travel, and burgeoning numbers of automobiles,
personal computers, and electronic devices are
only a few areas in which the impacts from consumer demand for food, energy, materials, and
infrastructure are poised to escalate. The global
consumer society emerging in our time invites recognition that stabilizing and eventually reducing
the global population is crucial for lowering total
consumption. Reducing excessive per-person consumption, along with other shifts and policies, is
also needed (see below).
The second way of rethinking the population
question is in conjunction with the requirements of
biodiversity. Until recently, the question of whether
and to what extent population growth is sustainable has tended to overlook ecological constraints
for avoiding extinctions and for sustaining wild species at healthy levels of abundance, metapopulation
structure, and dispersal or migratory needs (14, 21).
If protection of Earth’s remaining species, genetic
heritage, and natural ecosystems were to be included as a constraining factor of human development, the question of how many people can
sustainably inhabit the planet would appear in a
new light. Denials of identifiable limits to human
numbers (or affirmations of their great elasticity)
typically ignore the fact that extending humanity’s
carrying capacity has succeeded by means of usurping resources from other species. For example, the
transformation of the American plains for food production wiped out 99% of the grasslands biome
along with the great diversity of plants, animals, and
other organisms that constituted it (60). Looking
toward the future, conservation scientists contend
that if humanity wishes to conserve Earth’s biodiversity, protecting large areas of terrestrial and
marine habitats is needed (61, 62). Such large-scale
protection of nature from intensified agriculture,
industrial fishing, road building, and other high-impact development—if embraced—would imply
limits to how many people, at an equitable standard of living, a biodiverse Earth can support.
Some may object that biodiversity protection is
not a universal value, and therefore should not
be included as a constraining factor of development and population size. Contrary to such an
objection, averting the anthropogenic mass extinction now under way and conserving natural ecosystems, species, healthy populations of biota,
and robust ecological and evolutionary processes
are needed to ensure a better and safer future.
Whether people value the natural world for its
intrinsic standing or for the ecological services it
provides humanity (e.g., food, clean water, climate
regulation, crop pollination, recreational spaces),
sustaining Earth’s biological wealth is an ecumenical
good (63, 64). Addressing catastrophic biodiversity
decline is therefore an inescapable responsibility.
Undertaking this responsibility requires that
a sufficient amount of land and ocean be protected
for the livelihood of other species (61, 62, 65, 66).
Such division of Earth’s sources of livelihood, to
facilitate both humanity’s and nature’s well-being,
has led to proposals of a sustainable human population ranging between 1.5 billion and 5 billion
people (depending on per-person levels of consumption assumed) (67). Daily et al. estimated a
global population of roughly 2 billion as more
“optimum”—where optimum means human numbers that can accommodate large-scale nature
protection and secure a high-quality life for all
people, while sustaining a globally interconnected
What a sustainable population on a biodiverse planet will be is difficult to foresee, for it
depends on the standard of living that future
people will choose, the food system and diet they
will gravitate toward, and technological developments that are difficult if not impossible to predict.
But from the present vantage point of a mass
extinction event (2), it is clear that consumption
levels that can accommodate conserving Earth’s
biological wealth have been exceeded. What makes
the case starkly is the impact of food production—
the consumption activity most directly linked
with human numbers.
Food production and biodiversity
The impact of food production on biodiversity
affects every system of the planet. Land conversion
for crop and animal agriculture is the chief driver
of habitat loss, which, along with direct killing,
continues to be the leading threat to biodiversity
(48, 52, 69) (Fig. 3). Of the freshwater resources
appropriated for human use, 80% is claimed by
agriculture (4). At least one-fifth of anthropogenic
greenhouse gases are attributed to the food system
(70, 71). Agriculture is also largely responsible for
the world’s 400 dead zones, which have been
increasing in number and extent since the 1960s
(72), and agricultural pollution also affects freshwater systems worldwide (40).
Many ecosystems and biomes have fallen to
food production. Temperate grasslands are among
the hardest hit, with habitat conversion exceeding
habitat protection by a ratio of 8:1 (73). More than
half of the world’s species-rich wetlands have been
drained over the past century, largely for repurposing into agriculture (74, 75). Aquaculture operations
are driving mangrove declines (76). Freshwater biodiversity is endangered worldwide, with an estimated 10,000 to 20,000 freshwater species at risk of
extinction; river biodiversity is most threatened in
regions of intensive agriculture and dense settlement (77). Most tropical deforestation since the
1980s is due to the expansion of plantations and
ranches (50, 78). Coastal seas are critically endangered and continental shelves are endangered primarily as a result of overfishing (79), while trawlers
have additionally turned to seamount habitats (80).
Many commercial fisheries are depleted, most big
fish are gone, and large-scale aquaculture and livestock operations contribute to the depletion of the
ocean’s small fish for feed (80–86). Scientists recently warned of a high risk of extinction of large
ocean animals, with the chief culprit being industrial fishing (87 ). On land, steep population declines
of big herbivores and carnivores are also linked to
crop and animal agriculture (88–91). Indeed, the
two least disturbed biomes on Earth—boreal forests
and tundra (73)—are, tellingly, two biomes where
large-scale food production does not occur.
The adverse impact of livestock on the biosphere
has been extensively documented (70, 92–95).
Many people in the developed world could eat
fewer animal products, and advocacy on that front
is valuable, but for the foreseeable future the trends
point toward increasing consumption of meat, fish,
dairy, and eggs, especially in the developing world.
At the same time, so-called “landless” livestock operations (concentrated animal feeding operations)
are not a solution. Most cereal croplands in the
developed world are devoted to feed production,
whereas the livestock facilities themselves are
resource-intensive (especially in water use); polluting of air, groundwater, and waterways; vulnerable to diseases and epidemics; and ethically unjust
in the treatment of farm animals (95–97).
The intent of sustainable intensification is laudable, but we argue that its reasoning is flawed in
two ways: (i) in its apparent acceptance of the
Fig. 2. World population growth in the 21st century: Four projections indicating range of
possibilities and what is “80 percent probable.” Source: (34).