4. DISCUSSION
Despite the fact that tropical areas host the majority of insect diversity, they are generally vastly undersampled compared to temperate ecosystems (Hellmann & Sanders, 2007). This knowledge gap is particularly undesirable in Southeastern Asia, which is experiencing a rapid economic growth that may imperil its rich insect biodiversity. In this study, we report for the first time a large sampling of the whole order Coleoptera carried out across Laos, which enabled the investigation of the drivers of beetle diversity at a large spatial and taxonomical scale. This is a step towards a better understanding of insect diversity in Asian tropical forests, and of the threats they may face.
The present study first confirmed our expectation that beetle diversity is high in the country. From the ca. 20,000 collected specimens, we recorded 64 beetle families. Moreover, more than 50 beetle specimens remained impossible to assign to any known family and were thus excluded from this study. Some of them may belong to additional beetle families, suggesting that the diversity of beetle families in the region may be even larger. It is however hard to reliably estimate the actual number of families living across the various landscape contexts of the country without a meticulous approach involving standardized sampling protocols and molecular taxonomy (García-Robledo et al., 2020). The data presented here reveals slightly higher number of families than that of earlier recorded data in surrounding countries such as Thailand, Vietnam, and Hong Kong (Rattanawannee et al., 2013; Thinh et al., 2004; Zhao et al., 2022). Overall, 177 Coleoptera families were recorded globally, meaning that we recorded 36% of all known beetle families (Moodley et al., 2022). However, due to practical constraints, the present study described beetle communities at the family level only. Indeed, beetle taxonomy is notoriously difficult in the absence of detailed identification keys or molecular tools (see e.g. Jin et al., 2020; Sabatelli et al., 2021). The actual diversity at the species level, i.e. species richness, is thus much more important, and may include endemic or undescribed species. It is widely accepted that Laos is a hotspot for the biodiversity of beetles, insects, and other organisms. By providing a first countrywide view of beetle family diversity of Laos, we aim to provide a basis for future studies investigating the impact of an extremely rapid economic change, associated with land use change, on beetle diversity in Laos.
According to our results, beetle abundance was reduced in plantations compared to natural forests, which is in line with global patterns which show that insects are sensitive to habitat disturbance from human activities such as agricultural expansions or settlements (New et al., 2021; Hansen et al., 2012; Sánchez-Bayo & Wyckhuys, 2019). Natural forest ecosystems play an important role in species diversity worldwide (Gibson et al., 2011), whereas intensification of agriculture is identified as a major cause of insect diversity decline and extinction (Sánchez-Bayo & Wyckhuys, 2019), but also soil carbon loss (Guo et al., 2022). A strong impact of land-use intensification has been reported for beetles and other insects in tropical forests in Asia, as well as in Africa and America (Phillips et al., 2017). Several observational and experimental studies have revealed that the conversion of natural forests into plantations is harmful to species that cannot adapt to their new environmental conditions (Uribe et al., 2021; Warren-Thomas et al., 2015); our results show that Laos is no exception. Therefore, a large number of insect communities may be, currently or in the near future, at risk from land-use intensification in Laos, even though the region is still mainly covered by mountains and forests.
A more in-depth understanding of the response of beetle diversity to current and future economic development in Laos is needed to implement practical conservation actions. In this regard, this study clearly demonstrates that the rapid and continuous land-use changes the country is experiencing may threaten beetle communities: not only their abundance declined in plantations, but across the whole survey we also sampled fewer families in plantations compared to natural forests (48 vs. 59 observed families, respectively). This finding is in line with other studies showing the impact human-modified landscapes can have on beetle biodiversity; for example, dung beetle communities are well recognized as a good indicator to estimate the influence of anthropogenic habitats in tropical forests (Gardner et al., 2008; Halffter & Arellano, 2002). In the present study, we investigated beetle communities across a large countrywide latitudinal gradient, in which the northern to center parts are facing a modernization of the road network in addition to the conversion of forests into plantations. In this regard, a recent report by Danyo et al. (2018) pointed out the potential risks of forest and biodiversity loss resulting from road improvement in Laos. We believe that our findings will therefore be useful and important in order to predict properly the conservation issues arising from the economic growth of the region.
However, unexpectedly, the observed difference in abundance between plantations and natural forests did not correspond to differences in terms of family composition. Similarly, although fewer families were sampled in plantations at the scale of the whole country, anthropization does not seem to reduce community diversity when analyzed at the scale of each sampling site. Logically, this must be caused by a higher homogeneity of communities located in plantations, i.e. the same set of families are found in all plantations, while forest communities are more diverse from each other (despite a similar local diversity). Biotic homogenization, in which a few common species take over specialist species, is frequently observed in human-modified landscapes (McKinney & Lockwood, 1999), and has also been observed in beetles (Ramírez-Ponce et al., 2019). However, results may vary at a lower taxonomic level; it is likely that species diversity is actually reduced along human activities, in accordance with previous studies (e.g. Jung & Lee 2016; Vanbergen et al., 2005). Here, beetle individuals were assigned to the family level, since we expected that it would be sufficient to investigate the impact of rapid change caused by anthropogenic pressures associated with land use change. The beetle collection contained a huge number of specimens, and the taxonomy of many groups is challenging due to their complex diagnostic morphological characters and their small body size. An improved dataset that would distinguish individuals at the species-level may reveal a slightly picture, including perhaps an effect of agricultural development on species diversity at the local scale.
Despite the geographical scale of the study, we did not find an effect of climate on community composition. However, again, it could be because the present study has investigated families only and not species. Generally, assessments of climatic niches are considerably more precise when carried out at lower taxonomic levels (Bayliss et al., 2022; Gonzalez et al., 2011), meaning that each individual species may have vastly different responses to climatic variables that would be masked when merged into whole families. Still, the present study found more individuals in more humid areas, revealed by a positive effect of the precipitation variable on beetle abundance. This general effect may vary depending on the family composition, as evidenced by different responses of beetle abundance to temperature and precipitation depending on the family considered; the actual variation of diversity across climate gradients may thus be more complex.