Agricultural expansion; crops HIstory +3 3rd Semester
Introduction
The recent intensification of agriculture, and the prospects of future
intensification, will have major detrimental impacts on the nonagricultural
terrestrial and aquatic ecosystems of the world. The doubling of agricultural
food production during the past 35 years was associated with a 6.87-fold
increase in nitrogen fertilization, a 3.48-fold increase in phosphorus
fertilization, a 1.68-fold increase in the amount of irrigated cropland, and a
1.1-fold increase in land in cultivation. Based on a simple linear extension of
past trends, the anticipated next doubling of global food production would be
associated with approximately 3-fold increases in nitrogen and phosphorus
fertilization rates, a doubling of the irrigated land area, and an 18% increase
in cropland. These projected changes would have dramatic impacts on the
diversity, composition, and functioning of the remaining natural ecosystems of
the world, and on their ability to provide society with a variety of essential
ecosystem services. The largest impacts would be on freshwater and marine
ecosystems, which would be greatly eutrophied by high rates of nitrogen and
phosphorus release from agricultural fields. Aquatic nutrient eutrophication
can lead to loss of biodiversity, outbreaks of nuisance species, shifts in the
structure of food chains, and impairment of fisheries. Because of aerial
redistribution of various forms of nitrogen, agricultural intensification also
would eutrophy many natural terrestrial ecosystems and contribute to
atmospheric accumulation of greenhouse gases. These detrimental environmental
impacts of agriculture can be minimized only if there is much more efficient
use and recycling of nitrogen and phosphorus in agroecosystems.
The
agricultural achievements of the past 35 years have been impressive. Grain
production, mainly from wheat, rice, and maize, has increased at a rate greater
than human population. This has decreased the number of malnourished people
even as the earth’s human population doubled to 5.8 billion. Although the
estimates vary widely, world population is projected to increase about 75%
before leveling off at about 10 billion. In combination with increasing demand for
meat in developing countries and the use of grains as livestock feed, this
increased population density should cause world demand for grain production to
more than double. This raises several important questions. If it is possible
for world food production to double, again, within the next four or five
decades, what impacts would this doubling have on the functioning of the
nonagricultural ecosystems of the world, and on the services they provide to
humanity? What routes might be used to decrease such impacts? I explore these
questions first by asking what the global ecological impacts of “more of the
same” agriculture might be, and then by considering practices that might
decrease such impacts. In particular, insights are sought in the parallels
between natural and agricultural ecosystems, but no easy answers are uncovered.
Rather, a new long-term, multidisciplinary research program is needed to
develop agricultural methods that can feed a growing world and still preserve
the vital services provided to humanity by the world’s natural ecosystems.
Current agricultural practices involve deliberately maintaining
ecosystems in a highly simplified, disturbed, and nutrient-rich state. To
maximize crop yields, crop plant varieties are carefully selected to match
local growing conditions. Limiting factors, especially water, mineral nitrogen,
and mineral phosphate, are supplied in excess, and pests are actively
controlled. These three features of modern agriculture—control of crops and
their genetics, of soil fertility via chemical fertilization and irrigation,
and of pests (weeds, insects, and pathogens) via chemical pesticides—are the
hallmarks of the green revolution. They have caused four once-rare plants (barley,
maize, rice, and wheat) to become the dominant plants on earth as humans became
the dominant animal. Indeed, these four annual grasses now occupy,
respectively, 67 million hectares, 140 million hectares, 151 million hectares,
and 230 million hectares, each, worldwide, which is 39.8% of global cropland.
For comparison, the total forested area of the United States, including Alaska,
is 298 million hectares. Entire regions of the world now are dominated by
virtual monocultures of a given crop. These monocultures have replaced natural
ecosystems that once contained hundreds to even thousands of plant species,
thousands of insect species, and many species of vertebrates. Thus, agriculture
has caused a significant simplification and homogenization of the world’s
ecosystems.
The Ecology of Doubling Crop Production
The Food and Agriculture Organization (FAO)
database provides a wealth of information on agricultural activities for
individual nations, regions, and the world from 1961 to the present. Using the
FAO data, let’s look at the pattern of world food production during this period
and the factors that allowed it to almost double. The majority of the food
crops grown on the arable lands of the earth are cereals (barley, maize, rice,
and wheat), coarse grains, and root crops. For convenience, I will call the sum
of these world food production. In 1996, cereals comprised 57% of this total,
coarse grains 25%, and root crops 18%. By using this measure, world food
production, as estimated from the FAO database almost doubled (increased
1.97-fold) from 1961 to 1996 Comparable patterns, and comparable ecological
implications, occur if just cereal production was considered, or if production
for just Europe and the United States, for which better data are available, was
considered.
Ecological Impacts of Doubling Global Food
Production
If these simple extrapolations of past
practices are any indication, doubling global food production will triple the
annual rates of nitrogen and phosphorus release to the globe. Current rates of
agricultural nitrogen production, via both production of fertilizer and
cultivation of legume crops, already approximately equal the natural
(preindustrial) rate of addition of biologically active nitrogen to the globe (4). Point-source releases of
phosphorus are tightly regulated in developed nations because phosphorus is a
major limiting nutrient in aquatic ecosystems and increases in its supply rate
harm water quality and aquatic foodweb structure. A tripling of global
phosphorus supply rates is likely to adversely impact many aquatic ecosystems,
especially those that have significant inputs of eroded agricultural soils or
phosphorus-rich wastes from livestock and poultry. Nitrogen is much more motile
in soil than phosphorus because soil bacteria can convert ammonia to nitrate
and nitrite, which are readily leached from soil Denitrification by bacteria also can convert
nitrate into nitrous oxide, a potent greenhouse gas. In addition, ammonia,
which is both directly applied as fertilizer and created via bacterial
degradation of animal waste and other organic compounds, is highly volatile. It
is transported via air and deposited on other ecosystems with precipitation.
These numerous modes of transport mean that agricultural nitrogen, less than
half of which stays in a field or is harvested with a crop, impacts both
terrestrial and aquatic ecosystems as a eutrophier, and impacts global climate
because of is role as a greenhouse gas. Indeed, there is a direct and
quantitative link between the amounts of nitrogen in the major rivers of the
world and the magnitude of agricultural nitrogen inputs to their watersheds.
Agriculture and the Loss of Ecosystem Services
A doubling of global food production would have major impacts on
the ability of nonagricultural ecosystems to provide services .vital to humanity.
Existing nonagricultural ecosystems provide, at no cost, pure, drinkable water.
In contrast, the groundwater associated with intensive agricultural ecosystems
often contains sufficiently high concentrations of nitrite and nitrates or of
pesticides and their residues as to be unfit for human consumption. Expensive
treatment is required to make it potable. The biodiversity of nonagroecosystems
provides many services to agriculture. For instance, the genetic diversity of
both wild relatives of crop plants and unrelated organisms is used to increase
yields and to reduce impacts of agricultural pests and pathogens. However, the
maintenance of the wild biodiversity needed for future development of crops and
medicines occurs mainly in nonagricultural ecosystems, the very ecosystems
threatened by agricultural expansion and nutrient release. Agriculture depends
on soil fertility, fertility created by the ecosystems destroyed when lands are
converted to agriculture. Especially on sandy soils, the best way to regain soil
fertility lost because of tilling is to allow re-establishment of the native
ecosystems. Many agricultural crops depend on the pollination services provided
by insects, birds, or mammals that live in nearby nonagricultural
ecosystems.Similarly, agricultural crops benefit from biocontrol agents, such
as parasitic and predatory insects, birds, and bats, that live in neighboring
nonagricultural ecosystems and that decrease outbreaks of agricultural pests.
Nonagricultural ecosystems, such as forests on slopes and wetlands, help meter
the release of water into streams and rivers, and thus help in flood control.
If properly managed, natural ecosystems also can produce a sustainable supply
of goods used by society, including timber and fiber, fish, and game.
Ecological Insights into Agricultural Impacts and Sustainability
What might be done to decrease the
environmental impacts of agriculture while maintaining or improving its
productivity, stability, or sustainability? This major challenge will have no
single, easy solution. Partial answers will come from increases in the
precision and efficiency of nutrient and pesticide use, from advances in crop
genetics including advances from biotechnology, and from a variety of
engineering solutions. Some additional insights may come from a consideration
of the principles that govern the functioning of all ecosystems, including
agroecosystems. Ecosystem functioning is known to depend on the traits of the
species ecosystem’s contain (their composition), the number of species they
contain (their species diversity), and the physical conditions they experience,
especially disturbance regimes. A consideration of the principles governing the
impacts of composition, diversity, and disturbance on ecosystems may suggest
ways to decrease impacts of agriculture or to make it more productive, stable,
or sustainable. It is critical to realize that these principles apply within a
given ecosystem type. They describe differences in functioning of otherwise
identical ecosystems that share the same species pool and differ only in which
and how many species they contain. These principles were not derived from, and
do not apply to, comparisons among different ecosystem types, such as cattail
swamps versus prairies, or mangrove versus upland forest, or tropical versus
temperate forests.
Conclusions
A hallmark of modern agriculture is its use of monocultures grown
on fertilized soils. Ecological principles suggest that such monocultures will
be relatively unstable, will have high leaching loss of nutrients, will be
susceptible to invasion by weedy species, and will have high incidences of
diseases and pests—all of which do occur. Although ecological principles may
predict these problems, they do not seem to offer any easy solutions to them.
Agriculture, and society, seem to be facing tough tradeoffs. Agricultural
ecosystems have become incredibly good at producing food, but these increased
yields have environmental costs that cannot be ignored, especially if the rates
of nitrogen and phosphorus fertilization triple and the amount of land
irrigated doubles. The tradition in agriculture has been to maximize production
and minimize the cost of food with little regard to impacts on the environment
and the services it provides to society. As the world enters an era in which
global food production is likely to double, it is critical that agricultural
practices be modified to minimize environmental impacts even though many such
practices are likely to increase the costs of production.
