Pleistocene megafaunal interaction networks became more vulnerable after human arrival

Abstract

The end of the Pleistocene was marked by the extinction of almost all large land mammals worldwide except in Africa. Although the debate on Pleistocene extinctions has focused on the roles of climate change and humans, the impact of perturbations depends on properties of ecological communities, such as species composition and the organization of ecological interactions. Here, we combined palaeoecological and ecological data, food-web models and community stability analysis to investigate if differences between Pleistocene and modern mammalian assemblages help us understand why the megafauna died out in the Americas while persisting in Africa. We show Pleistocene and modern assemblages share similar network topology, but differences in richness and body size distributions made Pleistocene communities significantly more vulnerable to the effects of human arrival. The structural changes promoted by humans in Pleistocene networks would have increased the likelihood of unstable dynamics, which may favour extinction cascades in communities facing extrinsic perturbations. Our findings suggest that the basic aspects of the organization of ecological communities may have played an important role in major extinction events in the past. Knowledge of community-level properties and their consequences to dynamics may be critical to understand past and future extinctions.

Exceptional body sizes but typical trophic structure in a Pleistocene food web

“…We analysed body size and feeding habits in Pleistocene mammal species found in Uruguay and Buenos Aires Province, Argentina, possibly conforming interacting communities [13,14,[16][17][18]. Detailed information on feeding habitats is not available for Pleistocene mammals, thus species were classified according to their feeding habits into two trophic positions: carnivorous or non-carnivorous.…”

“…Specifically, we analyse the relationships between body size, and (i) the probability of being carnivorous and (ii) the trend in species vulnerability to predation. However, the existence of larger predators than those found in modern fauna, such as sabre-toothed cats, also suggests that the predation rate on Pleistocene herbivores was similar to contemporary rates [13]. Indeed, it has been argued that large mammals were subject to heavy predator control [19].…”

“…The South American Pleistocene megafauna, a singular array of numerous giant mammals [11], is interesting from a food web perspective [13,14]. As these species became larger, their diet richness and range of prey body sizes consistently increased, as did their trophic positions [3,6,15].…”

“…The central role of body size on food web structure sheds light on the processes that moulded extinct faunas, in which community size structure strongly departed from contemporary examples [11], even though ancient and modern faunas share remarkable similarities [12]. Nevertheless, exceptionally large body sizes of extinct species such as those observed in Pleistocene mammals could give rise to unusual interaction patterns [13].…”

“…Thus, species of intermediate body size are expected to achieve higher trophic positions. Here we aim at elucidating diet -body size relationships and their ecological consequences for that peculiar megafauna [13] with lack of modern counterparts [16] and potential existence of a disproportionately low number of predators ( [16][17][18], but see [19]), based on their exceptional body sizes and the emerging principles of trophic interactions observed in modern biotas. Specifically, we analyse the relationships between body size, and (i) the probability of being carnivorous and (ii) the trend in species vulnerability to predation.…”