Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation

Abstract

Until recently in Earth history, very large herbivores (mammoths, ground sloths, diprotodons, and many others) occurred in most of the World's terrestrial ecosystems, but the majority have gone extinct as part of the late-Quaternary extinctions. How has this large-scale removal of large herbivores affected landscape structure and ecosystem functioning? In this review, we combine paleo-data with information from modern exclosure experiments to assess the impact of large herbivores (and their disappearance) on woody species, landscape structure, and ecosystem functions. In modern landscapes characterized by intense herbivory, woody plants can persist by defending themselves or by association with defended species, can persist by growing in places that are physically inaccessible to herbivores, or can persist where high predator activity limits foraging by herbivores. At the landscape scale, different herbivore densities and assemblages may result in dynamic gradients in woody cover. The late-Quaternary extinctions were natural experiments in large-herbivore removal; the paleoecological record shows evidence of widespread changes in community composition and ecosystem structure and function, consistent with modern exclosure experiments. We propose a conceptual framework that describes the impact of large herbivores on woody plant abundance mediated by herbivore diversity and density, predicting that herbivore suppression of woody plants is strongest where herbivore diversity is high. We conclude that the decline of large herbivores induces major alterations in landscape structure and ecosystem functions.browsers | ecosystem functions | herbivore diversity | landscape structure | megaherbivore During the late Quaternary, megafaunas were drastically reduced in most regions (1, 2), representing the start of an ongoing trophic downgrading that has resulted in the loss of entire functional guilds and relaxation of top-down control in today's ecosystems (3). A high proportion of the large herbivores that have survived into the Anthropocene (4) are now drastically reduced in range and abundance, rendering them functionally extinct, or have been replaced by livestock in much of their historic ranges (5-8). How has this loss of wild-living large herbivores affected landscape structure and ecosystem functioning?Contemporary large herbivores have strong effects on the abundance of woody species, plant diversity, nutrient cycling, and other biota (9). Most likely, the ecological effects of preextinction herbivores were as large, possibly much more so given the great size and diversity of the lost large herbivore assemblages (10, 11). We hypothesize that Pleistocene herbivore assemblages, including large and megaherbivore browsers, would have greatly reduced woody plant abundance and altered species composition and landscape structure, if present at sufficient densities. We review the impact of large herbivores (≥45 kg in body weight) on woody vegetation, with a focus on megaherbivores (≥1,000 kg), and combine information from ...

Megafauna and ecosystem function from the Pleistocene to the Anthropocene

“…A range of tools have been applied to tackle this topic, including ecological observations and experimental manipulations in the remaining megafauna-rich regions (18,49). The impacts of past extinctions have also been explored through paleoecological studies using pollen grains, charcoal particles, and beetle communities as indicators of ecosystem structure (14).…”

“…The most immediate and direct environmental effect of terrestrial megafauna is on the physical structure of ecosystems. Bakker et al (18) review the various ways in which megafauna shape ecosystem structure. Megafaunal browsers, and proboscideans in particular, are shapers of habitat structure through destruction of vegetation, through either their high rates of consumption or through breakage and trampling, and also by reducing productivity through damage to plants.…”

“…Exclosure studies and new extensive airborne lidar surveys in African savannas reveal that megafauna, especially elephants, reduce woody species cover by 15-95% (18,49,52), the extent depending on body size, feeding mode, and local environmental factors. African bush elephants in Kruger National Park uproot up to 1,500 adult trees per elephant per year (52).…”

“…Only recently has work begun to address the environmental consequences of this dramatic transition from a megafaunal to a nonmegafaunal world on Earth's ecology, as manifested through vegetation cover (16), plant-animal interactions (17), ecosystem structure (16,18), trophic interactions (7), fire regimes (19), biogeochemical cycling (20), and climate (21,22).…”

Spiny plants, mammal browsers, and the origin of African savannas

“…He argued that increased forest cover from the last glacial to interglacial conditions was partly the result of extinction of the Pleistocene fauna (13). Many experimental and observational studies have shown that mammals have the capacity to create open ecosystems by reducing tree biomass whether in the tropics or temperate and boreal regions (14,15). Although Owen-Smith (13) emphasized megaherbivores (animals >1,000 kg) as primarily responsible for open habitats, mesobrowsers (4-450 kg), such as deer, antelope, and caprids, are very effective at preventing woody plants from escaping the "browse trap" and growing into larger size classes.…”