Upstream colonisation from the Indo-Australian Archipelago
While there is strong evidence that upstream colonisation from islands
to continents does occur, it has until recently been considered an
anomalous trajectory (Bellemain & Ricklefs, 2008). The islands of the
IAA appear to be the source area for at least one global radiation of
birds (Jønsson et al. , 2010). Recent macroevolutionary analyses
of Columbiformes have also emphasised the importance of islands in their
early evolutionary history, but did not focus on spatial or temporal
patterns of dispersal. Conversely, other data indicate that the biota of
continental Asia serves as a strong filter on upstream colonisation by
either blocking successful colonisation completely (Oliver et
al. , 2018b; White et al. , 2021) or by strongly filtering taxa by
ecology (Letsch et al. , 2020).
Limitations of our phylogeny (70% sampling) and poor resolution at
basal nodes) and historical reconstruction methods mean that we are able
to give an accurate summary of trends, but not a detailed
reconstructions of all events. Nonetheless, in support of Lapiedra (et
al. 2021) we found that in the extant radiation of Columbiformes
upstream colonisations from the IAA dominated downstream colonisations
through much of the Oligo-Miocene. The importance of islands in the IAA
is particularly striking given their comparatively small areal extent
when compared to nearby continents (Fig. 2). The regional geography and
ecological context of the earliest inferred upstream shifts into the Old
World are relatively unclear, with the alternative hypotheses being one
or two shifts from islands into the broader “Old World” in a weakly
supported clade spanning Treron -Turtur . The Oligo-Miocene
timing and direction of these colonisations is potentially comparable
with upstream colonisation of the Oscine passerines (Jønsson et
al. , 2010; Moyle et al. , 2016), although the outcomes in terms
of diversification differ starkly (less than 40 species versus over
5000). A more strongly resolved phylogenetic tree, and ideally fossils,
would be needed to shed more light on the trajectory of potential early
upstream shifts in these lineages.
The geographical, temporal and ecological context of more recent
inferred upstream colonisations in Columbiformes is clearer. The
specialist fruit-eating arboreal pigeons (fruit doves) of the
Ptilinopini have colonised nearby continents on at least ten separate
occasions (and probably more if missing taxa such as Ptilinopus
alligator and intraspecific populations are added to the phylogeny).
Ecological niche shifts to arboreality in the ancestor of the
Ptilinopini underpinned extensive radiation in the IAA and Pacific
(~100 species), and potentially also predisposed them to
repeatedly colonise nearby continents (Lapiedra et al. , 2021). In
contrast, speciation within continental regions appears to have been
very limited. Indeed, around half the inferred upstream shifts involve
species level taxa that occur across islands and continents. Furthermore
most Ptilinopini in continental areas (and especially Asia) remain
closely associated with coastal habitats and islands (e.g. Ducula
aenea , Ptilinopus jambu, P. melanospilus ), or other habitats
with often less biodiverse communities such as montane areas (e.g.Ptilinopus porphyreus , Ducula badia ) (Baptista et
al. , 1997). This conforms with a hypothesis that upstream colonisation
is challenging (White et al. , 2021), and even when frequent,
ecological filtering may limit new arrivals to comparatively species
poor environments on ‘mainlands’ (Mayr & Diamond, 2001).
Upstream colonisation by terrestrial-feeding Raphinini and Phabines has
been even more limited, again suggesting ecological filtering at
continental margins. Raphinini are entirely restricted to islands,
despite their evident history of dispersing across vast distances into
the Indian and Pacific oceans. The one striking outlier is provided by
the Phabines, which are inferred to have colonised Australia from
islands in the early Miocene and subsequently radiated across the
continent. The relative success of this upstream colonisation could have
been mediated by major climatic changes (aridification) and the
concomitant opening up of niches and expansion of key food sources for
terrestrial taxa, especially grasses (Toon et al. , 2015). Like
the Raphinini, the Phabines have been largely unable to colonise
continental Asia. The only exception is in the genus Geopelia ,
which originated in Australia, but has likely colonised the expanding
savannahs of the Sundaland only recently during the
Plio-Pleistocene. Thus, our data support the hypothesis that
islands have been a net source area for continents (Lapiedra et
al. , 2021), while simultaneously indicating that continents (and
especially the lowland rainforests of Asia) present a formidable
ecological filter to upstream colonisation (White et al. , 2021).