Introduction
Analysing biodiversity patterns is fundamental to understanding the underlying processes and causes of diversification (Holt et al. 2013). In comparison to plants and vertebrates, arthropods are fragmentarily known and lack comprehensive comparative data (Decaëns 2010; Beck and Kitching 2007; Stork et al. 2015; Nielsen 2019). This is particularly true for biodiversity hotspots (Myers et al. 2000). Restricted dispersal capacities of arthropods (Gálvez-Reyes et al. 2020) and their occurrence in micro-niches result in fine scale high endemism and still unknown patterns (Buckley and Jets 2007; Daru et al. 2020; Baselga et al. 2022). However, data on arthropod biodiversity rely on museum collections and suffer largely from sampling bias (Santos and Quicke 2011; Echevarría Ramos and Hulshof 2019). This is true for even relatively large-bodied taxa such as phytophagous scarab beetles (Coleoptera: Scarabaeidae).
Diversity patterns of phytophagous beetles are known to be generally linked to species turnover of their host plants (Ødegaard 2006; Kemp et al. 2017; Luo et al. 2021) and their distribution is correlated with the region and forest type (Yotkham et al. 2021). Other guilds, such as dung feeding beetles, respond to shade cover rather than plant species composition. Further, their occurrence and relative abundance vary according as responses to microclimate (light intensity, temperature, humidity) (Davis et al. 2013) or other factors (rainfall, temperature, and host density/diversity) varying from regional to local scale in relation to actual local functional ecological conditions (Tshikae et al. 2013). Such correlation of occurrence and abundance with environmental conditions, suggest that a strong role of lineage- or species-specific traits such as dispersal capabilities or body size determines local community composition (Murria et al. 2017). Insect body size is modulated by many climatic factors along species ranges, especially when they are distributed across climatic gradients at large spatial scales (Lira et al. 2021; Romero et al. 2016; Brehm et al. 2019). Changes in body size may affect fertility, lifespan, population dynamics, and species composition (Garcia-Robledo et al. 2020).
In contrast to most other herbivore insects being rather host-specific, phytophagous scarab chafers (Coleoptera: Scarabaeidae), with ca 30,000 extant species worldwide, feed unspecifically on leaves of a vast variety of angiosperm plants as adults, on soil humus or roots as larvae (Ritcher 1958). They have had a very successful follow-up evolution with angiosperms (Ahrens et al. 2014). However, very little is known about their actual assemblages responding to habitat differences (Eberle et al. 2017). since only few studies have comparatively investigated their quantitative composition (Ahrens et al. 2009; García Lopez et al. 2010, 2013), and often these studies include either only a part of the assemblage (Ahrens et al. 2007), and/or consider separate localities rather than habitats (Ahrens et al. 2009; García-Lopez et al. 2013).
To close this gap, we investigated here patterns of species diversity and turnover in tropical phytophagous chafers in Sri Lanka, a global biodiversity hotspot (Myers 2000) across different forest types, elevation zones, localities, and habitats. We attempt to explore to which extent each of these spatial components determines assemblage composition. In this context, we also assessed their influence on different lineages and the role of body size in shaping species composition. If body size (as proxy of dispersal capability) had an impact on assemblage composition, we would expect contrasting patterns between entities of different spatial scales between smaller and larger species assemblies, also in respect to phylogenetically partitioned assemblages. This way, we expect to elucidate the dynamics of community assembly and differentiation and to explain the high species richness and endemism in tropical chafers.