Long seed dispersal between the island groups
In phylogenetic tree analysis, ecotypes S in the Chichijima Islands, SG
in the Hahajima Islands, and Sm in the Mukojima Islands together with
subclade 1–2, whereas ecotypes ST in the Hahajima Islands and STm in
the Mukojima Islands formed another subclade (Fig. 3b), indicating
potential interisland group migration. The primary seed disperser ofCallicarpa is the brown-eared bulbul (Hypsipetes
amauroti s). Furthermore, metabarcoding from the feces of the Japanese
wood pigeon (Columba janthina nitens ), endemic to the Bonin
Islands, occasionally showed the presence of Callicarpa seeds
(Ando et al., 2016). Although pigeons are not considered efficient seed
dispersers due to seed crushing in their gizzards, seeds up to 3 mm in
size remain intact in their feces (Shibazaki & Hoshi, 2006).
Additionally, until 1920, the jungle crow (Corvus macrorhynchos )
was present in the Bonin Islands (Higuchi, 1984). Estimated seed
dispersal distances for these bird species, based on body size (Dunning
Jr, 2007), and calculated using a phylogenetic generalized least squares
model for seed retention time (Yoshikawa et al., 2019) and flight speed
(Tennekes, 2009), are as follows: brown-eared bulbul, 39.7 km; Japanese
wood pigeon, 88.8 km; and jungle crow, 110 km. The current distances
between the Mukojima and Chichijima, Chichijima and Hahajima, and
Mukojima and Hahajima Islands are 32, 35, and 110 km, respectively. The
presence of the same ecotypes in different island groups is likely a
result of long-distance seed dispersal by these birds. Particularly,
ecotype ST, inhabiting the mesic forests of the Hahajima Islands, occurs
in the Mukojima Islands but not in the Chichijima Islands, despite
similar mesic forests in the latter. It is speculated that ecotype ST
migrated between the Hahajima and Mukojima Islands through birds with
high flight ability, bypassing the Chichijima Islands. However, due to
genetic differentiation even among the same ecotypes in different island
groups, the occurrence of long-distance seed dispersal between islands
is expected to be extremely rare.