Defaunation in the Anthropocene
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Rodolfo Dirzo
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, Hillary S Young
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, Mauro Galetti
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, Gerardo Ceballos
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, Nick JB Isaac
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,
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Ben Collen
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1
Department of Biology, Stanford University, Stanford, CA 94305, USA
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University of California Santa Barbara, Santa Barbara, CA 93106, USA
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Departamento de Ecologia, Universidade Estadual Paulista, Rio Claro, SP, 13506-900,
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Brazil
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Instituto de Ecología, Universidad Nacional Autonoma de Mexico, AP 70-275, Mexico
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D.F. 04510, Mexico
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NERC Centre for Ecology & Hydrology, Benson Lane, Crowmarsh Gifford,
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Oxfordshire, OX10 8BB, UK
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Centre for Biodiversity & Environment Research, Department of Genetics, Evolution &
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Environment, University College London, Gower Street, London WC1E 6BT, UK
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To whom correspondence should be addressed. E-mail: rdirzo@stanford.edu
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Word count: 4807 words, 80 references, 5 figures
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We live amidst a global wave of anthropogenically driven biodiversity loss: species
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and population extirpations and, critically, declines in local species abundance. Human
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impacts on animal biodiversity, particularly, are an under-recognized form of global
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environmental change. Among terrestrial vertebrates 322 species have become
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extinct since 1500, while populations of the remaining species show 25% average
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decline in abundance. Invertebrate patterns are equally dire: 67% of monitored
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populations show 45% mean abundance decline. Such animal declines will cascade
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onto ecosystem functioning and human well-being. Much remains unknown about
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this “Anthropocene defaunation”; these knowledge gaps hinder our capacity to
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predict and limit defaunation impacts. Clearly, however, defaunation is both a
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pervasive component of the planet’s sixth mass extinction, and also a major driver of
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global ecological change.
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In the past 500 years, humans have triggered a wave of extinction, threat, and local
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population declines that may be comparable in both rate and magnitude to the five
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previous mass extinctions of Earth’s history (1). Similar to other mass extinction events,
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the effects of this “sixth extinction wave” extend across taxonomic groups, but are also
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selective, with some taxonomic groups and regions being particularly affected (2). Here,
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we review the patterns and consequences of contemporary anthropogenic impact on
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terrestrial animals. We aim to portray the scope and nature of declines of both species and
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abundance of individuals, and examine the consequences of these declines. So profound
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is this problem, that we have applied the term defaunation to describe it. This recent pulse
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of animal loss, hereafter referred to as the Anthropocene defaunation, is not only a
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conspicuous consequence of human impacts on the planet, but also a primary driver of
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global environmental change in its own right. In comparison, we highlight the profound
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ecological impacts of the much more limited extinctions, predominantly of larger
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vertebrates, that occurred during the end of the last Ice Age. These extinctions altered
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ecosystem processes and disturbance regimes at continental scales, triggering cascades of
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extinction thought to still reverberate today (3, 4).
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The term defaunation, used to denote the loss of both species and populations of
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wildlife (5), as well as local declines in abundance of individuals, needs to be considered
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in the same sense as deforestation, a term that is now readily recognized and influential in
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focusing scientific and general public attention on biodiversity issues (5). However,
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whilst remote sensing technology provides rigorous quantitative information and
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compelling images of the magnitude, rapidity and extent of patterns of deforestation,
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defaunation remains a largely cryptic phenomenon. It can occur even in large protected
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habitats (6) and, yet, some animal species are able to persist in highly modified habitats,
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making it difficult to quantify without intensive surveys.
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Analyses of the impacts of global biodiversity loss typically base their
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conclusions on data derived from species extinctions (1, 7, 8) and typically evaluations of
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the effects of biodiversity loss draw heavily from small scale manipulations of plants and
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small sedentary consumers (9). Both of these approaches likely underestimate the full
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impacts of biodiversity loss. While species extinctions are of great evolutionary
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significance, declines in the number of individuals in local populations and changes in the
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composition of species in a community will generally cause greater immediate impacts
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on ecosystem function (8, 10). Moreover, while the extinction of a species often proceeds
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slowly (11), abundance declines within populations to functionally extinct levels can
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occur rapidly (2, 12). Actual extinction events are also hard to discern, and IUCN threat
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categories amalgamate symptoms of high risk, conflating declining population and small
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populations, such that counts of threatened species do not necessarily translate into
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extinction risk, much less ecological impact (13). Whilst the magnitude and frequency of
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extinction events remain a potent way of communicating conservation issues, they are
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only a small part of the actual loss of biodiversity (14).
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The Anthropocene Defaunation Process
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Defaunation: a pervasive phenomenon
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Of a conservatively estimated 5-9 million animal species on the planet, we are likely
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losing ~11,000 to 58,000 species annually (15, 16). However, this does not consider
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population extirpations and declines in animal abundance within populations.
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Across vertebrates, 16% to 33% of all species are estimated to be globally
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threatened or endangered (17, 18), and at least 322 vertebrate species have become
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extinct since 1500 (a date representative of onset of the recent wave of extinction, as
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formal definition of the start of the Anthropocene still being debated) (17, 19, 20) (Table
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S1). From an abundance perspective, vertebrate data indicate a mean decline of 28% in
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number of individuals across species in the last four decades (14, 21, 22) (Fig S1A), with
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populations of many iconic species such as elephant (Fig S1B) rapidly declining towards
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extinction (19).
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Loss of invertebrate biodiversity has received much less attention and data are
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extremely limited. However, data suggest that the rates of decline in numbers, species
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extinction, and range contraction among terrestrial invertebrates are at least as severe as
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among vertebrates (23, 24). Although less than 1% of the 1.4 million described
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invertebrate species have been assessed for threat by the IUCN, of those assessed, around
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40% are considered threatened (17, 23, 24). Similarly, IUCN data on the status of 203
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insect species in five orders reveals vastly more species in decline than increasing (Fig
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1A). Likewise, for the invertebrates where trends have been evaluated in Europe, there is
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a much higher proportion of species with numbers decreasing rather than increasing (23).
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Long term distribution data on moths and four other insect Orders in the UK show that a
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substantial proportion of species have experienced severe range declines in the last
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several decades (19, 25) (Fig 1B). Globally, long-term monitoring data on a sample of
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452 invertebrate species indicate that there has been an overall decline in abundance of
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individuals since 1970 (19) (Fig 1C). Focusing on just the Lepidoptera (butterflies and
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moths), for which the best data are available, there is strong evidence of declines in
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abundance globally (35% over 40 years, Fig 1C). Non-Lepidopteran invertebrates
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declined significantly more, indicating that estimates of decline of invertebrates based on
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Lepidoptera data alone are conservative (19) (Fig 1C). Likewise, among pairs of
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disturbed and undisturbed sites globally, Lepidopteran species richness is on average 7.6
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times higher in undisturbed than disturbed sites, and total abundance is 1.6 times greater
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(19) (Fig 1D).
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Patterns of defaunation
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Though we are beginning to understand the patterns of species loss, we still have a
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limited understanding of how compositional changes in communities following
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