Guardians of Balance: the role of megafauna in healthy ecosystems
I was about five years old, in 2002, when my parents took me to the cinema to watch one of the latest animated movies: Ice Age. I remember being instantly mesmerized and enthralled by the main characters, it was probably the first time I had seen such strange, large animals like Manny (the mammoth) and Diego (the sabre-toothed tiger), which I instantly adored. I couldn’t quite put my finger on why they gave me such a nostalgic feeling, though, and it wasn’t until later - maybe when I was seven or eight - that I realized these animals no longer existed. They were extinct.
Although the movie effectively shows the migration of these animals in response to climate changes (glaciation and subsequent ice-sheet melting), it does not portray their fate thousands of years later: due to a combination of rapid environmental shifts and growing human pressure, these giants were ultimately lost. I then realized that watching Ice Age was not so different from watching Jurassic Park: both depict megafauna that once existed, but no longer do. And it seems that the further back in time we go, the larger these animals were. The biggest animals alive today are small compared to the megafauna of the dinosaur era or even the Pleistocene, which included larger relatives of familiar creatures such as elephants and lions, as well as extraordinary species like giant ground sloths, car-sized glyptodonts in the Americas, rhino-sized marsupials in Australia, and gorilla-sized lemurs in Madagascar.
Figure 1 - Vintage drawing of Glyptodon in the Pleistocene. Source: J. Lange, n.a.
In this sense, interacting with megafauna can feel like a form of time travel: just as walking through the Parthenon in Athens offers a glimpse into the distant past, traveling to places like Africa to observe these increasingly rare giants, now so unusual to encounter, feels like peeking into an ancient version of our planet. This highlights the importance of Africa’s role in megafauna preservation: some African savannas and woodlands are among the few places where megafauna persist at high abundances.
Since the Late Pleistocene, large herbivores and carnivores have undergone widespread decline and extinction, marking a profound ecological transition from a megafauna-dominated world to one largely devoid of it. Research suggests that this shift involved the loss of up to a billion large terrestrial animals and has fundamentally altered Earth’s ecological functioning as manifested through different factors explained below:
Ecosystem Trophic Structure: Megafauna play a central role in structuring food webs through their position as large herbivores and apex predators. Their loss disrupts trophic interactions, often triggering cascading effects that disrupt predator-prey dynamics and can lead to population imbalances and secondary extinctions.
Ecosystem Physical Structure and Fire Regimes: Large herbivores maintain open landscapes by browsing, trampling vegetation, and reducing woody plant density. Their decline has allowed forests and shrublands to expand into formerly open systems, altering habitat structure and landscape heterogeneity, which in turn influences fire regimes: increased woody biomass and fuel loads can lead to more intense or more frequent fires.
Vegetation Composition: Many plant species evolved alongside large animals, relying on them to control competitors, disperse large seeds, and maintain plant diversity. Therefore, the disappearance of megafauna leads to shifts in plant communities and the dominance of species less dependent on animal-mediated dispersal or disturbance, reshaping ecosystems at the foundation of the food web.
Ecosystem Biogeochemistry: By consuming vegetation and transporting nutrients through dung, urine, and carcasses across large distances, megafauna connects ecosystems that would otherwise remain isolated. Their loss has severely reduced the redistribution of key nutrients such as phosphorus across ecosystems.
Regional and Global Climate: By shaping vegetation cover, influencing fire regimes, and regulating carbon storage and albedo effects, large animals indirectly affected Earth’s energy balance and atmospheric processes. This highlights megafauna as active agents in regulating Earth’s climate at large spatial and temporal scales.
Together, these factors reveal that megafaunal extinction was not merely a loss of species, but a reorganization of Earth’s ecological processes at planetary scale. And this is not something confined to the distant past: it still happens today. A clear contemporary example is the decline of African elephants and other large herbivores, mainly driven by poaching, habitat loss, and human land conversion. As mentioned, elephants maintain open savanna landscapes by knocking down trees and browsing shrubs, so when their population decreases, woody vegetation expands unchecked, transforming open grasslands into dense shrublands - a process known as bush encroachment. This structural shift cascades through the ecosystem: grazing species such as zebras and wildebeest decline as grasslands disappear, while dense vegetation provides ideal shelter for small mammals. Studies in East Africa show that the loss of large wildlife leads to sharp increases in rodent populations, which in turn host more fleas and ticks carrying zoonotic pathogens such as Bartonella, a bacterium linked to human febrile illnesses. In this way, the removal of megafauna indirectly increases disease risk for local human communities, illustrating how their decline can reshape ecosystems, alter species interactions, and feed back into human health and livelihoods.
This is only one example of the ripple effects of losing giants, which reinforces the importance of conservation efforts to stop the decline of these species’ populations, as well as rewilding efforts to reintroduce them into certain habitats.
It’s too late to bring Manny and Diego back, but we’re still on time to prevent future generations from watching movies like The Lion King and discovering that species such as elephants and lions exist only in fiction.
