Does gut passage affect seed predation in wild chilies (Capsicum annuum)?

Background/Question/Methods

Granivores have significant impacts on seedling recruitment in many species. Capsaicin in wild chilies (Capsicum annuum) deters mammalian granivores but does not deter consumption by birds or insects. This study examined whether gut passage by frugivorous birds influences post-dispersal seed predation. C. annuum seeds were collected from defecations of common fruit eating birds after the birds had been fed C. annuum fruits 24 hours earlier. The Capsaicin-stripping Hypothesis posits that seeds passed through birds (“passed seeds”) will be removed by granivores at higher rates than those removed directly from fruits (“non-passed seeds”) because gut passage will reduce the amount of capsaicin on the seeds.  To test this hypothesis 122 passed seeds and 122 non-passed seeds were placed side-by-side in 122 separate trials in C. annuum habitat on Seahorse Key, Florida.  Seeds were monitored for removal for 16 days. The Insect-granivore Hypothesis posits that insects are the major post-dispersal predator of chili seeds. To test this hypothesis, 40 passed seeds were placed on the ground under a cage (“caged seed”) near 40 passed seeds that were left exposed (“exposed seed”) and seed removal monitored for 15 days. The cages were accessible to insect granivores but not larger granivores.  One year later, both hypotheses were retested using a 2x2 factorial design (passed/unpassed x caged/exposed; n = 64).

Results/Conclusions

The Capsaicin-stripping Hypothesis was not supported.  In the first experiment, 25% more passed seeds remained than non-passed seeds after 1 day -- opposite the expected pattern.  This difference decreased to 18% after sixteen days (X²=6.54, d.f.=1, p=0.011).  However, no significant difference in seed removal of passed versus non-passed seeds was detected in the second year (X²=1.46, d.f.=1, p=0.23; 14 days).  The Insect-granivore Hypothesis was supported: after two weeks removal of caged and exposed seeds did not differ in the first year (X²=0.46, d.f.=1, p=0.50) and in the second year (X²=1.46, d.f.=1, p=0.227).   The results indicate that gut passage indeed affects post-dispersal seed predation, and multiple field seasons are needed to accurately gauge the nature of seed predation.