REVIEW
Defaunation in the Anthropocene
Rodolfo Dirzo,1 Hillary S. Young,2 Mauro Galetti,3 Gerardo Ceballos,4
Nick J. B. Isaac,5 Ben Collen6
We live amid a global wave of anthropogenically driven biodiversity loss: species
and population extirpations and, critically, declines in local species abundance.
Particularly, human impacts on animal biodiversity are an under-recognized form of
global environmental change. Among terrestrial vertebrates, 322 species have
become extinct since 1500, and populations of the remaining species show 25%
average decline in abundance. Invertebrate patterns are equally dire: 67% of
monitored populations show 45% mean abundance decline. Such animal declines
will cascade onto ecosystem functioning and human well-being. Much remains unknown
about this “Anthropocene defaunation”; these knowledge gaps hinder our capacity
to predict and limit defaunation impacts. Clearly, however, defaunation is both a
pervasive component of the planet’s sixth mass extinction and also a major driver of
global ecological change.
In the past 500 years, humans have triggered a wave of extinction, threat, and local popu- lation declines that may be comparable in both rate and magnitude with the five previous mass extinctions of Earth’s history (1). Similar
to other mass extinction events, the effects of this
“sixth extinction wave” extend across taxonomic
groups, but they are also selective, with some taxonomic groups and regions being particularly
affected (2). Here, we review the patterns and consequences of contemporary anthropogenic impact
on terrestrial animals. We aim to portray the scope
and nature of declines of both species and abundance of individuals and examine the consequences
of these declines. So profound is this problem that
we have applied the term “defaunation” to describe
it. This recent pulse of animal loss, hereafter referred to as the Anthropocene defaunation, is not
only a conspicuous consequence of human impacts
on the planet but also a primary driver of global
environmental change in its own right. In comparison, we highlight the profound ecological impacts
of the much more limited extinctions, predominantly of larger vertebrates, that occurred during
the end of the last Ice Age. These extinctions altered ecosystem processes and disturbance regimes
at continental scales, triggering cascades of extinction thought to still reverberate today (3, 4).
The term defaunation, used to denote the
loss of both species and populations of wildlife
(5), as well as local declines in abundance of
individuals, needs to be considered in the same
sense as deforestation, a term that is now read-
ily recognized and influential in focusing scien-
tific and general public attention on biodiversity
issues (5). However, although remote sensing
technology provides rigorous quantitative in-
formation and compelling images of the mag-
nitude, rapidity, and extent of patterns of
deforestation, defaunation remains a largely
cryptic phenomenon. It can occur even in large
protected habitats (6), and yet, some animal
species are able to persist in highly modified
habitats, making it difficult to quantify without
intensive surveys.
Analyses of the impacts of global biodiversity
loss typically base their conclusions on data derived from species extinctions (1, 7, 8), and typically, evaluations of the effects of biodiversity
loss draw heavily from small-scale manipulations
of plants and small sedentary consumers (9). Both
of these approaches likely underestimate the full
impacts of biodiversity loss. Although species extinctions are of great evolutionary importance,
declines in the number of individuals in local
populations and changes in the composition of
species in a community will generally cause greater
immediate impacts on ecosystem function (8, 10).
Moreover, whereas the extinction of a species often
proceeds slowly (11), abundance declines within
populations to functionally extinct levels can occur rapidly (2, 12). Actual extinction events are
also hard to discern, and International Union for
Conservation of Nature (IUCN) threat categories
amalgamate symptoms of high risk, conflating
declining population and small populations so that
counts of threatened species do not necessarily
translate into extinction risk, much less ecological
impact (13). Although the magnitude and frequency of extinction events remain a potent way of
communicating conservation issues, they are only
a small part of the actual loss of biodiversity (14).
The Anthropocene defaunation process
Defaunation: A pervasive phenomenon
Of a conservatively estimated 5 million to 9 mil-
lion animal species on the planet, we are likely
losing ~11,000 to 58,000 species annually (15, 16).
However, this does not consider population ex-
tirpations and declines in animal abundance
within populations.
Across vertebrates, 16 to 33% of all species
are estimated to be globally threatened or endangered (17, 18), and at least 322 vertebrate
species have become extinct since 1500 (a date
representative of onset of the recent wave of extinction; formal definition of the start of the
Anthropocene is still being debated) (table S1)
(17, 19, 20). From an abundance perspective,
vertebrate data indicate a mean decline of 28%
in number of individuals across species in the
past four decades (fig. S1, A and B) (14, 21, 22),
with populations of many iconic species such
as elephant rapidly declining toward extinction (19).
Loss of invertebrate biodiversity has received
much less attention, and data are extremely
limited. However, data suggest that the rates of
decline in numbers, species extinction, and range
contraction among terrestrial invertebrates are
at least as severe as among vertebrates (23, 24).
Although less than 1% of the 1.4 million described invertebrate species have been assessed
for threat by the IUCN, of those assessed, ~40%
are considered threatened (17, 23, 24). Similarly,
IUCN data on the status of 203 insect species in
five orders reveal vastly more species in decline
than increasing (Fig. 1A). Likewise, for the invertebrates for which trends have been evaluated
in Europe, there is a much higher proportion of
species with numbers decreasing rather than
increasing (23). Long-term distribution data on
moths and four other insect orders in the UK
show that a substantial proportion of species
have experienced severe range declines in the
past several decades (Fig. 1B) (19, 25). Globally,
long-term monitoring data on a sample of 452
invertebrate species indicate that there has been
an overall decline in abundance of individuals
since 1970 (Fig. 1C) (19). Focusing on just the
Lepidoptera (butterflies and moths), for which
the best data are available, there is strong evidence of declines in abundance globally (35%
over 40 years) (Fig. 1C). Non-Lepidopteran invertebrates declined considerably more, indicating that estimates of decline of invertebrates
based on Lepidoptera data alone are conservative (Fig. 1C) (19). Likewise, among pairs of
disturbed and undisturbed sites globally, Lepidopteran species richness is on average 7.6
times higher in undisturbed than disturbed
sites, and total abundance is 1.6 times greater
(Fig. 1D) (19).
Patterns of defaunation
Although we are beginning to understand the
patterns of species loss, we still have a limited
understanding of how compositional changes in
communities after defaunation and associated
disturbance will affect phylogenetic community
structure and phylogenetic diversity (26). Certain
lineages appear to be particularly susceptible to
human impact. For instance, among vertebrates,
more amphibians (41%) are currently considered
1Department of Biology, Stanford University, Stanford, CA
94305, USA. 2Department of Ecology, Evolution, and Marine
Biology, University of California Santa Barbara, Santa
Barbara, CA 93106, USA. 3Departamento de Ecologia,
Universidade Estadual Paulista, Rio Claro, SP, 13506-900,
Brazil. 4Instituto de Ecología, Universidad Nacional Autónoma
de México, AP 70-275, México D.F. 04510, Mexico. 5Natural
Environment Research Council (NERC) Centre for Ecology
and Hydrology, Benson Lane, Crowmarsh Gifford,
Oxfordshire, OX10 8BB, UK. 6Centre for Biodiversity and
Environment Research, Department of Genetics, Evolution
and Environment, University College London, Gower Street,
London WC1E 6BT, UK.
*Corresponding author. E-mail: rdirzo@stanford.edu
