Peninsular India: how biodiversity changed through millions of years
Biodiversity of peninsular India — home to unique plants and animals, and a dramatic geological history — has responded differentially to geoclimatic history, with a few groups remaining unperturbed by these events while others exhibiting varying rates of speciation
Peninsular India extends from the Aravalli Mountains southwards, including the Satpuras and Vindhyas, the Eastern Ghats, the Deccan Plateau, and a global biodiversity hotspot — the Western Ghats. It houses very diverse groups of unique animal and plant species, many of which are not found anywhere else in the world, i.e., they are endemic. These endemic species are found across a range of habitat types, including tropical evergreen forests, deciduous forests, and savannas. In a recent study published in Ecology Letters, we employed mathematical tools and existing data on species diversity, age, and evolutionary relationships to decipher the patterns through which this high endemic diversity was generated and the factors that contributed to it.
Determining diversity
Biodiversity on Earth is shaped by two key processes: the formation of new species (speciation) and the disappearance of species (extinction). These processes have played out over millions of years and across the globe. The balance between speciation and extinction determines the diversity observed in different regions and ecosystems. Importantly, the speed at which species form or go extinct can vary across time periods, types of organisms or regions. This leads to different patterns of biodiversity build-up across the tree of life and the globe.
For nearly 200 years, scientists have been fascinated by the evolution of life over time. Today, scientists study these patterns by building evolutionary trees using both DNA and fossils to understand the relationships between species.
Interesting diversification in peninsular India
About 200 million years ago, the peninsular Indian plate was part of the Gondwanaland supercontinent, which also included other southern continents such as Africa, South America, Madagascar, Australia, and Antarctica. Following the breakup of Gondwana around 100 million years ago, the Indian plate drifted in isolation for nearly 50 million years before eventually colliding with Asia. This long northward journey caused significant latitudinal shifts, resulting in geo-climatic and habitat changes that likely influenced speciation and extinction patterns within the region.
Today, peninsular India is home to a wide variety of unique and highly diverse plant and animal groups, many of which are endemic. These groups are believed to have descended from a common ancestor that once existed in peninsular India. Some of these ancestors were present even before the breakup of Gondwana, while others arrived later. Over millions of years, their descendants spread across the region and diversified into multiple species, many of which persist to this day.
Given that tropical regions are a central focus of diversification studies, peninsular India — with its rich biodiversity and complex geological history — offers an excellent natural laboratory to explore how biodiversity originates and evolves.
Estimating the rates of speciation, extinction
We wanted to find out how species in peninsular India evolved over time and what factors may have influenced their rise and disappearance. To do this, we analysed the evolutionary trees of 33 well-known animal and plant groups to determine when new species emerged (speciation) and when species became extinct.
The rates of speciation and extinction for each group were estimated using multiple mathematical models, which consider both the age of the taxa and the frequency of speciation events. A higher number of speciation events generally indicates a higher speciation rate and lower extinction rate, and vice versa. Since the groups differ in evolutionary age, both the number and timing of events (speciation and extinction) are essential in determining these rates.
Importantly, within a single group, speciation and extinction rates can change over time. For example, a climatic event might trigger a burst of speciation at a specific time. The models we used can detect such time-dependent changes probabilistically.
By applying models that account for such time-based changes, we learned how speciation and extinction shaped the diversity we see in peninsular India today. For each group, the speciation and extinction rates averaged over time were used to compare the overall pace of diversification across different groups.
High disparity in diversification rates
We found a high disparity in the overall diversification rates across life forms. Among peninsular Indian clades, angiosperms diversified the fastest, whereas invertebrates diversified the slowest. We found that evolutionarily related groups, such as different types of ground-dwelling arthropods or various types of lizards, exhibited similar rates of diversification. Also, younger groups were found to have diversified at a higher pace than older groups. These relatively young groups, such as Hemidactylus, Geckoella, and Ophisops lizards, which have dispersed into India from Asia after the India-Eurasia collision, occupy a wider range of habitats, including wet and dry forests, rocky outcrops, dry grasslands, and subterranean habitats. On the other hand, some of the most ancient groups, such as Scolopendrid centipedes, Heterometrinae scorpions, Gerrhopilus blindsnakes, and Gegeneophis caecilians, have a Gondwanan origin. They live in the soil under leaf litter, rocks, or logs; some of them can even burrow under the soil. This could have restricted their pace of diversification.
Furthermore, half of the animal and plant groups showed a gradual accumulation of diversity characterised by a constant rate of speciation throughout their evolutionary history. Such steady diversification, for example, in ancient groups like the Digitipes centipedes and young Acanthaceae plant species, is attributed to the prolonged stability of tropical forest ecosystems in peninsular India, which may have served as refuges shielding species from severe climate fluctuations. Additionally, many groups also experienced fluctuations in their rates of speciation and extinction across time. Global temperatures have been highly dynamic over several million years, and our analyses strongly suggest that these fluctuations in paleotemperature have influenced fluctuations in diversification rates of many groups, such as the Cnemaspis geckos or the Memecylon plants.
Many of these groups, such as the Ophisops lizards and the Raorchestes bush frogs, experienced sudden, abrupt changes in speciation rates within the time range of 11-3 million years ago. This geological period experienced high aridification and monsoon seasonality. Additionally, peninsular India underwent a major expansion of grasslands, resulting in the shrinkage of evergreen forests in the Western Ghats and on the mountaintops of the Eastern Ghats. These events certainly inflicted changes in the ranges and habitats of organisms, disrupting their earlier evolutionary trends.
Idiosyncrasies unveiled
The study found the peninsular India plate had experienced “idiosyncratic” biotic diversification — varied trajectories of biotic diversification, few of which were influenced by the stability of tropical ecosystems, while few others were driven by fluctuations in paleotemperature and geo-climatic changes that occurred between 11 million and 3 million years ago. This study is unique, as it is the first synthesis of biotic diversification in Southern Asia and Asia as a whole. It is poised to have implications for the conservation of biodiversity, given the changing global and regional climate.
