How ancient forests became grasslands: Study traces 3,300 years of Assam’s Kaziranga ecology
Guwahati: The sweeping grasslands and dense populations of one-horned rhinoceros that define Kaziranga National Park today are the result of thousands of years of ecological change—driven not just by climate, but by animals themselves.
A new scientific study has reconstructed nearly 3,300 years of Kaziranga’s ecological history, revealing how forests, floods, and large herbivores together shaped one of India’s most iconic landscapes.
Published in the international journal CATENA, the study draws on microscopic evidence preserved deep beneath Kaziranga’s floodplains—pollen grains and dung-associated fungal spores locked inside ancient sediments. Led by palaeoscientist Sadhan K. Basumatary from the Birbal Sahni Institute of Palaeosciences, with a team of researchers, the work is the first to combine palaeoecology and palaeoherbivory to tell a long-term story of vegetation and wildlife change inside the park.
"Wildlife, especially rhinoceros, migrated from the western and northern parts of the region into the northeastern part of the Indian subcontinent in response to climatic amelioration and increased human activity during the late Holocene period. This study will help provide a better understanding of the palaeoecological and palaeoherbivory history of this region and offer greater insight into the causes of wildlife migration and subsequent extirpation," the authors say.
The researchers selected the Sohola swamp as an ideal site to study palaeovegetation and palaeoherbivory in Kaziranga National Park. A 110 cm-deep sedimentary profile was trenched on the northwestern side of the Sohola swamp, and soil samples were collected at 5 cm intervals for palynological analysis and radiocarbon dating.
The findings challenge the assumption that Kaziranga has always been dominated by open grasslands. According to the study, between about 3,300 and 1,700 years ago, much of the park was covered by dense tropical mixed forests under warm, humid conditions.
Tree species such as Mesua, Cinnamomum, Duabanga, and Lagerstroemia were widespread, while grasslands were relatively limited. Evidence from dung-loving fungal spores suggests that large herbivores were present, but in much lower numbers than today.
“Our data show that this early phase was characterised by dense forest cover and comparatively low wildlife activity,” the researchers note, adding that high monsoon rainfall and extensive wetlands defined the landscape during this period.
Forests thin, grasslands expand
A major ecological shift began around 1,700 years ago. Evergreen forest species started to decline, deciduous trees became more common, and grasslands expanded steadily across the floodplain.
At the same time, the researchers recorded a clear rise in coprophilous fungal spores—species that grow on animal dung and are widely used as indicators of herbivore presence. This suggests that populations of large grazing animals were increasing alongside the spread of open habitats.
This phase also marks the first appearance of Mimosa in the sediment record—an invasive plant species that continues to pose management challenges in Kaziranga today. Its arrival, the study suggests, coincided with forest degradation and expanding open land.
Rhinos as architects of the landscape
The most striking changes occurred over the last 640 years, when evidence points to a sharp rise in large herbivore populations. Fungal spores associated with dung—especially Sporormiella—increased dramatically during this period, indicating intense grazing activity.
Rather than being passive inhabitants, the study argues that animals such as rhinoceros, elephants, and wild buffalo actively shaped Kaziranga’s ecology. Grazing, trampling, and wallowing helped suppress forest regrowth, maintain open grasslands, and alter wetland dynamics.
“The significant increase in herbivore indicators implies that wildlife played a key role in driving vegetation dynamics,” the authors observe, describing large mammals as ecosystem engineers rather than mere consumers.
A refuge shaped by migration
The research also offers a broader explanation for why Kaziranga emerged as a stronghold for megafauna. Comparing Kaziranga’s ecological record with palaeoclimate data from western and northern India, the authors suggest that megaherbivores, particularly rhinoceros, gradually migrated eastwards during the late Holocene.
Climatic drying, weakening monsoons, and rising human pressures in parts of the Gangetic plains and western India likely reduced suitable habitats there, while the Brahmaputra floodplain offered water, forage, and relative refuge.
Today, Kaziranga supports some of the highest densities of one-horned rhinoceros in the world—a legacy shaped by both ancient climate shifts and long-term animal movement.
Lessons for conservation
Beyond its historical insights, the study has clear implications for conservation policy. By showing that Kaziranga’s grasslands are the product of long-term interactions between climate, vegetation, and wildlife, the research cautions against simplistic management approaches.
Understanding this deep ecological history, the authors argue, is essential as Kaziranga faces new pressures—from climate change and invasive species to altered flood regimes and growing human activity around the park.
Kaziranga’s landscape, the study makes clear, is not static or accidental. It is the living outcome of thousands of years of change—written in pollen, spores, forests, and the footsteps of rhinos.