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).