4.2 | Implication by the biased discard of dummy seeds
Although we could not track where the genuine seeds discarded because of
their small-size, the author M. Y. observed a part of the genuine seeds
carried into the nests are left in a corner of the nest (data not
shown). Thus, the ant nest could be one of the seed directions. The
dummy seeds coated with mixed odors were carried into the nest and
subsequently discarded out of the nest by the ants. Although 13 dummy
seeds were left inside the nest (17 were discarded outside), it appears
that seeds staying in the nest can also be considered as dispersed sinceN. flavipes nests are located under litter or on rotting trees
(Kallal & LaPolla, 2012). The direction of discards was biased, and the
dummy seeds were discarded mostly onto the water-moistened glass beads.
In the ant species Myrmica rubra , it is known that workers
discard inert items and corpses in different destinations (Diez et al.,
2012). In our study, it was also observed that ant corpses were dumped
into cotton wool compartments, where the dummy seeds were not dumped.
These results suggests that the ants differently use the discard
destinations depending on what they are discarding, and the seeds could
be discarded specific locations by the ants.
Mycorrhizal fungi of Russula species as the hosts of M.
humile inhabit the top shallow layer of the forest floor (Rachel, 2004;
Courty et al., 2008), and Imamura and Kurogi (2003) reported thatM. humile roots inhabit 5-10-cm depth as parasites on
Russulaceae fungi. Thus, where the
ant nests are located and where the seeds are discarded (moist soil near
the surface) is considered to be close to their hosts and to be suitable
for germination, growth and survival of the M. humile seeds
although the mycorrhizal roots have not been quantified before.
The dummy seeds were left inside the nest or most frequently discarded
onto the water-moistened glass beads section. This observation can
support two seed-dispersal hypotheses: 1) seeds relocated to the moist
environments including ant nests can escape desiccation and have
enhanced germination and seedling survival; 2) seeds relocated to ant
nests can also escape predators (modified from Hanzawa et al 1988).
Our results suggest that seed dispersal of M. humile by ants is
directional. Directed seed dispersal and its ecological significance
have rarely been verified in achlorophyllous herbaceous species
(reviewed by Giladi, 2006). With regards to myco-heterotrophic plants,
large parts of them produce dust-like and
anemochorous seeds, which require
host fungus in their germination, and which could disperse over long
distances (Merckx, 2013). Thus, there is a trade-off of seed dispersal
between distance and direction. Since the genus Monotropastrum ,
producing fleshy fruit, is thought to evolve from the relative and
anemochorous genus Monotropa , producing capsule fruit (Bidartondo
& Bruns, 2001), this speciation is considered as from anemochory
(distance dispersal) to zoochory including myrmecochory. Therefore, the
verification of the role of myco-heterotrophic herbaceous species in the
temperate forests of Asia, and of the role of seeds without specialized
rewards for ants, presents a new aspect of directed seed dispersal of
achlorophyllous and myco-heterotrophic plants involving ants.