Megafaunal hegemony and subsequent decline through prehistoric times
For hundreds of millions of years, including through multiple glacial–interglacial transitions, megafauna (literally, “large animals” weighing in over 45 kg) have been abundant across all landscapes and seascapes on Earth. Kingly roaming vast grasslands, they have long played crucial roles in the ecosystems they inhabit (1). The last 50,000 years, however, have been marked by a dramatic decline in megafaunal abundance and diversity. Leading up to the end of the Pleistocene era 12,000 years ago, co-incident waves of human arrival and overhunting (2), alongside climate change, intensified pressures on megafaunal populations to the point of precipitating vast extinctions (3). Approximately 1 billion large animals were lost from the Earth’s surface (4): In the Americas, all 27 megaherbivore species were lost (5), including 8 of the elephant family, while North America alone saw the extinction of all 9 species of its megacarnivores (5). This megafaunal decline tragically continues on land and in oceans to this day (6,7).
Megafaunal decline: Catastrophic consequences on ecosystems
Changes in megafaunal abundance have striking effects on ecosystem structure, species and vegetation compositions, biogeochemical dynamics, and climate change (5).
Ecosystem trophic structure. In prehuman times, a complete animal community consisted of multiple levels of both food- and predator-limited species in relative balance with each other (33), reflecting the baseline organizational dynamics of species as they co-evolved on land and in the sea. In trophic terms, megaherbivore populations within ecosystems have traditionally been limited from the bottom-up by food availability, while in turn controlling vegetation structure and composition from the top-down (8). As a result of late-Pleistocene megafaunal decline, in most land regions, this entire top trophic level (megafauna) was lost, causing the next trophic level (megacarnivores) to be either lost or heavily compromised – resulting in a profound trophic simplification of wild ecosystems. Further, the loss of top-down control from invasive medium-sized carnivores has also made ecosystems more vulnerable to invasive species: in a number of cases, restoring native top predators has helped suppress invasive species to the benefit of native species (9).
Ecosystem physical structure and fire regimes. Megafauna directly impact the core physical structure of global biomes. African savannas and woodlands are a natural product of and maintained by megafaunal presence. Exclosure and remote sensing LiDAR studies have revealed that megafauna – especially elephants – reduce woody species cover by 15-95% (10,11), while an average South African bush elephant in Kruger National Park uproots up to 1,500 adult trees every year (12). In high northern latitudes however (such as across northern Eurasia and Beringia), megafauna affect the balance between water-logged vegetation and cold, dry “mammoth steppe”, maintained by the suppression of tree growth, stimulation of deep-rooted grasses, and accelerated nutrient cycling (13,14). Concurrently, megafauna differentially affect the floral architecture and fire regimes of various types of ecosystems. In wetter climates, the loss of megafaunal grazing leads to closed canopy forests – the tree growth within which is supported by megafauna eating smaller growths, resulting in characteristically higher forest biomass in elephant-rich forests (15). In drier climates however, the loss of grazers increases grass density (think greater fuel loads) – resulting in a shift towards fire-dominated ecosystems.
Vegetation composition. Megafauna are key to the dispersal of large seeded fruits by facilitating long-distance seed dispersal and gene flow, the restriction of which impacts the distribution and genetic structure of remaining plant populations (16). In addition, by promoting browsing-tolerant vegetation, megafauna affect the composition of woody species (10) which evolve a certain degree of resistance to their predators. In pre-Polynesian New Zealand for example, where the main herbivore was the large wingless moa, plants evolved wide-angled branches and small leaves to confer resistance to their predation (17). Such effects differ in fascinating ways across global biomes from African and Neotropical savannas to boreal forests.
Ecosystem biogeochemistry. Megafauna accelerate biogeochemical cycling by unlocking and promoting the flow of nutrients. First, megafauna break down and digesting recalcitrant plant matter (otherwise resistant to decomposition) in African savannas (18). Second – and most importantly – land megafauna release nutrients otherwise trapped in leaves and stems through their consumption, digestion and excretion (19). As a result, lateral nutrient diffusion rates may be increased by at least one order of magnitude (20) – effects which are particularly important in nutrient-poor soils and low-productivity cold, dry climates. In parallel, marine megafauna are known to bring nutrients up to the ocean’s surface through defecation and hydrological mixing (21). Fascinatingly, with whales moving nutrients up to the sea surface, migrating fish and birds from the ocean to the land, and terrestrial megafauna away from hotspots such as river banks to the continental interior, megafauna may be key actors in a vast, interconnected nutrient pump reversing the otherwise entropic flow of nutrients from continents to oceanic sediments – all actors of which have been revealed to be in undeniable decline (22–24).
Regional and global climate. Pleistocene megafaunal loss at high latitudes led to a decrease in tree cover, making way for vast expanses of grasslands and highly reflective white snow. By virtue of a more uniform, light surface, these grasslands have a higher albedo, reflecting a greater fraction of incident solar energy, while, thanks to their deep roots, capturing vast volumes of carbon (25) – leading to a net global warming effect in response to megafaunal decline (26,27).
Megafauna rewilding as a strategy for functional ecosystem restoration
A new appreciation of the significant role of megafauna in ecosystems has galvanized the first attempts to model these terrestrial (20,28) and oceanic processes (22,29). Now explicitly quantitatively assessing megafaunal effects in Earth systems, new models will continue to deepen our ever-growing understanding of how megafaunal loss impacts global ecosystems and climate change (30), informing the increasingly evident benefits of megafaunal restoration in future ecosystems. Reintroducing key megafaunal species into their native ecosystems through megafaunal rewilding is a creative, empirically grounded ecological restoration strategy to restore the healthy trophic interactions embedded within inherently wise, self-regulating biodiverse ecosystems (31). Recently, the widespread re-expansion of large carnivores across North America and Europe has provided clear evidence in support of productive, sustainable human–megafauna coexistence at a global scale (32).
Regions of past and ongoing megafaunal decline have suffered from a catastrophic cascade of consequences in ecosystem structure, vegetation composition, and nutrient and energy flow: To re-establish healthy, vibrant and sustainable ecosystems, the time is now to reintroduce and ensure the sustainability of our charismatic, beloved megafauna.
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