Discussion
Our analyses of the effects of projected near-future climate change on
bats in South Asia show a general pattern of moderate potential loss in
all scenarios with relatively high degrees of retention in climatic
suitable areas. As expected, smaller losses were projected under the
‘middle-of-the-road’ SSP2-RCP4.5 scenarios compared to the more
pessimistic ’fossil-fuelled development’ SSP5-RCP8.5 scenarios,
reinforcing the need for changing behaviour and avoiding business as
usual (Peters et al., 2013). While loss was moderate, our results also
revealed variability among species and scenarios with winners and
losers. For example, Myotis csorbai , a species restricted to
Nepal, was projected to lose all its current suitable area in some
scenarios, while Saccolaimus saccolaimus , a species with a very
wide but fragmented distribution in South and Southeast Asia, was
projected to double its current suitable area in some scenarios.
Previous work has also reported varied responses of bats to climate
change including negative impacts (Rebelo et al., 2010; Hughes et al.,
2012; Bellard et al., 2013; Thapa et al., 2021), no or limited effects
(Bandara et al. 2022), and positive effects (Costa et al., 2018; Hayes
& Piaggio et al., 2018; Thapa et al., 2021). This variation likely
reflects differences in niche plasticity, robustness to changing
conditions, and migratory propensity among species. Additionally,
variation can occur due to data limitations and modelling choices.
Indeed, our results showed uncertainty in projections for some species:
many species involved small (< 30 occurrences) sample sizes,
and the minimum sample size of five occurrences has been used before but
may not be fully reliable (Pearson et al., 2007). However, most advanced
ENM algorithms, especially techniques such as MAXENT, can account for
fewer occurrences in species with low prevalence and narrow ranges
(Pearson et al., 2007; van Proosdij et al., 2016; Morales et al., 2017),
and ensemble modelling can provide higher predictive accuracy by
accounting for inter-model variation. Capturing and reporting this
variation by considering various scenarios, scales, and algorithms is
important to ensure conservation and management recommendations are not
misguided due to over- or under-fitting and a lack of reliability in
results.
Similar to other studies on bats (in Europe by Rebelo et al., 2010; in
Southeast Asia by Hughes et al., 2012; in Nepal by Thapa et al., 2021),
we also found a trend for likely shifts towards higher latitudes in
climatically suitable areas. In SSP2-45 scenarios, suitable areas for
most species were projected to be located north-northeast or northwest
from current areas; in SSP5-85 scenarios the shift was more generally
northeast. Overall, climate change is expected to lead to latitudinal
shifts towards polar regions, but arguably, our projected shifts could
also reflect the geographic configuration in our study area, where the
Southern region is mostly coastal. However, this is unlikely the reason
for the observed trend, as for species currently in northern and central
areas we also did not generally find projected southward shifts. Future
suitable areas were also usually not located far from current suitable
areas, and it is likely bats could track changes in climatic suitable in
many cases. However, there are several barriers that could limit shifts,
including the Himalayas in the north and northeast, the Thar Desert and
the Great Rann of Kutch in central India, and the hill ranges of south
and central India. Similarly, moving across large tracts of water may
prevent tracking of climatically suitable areas in some species.
Importantly, even if there are no strong geographical barriers, movement
may be prevented due to lack of other abiotic and biotic resources or
variation in dispersal behaviour.
Combining information from individual species we identified current and
future suitability hotspots - areas of climatic suitability for
>30% of study species. The current hotspots aligned with
the four biodiversity hotspots in the region defined by Myers et al.
(2000), but representation varied. While a large proportion of the
current suitability hotspots were projected to be retained into the
future, projected losses outweighed areas of projected gain. In the
Himalaya and Indo-Burma hotspots, projected losses would lead to high
fragmentation and isolation between hotspots (especially under the
SSP2-RCP4.5 scenarios). Movement corridors and continuous protected area
networks are likely to be critical to allow movement and occupation of
new suitable areas. Interestingly, new suitable areas were consistently
identified nearby rivers, around the northern regions of the Indus
valley, northwestern Pakistan, and southern Gujarat. The small areas of
suitability in southern Myanmar were projected to increase only in the
HadGEM3 scenarios, and in Had5-85 there were a few additional isolated
fragments of projected gain in suitability in Bangladesh and northeast
India, regions with typically moist deciduous and wet evergreen habitats
(Champion & Seth, 1968; Olson et al., 2001), quite similarly to
southern Myanmar. The Western Ghats suitability hotspot was mostly
retained and contiguous across all scenarios, and increases were
restricted to small areas on the margins, showing that the Western Ghats
are likely to remain stable as a suitability hotspot into the near
future. Decreases were consistently seen in the Nilgiri hills and in
northern Maharashtra - both regions with unique vegetation and habitat
structures, ranging from semi-arid scrublands in the north of the
Western Ghats to tropical moist deciduous forests in the Nilgiris. Many
regions in peninsular India which are known to be specific in geography
and habitat consistently showed patterns of projected loss in suitable
areas. Combined with a growing understanding of the effects of climate
change in the Himalayas and Western Ghats, and how it impacts
biodiversity (Srinivasulu et al., 2021; Thapa et al., 2022), our results
showing high degrees of retention combined with severe losses and
minimal gains further support the importance of these regions.
We evaluated projected impacts of climate change using ensemble
ecological niche modelling, an approach with good predictive accuracy
that captures uncertainty from algorithm choice (Hao et al., 2020).
However, this method has some limitations. First, ensemble ENM requires
considering the balance between selecting many algorithms and the
increased computational time and resources. Generally, the suggestion is
that if a smaller number of algorithms is used, the ones with higher
predictive accuracy and robustness are selected, as done here (Drake,
2014). A second limitation is the challenge to generate suitable
pseudoabsences for presence-only occurrence data (Engler et al., 2004;
Lütolf et al., 2006; Barbet-Massin et al., 2012). Our method for
generating pseudoabsences incorporates random sampling within geographic
limits based on spatial resolution and scale of the analysis and
estimated foraging distances of study species. We aimed to balance
statistical rigour and ecological realism while working with the
constraints of lacking accurate species-level data on bat ranging and
movement in South Asia. Further analyses for species with available data
could incorporate information on ecological distance and environmental
profiling as additional limiting factors to pseudoabsence generation
(Iturbide et al., 2015). Model certainty is also an issue with ENM that
greatly impacts the reliability and interpretability of these results.
For instance, our analysis showed high certainty of climatic
unsuitability in most of the study area (Supplementary Material 3).
However, it is important to note that per-species results are much more
informative and broad interpretations must be very cautious. Finally,
another limitation of ENM is the need to carefully select variables to
balance ecological importance, methodological constraints including the
effects that multicollinearity and collinearity shift have on different
algorithms, and model transferability (Feng et al., 2019). Here, we also
considered this balance selecting robust methods, reducing variables to
avoid high collinearity while prioritising ecologically relevant
information.
Understanding species responses to rapid climate change is vital in
conservation planning, especially in regions with high biodiversity and
rates of endemism (Warren et al., 2008; Quintero & Wiens, 2013; Raman
et al., 2023). This study is an initial assessment of potential effects
of near-future climate change on bat species in South Asia, offering
reasons for concern but also some hope. While climatically suitable
areas may be reduced, many currently suitable areas are likely to
remain, and shifts may be within the dispersal potential of many
species. Nonetheless, retention of suitable areas and moderate loss does
not necessarily ensure population persistence. Our initial assessment of
impacts focuses on abiotic climate effects, but climate change may
influence habitat and food resources differently and may result in
climatically suitable areas being effectively unable to support healthy
bat populations. Analysing climate effects alone ignores the potential
combined effects of climatic and ecogeographic impacts on species
distributions (Newbold, 2018), but it is a first step to understand
potential changes in areas where climate and ecogeographic data may not
be of the same quality, interpretability, or accessibility, possibly
biasing ecological interpretations. Future climatically suitable areas
may also be unsuitable due to geology and topography, factors linked to
human activities including human land use, human population densities,
and proximity to various infrastructures and developed areas. However,
while it is possible to model both types of factors together (e.g.
Hughes et al., 2012; Simões & Peterson, 2018; Raman et al., 2023), the
lack of accurate data on their interactions when it comes to bats in
this region could lead to overfitting, erroneous predictions, and
misinterpretations across highly diverse species and functional groups
(Araújo et al., 2007; Peterson et al., 2011; Fordham et al., 2012;
Newbold, 2018; Simões & Peterson, 2018). It is important to analyse the
effects of climate (and future climate change) and ecogeography
independently as well as together, and also study their interactions, in
order to truly understand the drivers of suitability for each species.
Future work that considers these additional factors and the current
intactness of future suitable areas would be important to inform
conservation actions for climate change mitigation. In addition, it is
important to consider that climatic niche modelling based on data from
current occupied areas may not reflect the full climatic limits or niche
of a species or its capacity to adapt (Hoffmann & Sgrò, 2011). Our
results offer a first evaluation that highlights the need to further
study climate change impacts in megadiverse regions such as South Asia
and to develop robust conservation plans that integrate this
information. Effective conservation requires the integrated study of
species responses to climate, biotic, geographic, and anthropogenic
factors, and effective communication with and outreach to policymakers
and stakeholders at all levels.