Introduction
Insects are a megadiverse group comprising an estimated 5.5 million
species globally (Stork, 2018). They play a central role in ecosystem
processes and functioning through pollination, herbivory, detritivory,
nutrient cycling, influencing the physiology and population dynamics of
plants, and providing a major food source for species from higher
trophic levels (Samways, 2007; Stork, 2018; Wagner, 2020; Dicks et al.,
2021; Seibold et al., 2021). Recent studies suggest that insect
populations are in a state of catastrophic collapse in many parts of the
world, evidenced by rapid declines in abundance as well as extinctions
(Hallmann et al., 2017; Didham et al., 2020; Wagner, 2020; Cardoso et
al., 2020; Goulson, 2021; Wagner et al., 2021). Yet the extent of the
global insect collapse is poorly understood (Wagner, 2020), given that
insects comprise only 8% of species formally assessed against IUCN Red
List extinction risk criteria (12,100 of 142,577 species;
https://www.iucnredlist.org/; accessed on 15 April 2022).
Despite their apparent importance, insects are often neglected in
studies on ecosystem function and conservation assessments (Dunn, 2005;
Samways, 2007; Di Marco et al., 2017; Taylor et al., 2018; Geyle et al.,
2021); however, butterflies, to some extent, are an exception because
the taxonomy, geographic distribution and status of many species are
relatively well known (Schulze et al., 2004, Schultz et al., 2019).
Butterflies are good indicators of habitat quality as well as general
environmental health (Dennis et al., 2003), are sensitive to disturbance
and changes in their habitat, and can severely be affected by
environmental changes, including abiotic factors - such as temperature,
light intensity, soil composition, radiation, humidity, photoperiod, and
changes in the forest structure (Whitworth et al., 2018). Even minor
changes in their habitat can lead to either migration or local
extinction (Thomas et al., 2006; Chowdhury et al., 2017). Recent studies
have revealed dramatic declines in the richness and abundance of
butterflies worldwide, with climate change and habitat alteration
emerging as critical threats (Forister et al., 2010; Oliver et al.,
2015; Forister et al., 2019; Soroye et al., 2020; Wagner, 2020; Halsch
et al., 2021; Hill et al., 2021; Raven and Wagner, 2021).
The world’s temperature has risen by ~ 1°C since 1880
(McGregor, 2018), and precipitation patterns are also changing (Dore,
2005). Climate change substantially impacts species’ biological systems,
leading to shifts in the timing of important life-history events,
changes in geographical distribution and phenotypical features,
migration, and even extinction (Walther et al., 2002; Lenoir et al.,
2010; Parmesan, 2006; Van der Putten, 2012; Gardner et al., 2011; Lenoir
and Svenning, 2014; Furlong and Zalucki, 2017; Oostra et al., 2018;
Lindestad et al., 2019; Merckx et al., 2021). In fact, climate change is
predicted to become the dominant driver of global species decline in the
coming future (Di Marco et al., 2019). With the increasing temperature,
species need to move polewards to track suitable environmental
conditions and prevent widespread extinction (Thomas et al. 2004), which
has been documented in many taxa (Parmesan 1996; Hill et al. 1999;
Parmesan et al. 1999; Parmesan & Yohe 2003; Wilson et al. 2005;
Hickling et al. 2006; Poyry et al. 2009; Amano et al., 2014; Garcia et
al., 2014; Warren et al., 2018). Climate-induced shifts in a species
distribution commonly involve movement toward higher latitude (with a
median rate of 1.69 km/year) and higher elevation (with a median rate of
1.1 m/year; Chen et al., 2011). Due to several anthropogenic stressors,
geographic range shifts required to track changing climate are about 2-5
times higher than the fastest range shifts observed in the fossil record
(Davis & Shaw, 2001).
Due to a short lifespan and being sensitive to environmental conditions,
butterflies have been widely considered a model group to assess climate
change impacts on ecosystems (Dennis et al., 2003; Nadeau et al., 2017).
About 29% of studies testing climate change impact on insects are on
butterflies (Andrew et al., 2013). Given climate change is a major
factor contributing to butterfly decline globally (Wagner, 2020; Halsch
et al., 2021), identifying the extent of climate change impact on
butterflies is a top conservation priority (Hill et al., 2021). When
climate changes, species move areas to match the suitable climate
(Thomas et al., 2004). There is mounting evidence showing species
redistribution as a response to climate change globally (Chen et al.,
2011; Taheri et al., 2021). Species distribution models (SDMs) are
widely used to assess climate change impact on species distributions and
to forecast changes in distributions under different climate change
scenarios (Pearson & Dawson, 2003; Araújo et al., 2005; Heikkinen et
al., 2006; Elith et al., 2010; Yates et al., 2010). With SDMs, we can
predict the probable abundance of species based on resource
distributions or climatic variables (Eskildsen et al., 2013; Araújo et
al., 2019). Species with wide and variable distribution and species that
have been accidentally or deliberately introduced to a new geographic
area and eventually have established and spread fit the model
description (Guisan and Zimmermann, 2000, Elith et al., 2006, Elith et
al., 2011).
Climate change impact differs markedly both taxonomically and
geographically with the temperature (Mikkonen et al., 2015; Amano et
al., 2020; Taheri et al., 2021), yet most studies are taxonomically and
geographically biased. Of the published literature on insect
redistribution as a response to climate change, about 50% are from
Europe (only four studies from south Asia) and 23.47% on birds (only
0.44% on insects; Taheri et al., 2021). A vast majority of insect
species are distributed in the tropics, yet how climate change impacts
tropical insects is largely unknown (Chen et al., 2011; García-Robledo
et al., 2020; Kwon et al., 2014; Taheri et al., 2021). Here, I assess
climate change impact on butterflies in a tropical country, Bangladesh
– a South Asian developing country. Bangladesh is located at the
transition between the Indo-Himalayan and Indo-Chinese subregions of the
Oriental region (Reza and Hasan, 2019) and forms part of the Indo-Burma
biodiversity hotspot (Mittermeier et al., 1998). Being the most densely
populated country (among countries with ≥10 million people), there is
always a high demand for the limited resources, which is causing rapid
habitat clearance, and natural resource extraction is a crucial economic
activity (Mukul et al., 2008; Watson et al., 2014; Chowdhury et al.,
2021b, f). Only about 11% of the original forest cover in Bangladesh
remains, and deforestation is continuing rapidly (Poffenberger, 2000;
Mukul and Quazi, 2009; Chowdhury et al., 2021b). About 62% of 305
butterflies found in Bangladesh are now threatened with local extinction
(IUCN Bangladesh, 2015).
Here I collate species distribution records from a range of sources
(online repository, social media, and published literature), and i)
develop climatic niche models for 242 Bangladeshi butterfly species
using current and four future climatic scenarios, ii) compare the
suitability maps to assess the impact of climate change, iii) develop
generalised linear mixed models (GLMMs) to assess if the shift in
suitable habitat is associated with the current amount of suitable
habitat and elevational distribution, and iv) compare the results
between migratory and non-migratory species to determine if migratory
species can adjust with the shift faster than the non-migratory species.
Finally, I discuss ways to mitigate climate change impact on tropical
insects. I chose butterflies as a model group because butterflies are
the most threatened assessed taxonomic group in Bangladesh (62% are
nationally threatened), the only assessed insect group in the most
recent national threatened species assessment (IUCN Bangladesh, 2015;
Chowdhury et al., 2021b).