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Seed collection by mutualistic ants and predaceous mice

A Bloodroot fruit pod, containing approximately 25 dark seeds, each with a white elaiosome attached.
On Oct 27, 2005, we tested whether the seeds of Sanguinaria canadensis would be collected by the animals in the Northwoods. These seeds have an attached elaiosome that is attractive to ants that can disperse the seeds (a dispersal strategy called myrmecochory). These seeds and [[elaiosomes may also be attractive to mice that eat and kill the seeds. We removed the elaisomes from half of our seeds, and placed petri dishes including approximately five elaiosome-bearing and five elaiosome-less seeds in each dish (avg = 4.6, n=20). We placed cages over half of these dishes, to selectively excluded rodents (mice) and birds, which we judged likely to act as S. canadensis seed predators. Ants could easily crawl through the cage mesh, resulting in two treatments: 1) seeds accessible only to ants and 2) seeds accessible to vertebrates and ants. We predicted that 1) ants would not remove seeds lacking elaiosomes, 2) that rodents would remove seeds regardless of whether they had an elaiosome attached, and 3) that rodents would collect seeds (i.e., more seeds would be removed when rodents had access). Seeds were placed in the dishes at 3pm, and inspected several times over a 30 hr interval. We used a paired t test to compare removal from dishes (paired by student researcher, each of whom set up a replicate of each treatment). Key findings:
  • More seeds were removed from dishes accessible to rodents and birds than in dishes accessible to only ants, supporting our hypothesis (marginally significant difference, one-sided paired t test, t = 1.39, p = .09). In the absence of collection by ants, seed predators remove seeds.
    ScribnerBloodroot.jpg
  • Elaisomes rotted over a matter of hours, making it impossible to distinguish seed treatments. Future studies should be performed on shorter time scales (hours, not days).
  • The majority of seed removal occurred at night in the 'all-accessible' treatment, suggesting that nocturnal rodents were the most common seed removers (rather than birds, which are predominantly active during the day).
  • Little ant activity in October, although it seems consistent over day and night. Perhaps this is the reason ant-dispersed seeds offer seeds in spring rather than fall!
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