Megafauna and ecosystem function from the Pleistocene to the Anthropocene

Abstract

Large herbivores and carnivores (the megafauna) have been in a state of decline and extinction since the Late Pleistocene, both on land and more recently in the oceans. Much has been written on the timing and causes of these declines, but only recently has scientific attention focused on the consequences of these declines for ecosystem function. Here, we review progress in our understanding of how megafauna affect ecosystem physical and trophic structure, species composition, biogeochemistry, and climate, drawing on special features of PNAS and Ecography that have been published as a result of an international workshop on this topic held in Oxford in 2014. Insights emerging from this work have consequences for our understanding of changes in biosphere function since the Late Pleistocene and of the functioning of contemporary ecosystems, as well as offering a rationale and framework for scientifically informed restoration of megafaunal function where possible and appropriate.For hundreds of millions of years, an abundance of large animals, the megafauna, was a prominent feature of the land and oceans. However, in the last few tens of thousands of years-a blink of an eye on many evolutionary and biogeochemical timescales-something dramatic happened to Earth's ecology; megafauna largely disappeared from vast areas, rendered either actually or functionally extinct (1, 2). Only in small parts of the world do megafauna exist at diversities anything close to their previous state, and, in many of these remaining regions, they are in a state of functional decline through population depletion and range contraction. In the oceans, a similar process has occurred over the last few hundred years: although there has been little absolute extinction, there has been a dramatic decline in the abundance of whales and large fish through overharvesting (3). Both on land and in oceans, declines continue today (4-7).Homo sapiens evolved and dispersed in a world teeming with giant creatures. Our earliest art forms, such as the haunting and mesmerizing Late Pleistocene cave paintings of Lascaux and Altamira, show that megafauna had a profound impact on the psyche and spirituality of our ancestors. To humans past and modern, they indicate resources, danger, power, and charisma, but, beyond these impacts, such large animals have profound and distinct effects on the nature and functioning of the ecosystems they inhabit.Martin (8) first posited a major human role in past megafaunal disappearances, and, since then, much has been written on their patterns and causes and the relative importance of human effects, climate change, and other factors (8)(9)(10)(11)(12)(13)(14)(15). 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).In this pap...

Many shades of green: the dynamic tropical forest–savannah transition zones

“…Decreasing atmospheric CO 2 and increasing daytime water stress favoured the C 4 photosynthetic pathway, which is physiologically competitive but metabolically expensive so cannot be sustained in shady conditions [25]. The opening up of forests into predominantly C 3 grasses occurred in the Early-Middle Miocene (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) [24,25]. As C 3 grasses do not have an intrinsic photosynthetic advantage over C 3 trees, this transition is unlikely to have been triggered by atmospheric CO 2 falling below a critical threshold value.…”

What North America's skeleton crew of megafauna tells us about community disassembly

“…All rights reserved. excellent natural experiment for examining massive community disassembly on a continental scale because a high proportion of large mammal species went extinct, probably due to the same stressors impacting vulnerable species today: rapid climate change and growing human populations [21]. Although the still debated causes of this extinction have received the most attention historically, researchers now recognize the important ecological consequences of megafaunal disassembly, which included major changes in vegetation cover, nutrient cycling, and possibly atmospheric composition [21,22].…”

“…There are few direct investigations of this relationship but because certain traits like large body size likely confer unique functional roles (e.g. ability to disperse larger seeds, higher trophic levels) and are known to increase extinction risk across a wide range of taxa, functional losses during disassembly should be greater than expected given the proportion of taxa going extinct [14,21]. If effect and response traits are not correlated, then functional diversity change during a real extinction should be indistinguishable from change given any equal drop in taxonomic richness, regardless of the species, something that is not expected by theory [5] but has been found in some habitat fragmentation studies [16].…”

The Plight of Reptiles as Ecological Actors in the Tropics

“…Since body size is such an important factor in species interactiveness, this misconception must be addressed: (i) several living giant snake species such as anacondas and pythons (e.g., Eunectes notaeus, E. murinus, Python molurus, P. reticulatus, P. sebae) occur in mainland environments, where they successfully compete with mammalian carnivores; (ii) Komodo dragons were formerly widespread over Sahul (Hocknull et al, 2009), where they competed with Thylacoleo carnifex, an extinct 100 kg-plus marsupial carnivore (Wroe et al, 1999), so that their current distribution is a misleading relict; (iii) other species of large lizards occur on mainland Asia, such as the water monitor, a species that exceeds 20 kg, which is smaller than only two of southeast Asia's largest mammalian predators, the tiger (Panthera tigris, 227 kg) and the leopard (Panthera pardus, 42 kg; Corlett, 2011); (iv) in the early Pleistocene, southeast Asia had giant species of monitors like Varanus sivalensis that coexisted with carnivore faunas similar to current ones (Hocknull et al, 2009). Despite this, reptiles are often completely ignored as predators in both present (Jorge and Galetti, 2013), and past ecological communities (Malhia et al, 2016). As suggested from this brief list, the popular notion of food chain apices occupied solely by mammalian carnivores must be reassessed in tropical ecosystems.…”