Abstract
The island monarch (Monarcha cinerascens) was an original example of the “supertramp strategy”. This involves well-developed dispersal specialisation, enabling a species to colonise remote islands but leaving it competitively inferior. Supertramps are hypothesised to be excluded from larger islands by superior competitors. It is the only original Melanesian supertramp to occur in Wallacea, home also to the sedentary pale-blue monarch (Hypothymis puella). We interrogate the supertramp strategy and its biogeographical underpinnings by assessing the population structure of these two monarchs. We sampled island and pale-blue monarchs in Wallacea, collecting DNA and morphological data. We investigated monarch population structure by applying ABGD and Bayesian and Maximum Likelihood methods to their ND2 and ND3 genes. We constructed linear models to investigate the relationships between genetic divergence, dispersal ability, and island area, elevation, and isolation. Wallacea’s deep waters restrict gene flow even in a supertramp, as the Wallacean and Melanesian island monarchs are likely separate species (mean genetic distance: 2.7%). This mirrors the split of the pale-blue monarch from Asia’s black-naped monarch (Hypothymis azurea). We found further population structure within Wallacean and Melanesian island monarch populations. Their genetic divergence was related to elevation, area, and isolation of islands, as well as dispersal ability of birds. However, dispersal ability was independent of island elevation and area. Rather than being r-selected on small, disturbance-prone islands, our results support the view that the island monarch’s supertramp lifestyle is a temporary stage of the taxon cycle, i.e. supertramps may transition into resident species after colonisation. Our models suggest that more dispersive monarchs reach more distant islands, and divergence is promoted on islands that are more distant or larger or more permanent, without selection against dispersal ability per se. We suggest that supertramp lifestyle helps determine the distribution of species across islands, not necessarily the divergence occurring thereafter.
Highlights
Animals vary in their ability to move or disperse through the environment, and these differences affect the range size of individual species (Sheard et al 2020), but broad patterns of speciation (Smith et al 2014, Manthey et al 2020) and even the biogeographic division of life between regions of the Earth (White et al 2021)
Classic supertramp theory describes certain species colonising distant islands due to superior dispersal abilities, but the “taxon cycle” hypothesis predicts they will lose these abilities and become differentiated (Le Pepke et al 2019). We tested this using morphological analyses on adult island monarchs from Wallacea, carried out in R version 4.0.2 (R Core Team, 2020). These analyses investigated whether genetic divergence of populations was accompanied by a reduction in dispersal ability
The genetic distance we found between island monarchs of Wallacea and Melanesia (2.7%) is closely equivalent to that between well-differentiated monarch species, as Monarcha castaneiventris (MOCA_H01) had a mean distance of 2.6% from our Wallacean samples and 2.8% from the Melanesian birds
Summary
Animals vary in their ability to move or disperse through the environment, and these differences affect the range size of individual species (Sheard et al 2020), but broad patterns of speciation (Smith et al 2014, Manthey et al 2020) and even the biogeographic division of life between regions of the Earth (White et al 2021). Diamond’s nine supertramps comprised a varied assortment of species: in updated taxonomy (this paper follows Gill et al 2021) they include three doves (Ptilinopus solomonensis, Ducula pistrinaria, and Macropygia mackinlayi), a kingfisher (Todiramphus tristrami stresemanni), and five passerines: one monarch flycatcher (Monarcha cinerascens), one white-eye (Zosterops griseotinctus), one whistler (Pachycephala melanura dahli), and two honeyeaters (Myzomela sclateri and Myzomela pammelaena) This disparate grouping allows us to compare supertramps to relatives with contrasting life histories. Linck et al (2016) found population structure in the Louisiade white-eye, one of Diamond’s original supertramps, with populations on larger and higher islands the most genetically distinct In their studies on cuckoo-shrikes, Pedersen et al (2018) and Le Pepke et al (2019) found some young taxa to be widespread and undifferentiated. We sought to build on this view by incorporating data on dispersal ability and Supertramp and sedentary monarchs extending it to another of the original supertramps, the island monarch (Monarcha cinerascens) (Temminck 1827), using data from Wallacea as well as some of the Melanesian islands where the theory originated
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