemical and physical changes that increase seed survival, and not the simple fact that birds deposit the seeds away from the dangers lurking near their parents.
To distinguish these possible benefits, Fricke and his collaborators fed chilli peppers to captive birds and then compared seed survival with that of unpassaged seeds. They found that passage through a bird's digestive system increased seed survival by an astonishing 370%! More surprisingly, this benefit had nothing to do with distance from the parental tree. So what is it about the bird gut that increased seed survival? It turns out that escape from predators and pathogens is crucial after all. Just not in quite the way Janzen or Connell envisaged.
Chilli seeds emit a unique cocktail of volatile odors that are particularly attractive to seed-eating ants. However, after traversing the alimentary canal of birds, these volatiles were reduced ~100-fold, rendering the seeds less detectable by ants. In addition, gut-processed seeds were significantly less infected with fungal pathogens. This roughly doubled seed survival in nature. Granivorous ants are a major seed predator of chilli seeds and Fusarium fungal infections are a major cause of seed mortality. Accordingly, the changes to seeds resulting from gut passage have a stunning overall effect on seed survival and plant fitness.
Bird-consumed seeds travel two important paths. Their first short trip through the bird's gut may be inhospitable but offers some predictability. By contrast, their second, longer trip from parental tree to the site of defecation or regurgitation is filled with biotic unknowns. The importance of this study is that it provides a neat link between these two seed voyages: the changes wrought during their first voyage improve survival at their final port of call. It also highlights the novel ways in which birds can mediate the interactions between pathogens, predators and fruiting plants.
