Discussion
Climate change is considered one of the most critical threats to
biodiversity, yet literature documenting climate change impact on
tropical insects remains scarce (Taheri et al., 2021). Given that
tropics contain enormous insect diversity, understanding climate
vulnerability to tropical species is essential. Here, comparing the
suitability distribution for 242 Bangladeshi butterfly species, I
revealed that climate warming could severely impact many butterfly
species. While the suitability might remain similar in some parts of the
country (e.g., north-east), the future suitability could expand (e.g.,
towards north-east) and contract (e.g., from the central) in other
parts. Although the direction of the shift in climatic suitability could
be multi-dimensional, most species might shift towards the north. This
finding is similar to VanDerWal et al. (2013), who analysed the
distribution changes of 464 Australian bird species and found that both
temperature and precipitation influence changes in species’
distributions, such that the resulting patterns of range shift are
complex. In addition, many Bangladeshi butterflies could shift towards
the north-east and south-east direction, mostly mountainous and forested
habitats, whereas the remaining parts of the country mainly consist of
grasslands (Chowdhury et al., 2021b). This finding supports that i)
there could be a decrease in grassland species and an increase in forest
species (Kwon et al., 2021) and that ii) elevational shifts are more
pronounced than poleward shifts for tropical butterflies (Colwell et
al., 2008; Chen et al., 2009).
Due to climate warming, the suitable habitat could primarily shift for
all the Bangladeshi butterfly species (except for Spindasis
elima ); however, there could be a striking difference between the
threatened and non-threatened species. While the suitable habitat could
increase for 58% of species and decrease for 42%, most of the
threatened species could experience range contraction. The centroid
shift for the threatened species (55km2) could be lower than the
non-threatened species (77km2); therefore, threatened butterfly species
could be more vulnerable to climate change if they cannot adapt to the
changing environment. This finding is similar to Mattila et al. (2011),
where authors analysed the distribution shift among the threatened and
non-threatened Finnish butterflies. Although the range of Finnish
butterflies shifted substantially, threatened species moved very little
(Mattila et al., 2011). Although Finnish threatened butterflies did not
show any consistent direction of the shift (Mattila et al., 2011), I
obtained contrasting result that most threatened Bangladeshi butterflies
could shift toward a south-south-west direction. Additionally, the mean
elevation of Bangladeshi butterflies could increase by about 2.4 times
in the future climatic condition, which could be more extreme for the
threatened butterflies. To safeguard the existential status of
Bangladeshi butterflies, especially threatened species, the conservation
managers, planners, and policymakers should take this into account in
their efforts.
Climate warming will lead many species to move towards higher elevations
to track suitable climatic niches (Walther et al. 2002). Nevertheless,
the ability to track climatic niche differ markedly depending on the
migratory behaviour of the species (Warren et al., 2001; Hill et al.,
2002). By analysing 46 British butterflies, Warren et al. (2001) showed
that the distribution range expanded for half of the mobile and
generalist butterfly species, whereas the other generalists and 89% of
the habitat specialists declined in distribution size. In another study,
Chowdhury et al. (2021a) showed that while expanding its geographic
range, the migratory tawny coster butterfly maintains its native
climatic niche. Here I found that the amount of suitable habitat for the
migratory species could increase significantly and that migratory
butterflies could experience a more pronounced shift in suitability in
the future climatic conditions comparing to the non-migrants. This
result supports the hypothesis that migratory species perform better in
tracking climatic niches (Warren et al., 2001).
The existing biodiversity is facing an existential crisis due to several
anthropogenic stressors; however, none is as pervasive as climate change
(Halsch et al., 2021). The current climate change has already impacted
27 million km2 (18.3% of land), which took place in all biomes (Elsen
et al., 2021). Climate change has negatively impacted the distribution
of 47% of 873 terrestrial non-volant threatened mammal species and
23.4% of 1.272 threatened birds species (Pacifici et al., 2017). Here,
I showed that the future climate could be unsuitable for 2-30% of
Bangladeshi butterfly species depending on the climate change scenarios.
If the climate changes at a similar rate – as it is now, many butterfly
species will go extinct. Wallace’s understanding of tropical climate as
“genial” dismisses the view that tropical climates can be vulnerable
to climate change (Colwell et al., 2008). While safeguarding tropical
insects, several steps need to be taken urgently.
Our current understanding of how climate change impacts tropical insects
is limited: Detailed studies are required to assess climate change
vulnerability (Basset & Lamarre, 2019; Janzen & Hallwachs, 2019;
Montgomery et al., 2020; Wilson and Fox, 2021) — identifying
species-level details on how climate change impacts physiology, life
history and evolutionary traits; diversity, abundance, and distribution.
Understanding the key drivers of species sensitivity and how it varies
among different groups (e.g., threatened and non-threatened; migratory
and non-migratory) is essential, where natural history collections can
provide important historical information on climate change impact
(Scheper et al., 2014; Kharouba et al., 2019; but see Ries et al.,
2019). Further, developing decisive conservation strategies for tropical
insects are needed, where spatial conservation prioritisation approaches
can be beneficial and identifying factors limiting species’ capabilities
to colonise areas beyond their historical limits is vital (Lewthwaite et
al., 2018). Developing efficient conservation strategies requires
long-term biodiversity data, yet such data is vastly unavailable from
most tropics. To solve the biodiversity data gap, citizen science
activities can be highly beneficial. A two-week citizen science project,
for instance, could generate comparable spatial coverage for social wasp
species as four decades of recording by expert amateurs (Sumner et al.,
2019; Wilson and Fox, 2020). Assessing climate change’s impact on
Bangladeshi butterflies was only possible because of utilising the
citizen science data, given a vast proportion of the collated species
distribution records were from Facebook. Scientists can engage the wider
community to document insect distribution data from the tropics;
however, developing standardised monitoring protocol with detailed
guidelines before organising such surveys.
The climatic niche evolution rate of species is slower compared to the
climate change rate (Quintero & Wiens, 2013), and failure to track
climatic conditions may lead to range contractions and/or local
extinctions (Moritz & Agudo, 2013; Wiens, 2016; He et al., 2018).
Besides, climate change is not just a future threat but impacts many
species in the current time (McCarty et al., 2001). Using the
niche-overlapping analysis, I showed that about 54% of the climatic
niche of Bangladeshi butterflies could shift in the future. Immediately
detecting climate change response is vital to improve (or initiate)
management strategies, counter species loss, and mitigate biodiversity
impacts. Here, specific management strategies should include
prioritising species-specific requirements. For example, butterflies
mostly are tied to specific habitat types, and therefore, to survive,
they will require coordinated movement of climatic conditions habitat
types (including vegetation structure) to track conditions spatially
(Peterson et al., 2004). Further, the adverse effects of climate warming
on pollinators can exacerbate by homogenous and fragmented landscapes,
limiting range shift opportunities and reducing micro-climatic buffering
(Vasiliev and Greenwood, 2021), which emphasises the importance of
considering landscape connectivity and habitat heterogeneity in
conservation planning (Vasiliev and Greenwood, 2021). Similarly,
increasing the number of floral resources could attract diverse insect
groups, creating connectivity between forest patches could support range
expansion, and protected habitat over elevational and vegetational
gradients could help range-restricted species.
Protected area (PA) coverage in the tropics is relatively low. For
example, only 4.6% of terrestrial land in Bangladesh is now protected,
and most are too small to support viable biota (Chowdhury et al.,
2021f). Increasing PA coverage could insulate species from human-induced
climate change and meet the Post 2020 biodiversity framework targets,
aiming to achieve 30% PA coverage by 2030 (Convention on Biological
Diversity, 2020). When proposing new PAs, scientists need to include
connectivity planning emphasising climate change impact, ensuring
landscape permeability, creating the network of habitats, future habitat
conditions, and prioritising intact habitats. In tropical mega-populated
countries like Bangladesh, many people live inside or surrounding PAs;
therefore, scientists need to engage local people in conservation.
This is the first study highlighting how climate change could impact the
suitability distribution of South Asian butterflies; however, these
results need to be interpreted cautiously. For example, the spatial
distribution data of Bangladeshi butterflies are highly biased and are
mostly distributed in the major cities (Chowdhury et al., 2021b, f),
which might impact findings. However, I followed two approaches to
handle sampling bias: i) spatial thinning process, where I filtered
single occurrence record from each raster pixel (Aiello-Lammens et al.,
2015), and ii) used the ‘checkerboard2’ evaluation method, which handles
overinflation of model performance, at least that portion resulting from
biased sampling (Muscarella et al., 2014; Chowdhury et al., 2021e).