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.