A phylogenetic study to assess the link between biome specialisation and diversification in swallowtail butterflies

The resource-use hypothesis, proposed by E.S. Vrba, states that habitat fragmentation caused by climatic oscillations would affect particularly biome specialists (species inhabiting only one biome), which might show higher speciation and extinction rates than biome generalists. If true, lineages would accumulate biome-specialist species. This effect would be particularly exacerbated for biomes located at the periphery of the global climatic conditions, namely, biomes that have high/low precipitation and high/low temperature such as rainforest (warm-humid), desert (warm-dry), steppe (cold-dry), and tundra (cold-humid). Here, we test these hypotheses in swallowtail butterflies, a clade with more than 570 species, covering all the continents but Antarctica, and all climatic conditions. Swallowtail butterflies are among the most studied insects, and they are a model group for evolutionary biology and ecology studies. Continental macroecological rules are normally tested using vertebrates, this means that there are fewer examples exploring terrestrial invertebrate patterns at global scale. Here, we compiled a large GIS database on swallowtail butterflies’ distribution maps and used the most complete time-calibrated phylogeny to quantify diversification rates. In this paper we aim to answer the following questions: 1) Are there more biome-specialists swallowtail butterflies than biome-generalists? 2) Is diversification rate related to biome specialisation? 3) If so, do swallowtail butterflies inhabiting extreme biomes show higher diversification rates? 4) What is the effect of species distribution area? Our results showed that swallowtail family presents a great number of biome specialists which showed substantially higher diversification rates compared to generalists. We also found that biome-specialists are unevenly distributed across biomes. Overall, our results are consistent with the resource-use hypothesis., species climatic niche and biome fragmentation as key factors promoting isolation.

Methods
The biomes inhabited by a species were determined by the overlap between the reported geographical distribution of each species and the biome map (Hernández Fernández, 2001). Here, we consider a biome as inhabited by a species if it constitutes 15% or more of its geographical range. For the cases where the species overlapped isolated, small and distinct biome patches, we also recorded the presence of a species in a biome if the species is present in 50% or more of that biome patch (Hernández Fernández, 2001). Further, for those species with presence in mountain environments, following Moreno-Bofarull et al. (2008) and Cantalapiedra et al. (2011), we considered the altitudinal vegetation belts (ETOPO2v2, NOAA National Geophysical Data Center, 2006), which were not included in Walter’s map, The overlap between species distribution ranges and biomes was calculated using ArcGIS software. These criteria allow to represent the adaptation capacity of species while maintaining their climatic specificity and, at the same time, allow us to compare our results with previous works using the same methodology (Hernández Fernández & Vrba, 2005; Moreno Bofarull et al., 2008; Cantalapiedra et al., 2011; Gómez Cano et al., 2013; Menéndez et al., 2021; Hernández Fernández et al., 2022). Then, we computed the Biomic Specialisation Index (BSI) defined by Hernández Fernández & Vrba (2005) as the number of inhabited biomes by a species. Biome-specialist species were defined as those occupying only one biome, with a BSI = 1 (Hernández Fernández & Vrba, 2005). In turn, species with BSI > 1 were considered as biome-generalists, differentiating between “semi-generalists” (1 < BSI < 5) and “extreme generalists” (BSI ≥ 5) (Hernández Fernández & Vrba, 2005)