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Agricultural Environments
Mark Blumler
SUNY-Binghamton
[email protected]
Word Count: 2197
Abstract
Agriculture arguably has been the major cause of ecological and evolutionary
change on the Earth during the 10,000 years since its inception. Much of the earth’s
terrestrial surface is agricultural today. Although originally based on renewable
resources, today it is utterly and unsustainably dependent upon fossil fuels. The
degree to which soil erosion and land degradation have resulted is debated, while
negative impacts on biodiversity seem to be proceeding rapidly.
Main Text
In nature, only certain ants, termites, and beetles practice agriculture; they
farm underground, and have relatively little impact on ecosystems. Humans,
already impacting environment especially through their use of fire, began to farm
about 10,000 years ago. Since then, agriculture arguably has been the major cause
of ecological and evolutionary change on the Earth. Agricultural environments are
discussed here from an ecological and an evolutionary perspective.
Farming began in several regions – the Fertile Crescent, China, Mexico, the
central Andes, and New Guinea - at about the same time. One or two additional
regions, notably the Sahel, developed largely indigenous agriculture somewhat later.
Farming enabled humans to obtain more food per unit area of land than huntergathering, and also allowed craft specialization and technological advances.
Consequently, farmers were able to spread geographically at the expense of huntergatherers, or to incorporate them into their system. The focus initially, and to a
large extent still today, was on fast-growing, mesic, high-yielding species. Annual or
short-lived perennial seed crops characterized most of the early agriculture centers,
though root crops were important in New Guinea and the Andes. The seed crops,
especially grains and legumes, tended to be derived from large-seeded wild
ancestors, possibly because seedlings from large seeds are easier to cultivate with a
primitive technology than plants from small seeds. Over time, there was an
expansion in the types of plants cultivated, and in their uses, to include textiles,
ornament, and today, even biofuel.
The agricultural origin centers tend to be located in the foothills of
mountainous regions, i.e., they have high habitat diversity and hence also high
biodiversity. In addition, there is some evidence that they have high local (alpha)
diversity. The Fertile Crescent and adjacent Mediterranean, for which biodiversity
patterns are best documented, has the highest herbaceous alpha-diversity on Earth.
Farming was inherently an attempt to simplify these ecosystems, to favor a few
staples at the expense of other species. Nonetheless, the origin regions remain
highly diverse today. Many species from those regions, including some crops, are
now serious overseas invaders, while the origin centers themselves have suffered
comparatively little from invading alien species.
Species that underwent domestication often hit the evolutionary jackpot: for
instance, wild emmer, the progenitor of most varieties of wheat, occurred naturally
only in the Fertile Crescent. Today, its domesticated derivatives are grown in huge
populations almost everywhere that agriculture is practiced in the temperate and
subtropical zones, and are even penetrating the tropics. When a species expands its
range so spectacularly, inevitably, other species must undergo range contractions or
even become extinct. The line in “America the Beautiful” about “amber waves of
grain” illustrates the extent to which farming landscapes have taken over and
replaced indigenous ecosystems. Global vegetation maps usually show “potential
natural vegetation” rather than the actual cover; if they were to show the latter,
agricultural plants would dominate over a major portion of the terrestrial regions.
While the domesticates benefited from their mutualistic relationship with
humans, other species were able to take advantage of the newly created agroecosystems, despite our best efforts to control or eliminate them. These unwanted
species, “weeds” and other “pests”, also benefited from agriculture at the expense of
other wild species, presumably. Some weeds became so successful as to overwhelm
the crop, typically as farming spread into a different environment less favorable for
the domesticate, at which point if the weed was edible, farmers began to harvest and
cultivate it (so-called secondary domestication). Meanwhile, the wild progenitor of
a domesticate frequently would evolve weed genotypes, and complex crop-weedwild interactions might develop (Harlan, 1975).
Certain families characteristically benefited from the adoption and spread of
agriculture. Proponents of the “Paleolithic Diet” argue that grains and legumes were
scarcely consumed before agriculture; regardless, they certainly are very important
today. The Cucurbitaceae (melons, squashes, and cucumber), mustard family, and
Solanaceae (potatoes, tomatoes, peppers, and eggplants), provided vegetable crops,
and the Rosaceae supplied a remarkable number of temperate fruits. Some families
were pre-adapted to the combination of fertile conditions and soil disturbance that
characterizes agriculture, and consequently became weeds, crops, or both: the
mustard family, spinach family, buckwheat family, etc. Others were attractive for
their nutritive qualities, such as the rose family.
As farming spread, humans had to transform environments so as to resemble
the origin center from which they were migrating. This is particularly evident for
spread from the Fertile Crescent. There, extreme summer drought favors
dominance by annual plants, and all the early domesticates were annuals. As
agriculture spread northwest into Europe into a region without seasonal drought, it
became necessary to plow to remove the native, perennial vegetation. Since even
the weeds were largely of Fertile Crescent origin, in effect an entire ecosystem was
transplanted. Fertile Crescent agriculture also spread east to India, where wheat,
chickpeas, lentils, peas, and other winter annuals are now major items of the diet.
But India has a monsoon climate, with rain in summer. To grow the Near Eastern
crops, Indian farmers developed the “rabe” system, from the ancient Indo-European
word for mustard, which also is from the Fertile Crescent. They dry fallow in
summer, plowing repeatedly to prevent weeds from utilizing the monsoon
moisture; the crops are sown in fall, and mature primarily on the stored moisture.
In the Old World, farming diffused until it occupied essentially all arable land.
Land that was too infertile, cold, or arid was reserved for pastoralism. Technologies
such as irrigation and terraces were massively developed in order to maximize the
arable land. In contrast, in the Americas population density is still relatively low;
arable land is currently expanding in much of Latin America. In particular,
whitewater portions of the Amazon are seeing massive penetration by peasant
farmers who clear the rain forest with fire. While swidden was long practiced
traditionally, it is unclear how intensively the Amazon was farmed before 1492:
there are indications of dense, permanent populations along the Amazon River,
before Old World diseases struck, and the widespread occurrence of terra preta, a
fertile, dark-colored, anthropogenic soil, also suggests that permanent agriculture
was a feature of the more fertile parts of the rain forest. In contrast, the blackwater
tributaries of the Amazon are simply too infertile to have supported other than
shifting, swidden agriculture.
Until the Industrial Revolution, agriculture was entirely based on renewable
resources, and thus in a sense sustainable; though the degree to which accelerated
soil erosion may have developed into land degradation/desertification is much
debated. The extent to which early Mesopotamian civilizations may have salinized
soils in the Euphrates-Tigris basin is in question; certainly much of the region is still
highly productive today. Erosion off slopes at times choked valley bottoms and
created wetlands that became malarial to the detriment of the local populations, but
such outcomes would not have lasted forever given the natural tendency of streams
to unclog their drainages. In desert regions, farmers such as the Nabataeans, Hopi,
and Mixtec intentionally caused slope erosion to concentrate soil in the wadis where
moisture was sufficient for crop growth. Properly maintained terraces typically
produced less erosion than would have occurred naturally; though during times of
war or other catastrophe, when terraces were neglected, erosion could become
highly accelerated. Bali represents one of the most extreme cases of land
transformation though terracing and complex water management. Almost the
entire surface is given over to agricultural plants, with terraced rice paddies, and
domesticated palms in unterraced spots. Rice geneticists attempted to spread the
Green Revolution to Bali, but later withdrew, admitting that the indigenous system
yields at least as well as a modern system would be able to do.
The infamous Dust Bowl of the 1930s had a major influence on beliefs about
soil erosion, and belief in widespread land degradation persists within the
environmental movement and the UN. But within geography, the consensus today is
that: 1) the Dust Bowl region was not permanently ruined; soils remain fertile on
the whole, though today if farmed they are irrigated. 2) There is no good evidence
for desertification anywhere except on a very local scale (Thomas, 1993), aside from
the Aral Sea debacle resulting from a misguided Soviet irrigation scheme. 3)
Increasingly, it is recognized that when soil is lost in one place it must be gained
somewhere else (though admittedly that is not always a good thing).
Farming can have major effects on off-farm ecosystems. Eutrophication of
waterways due to fertilizer runoff is one of the better known and more serious
examples. Construction of reservoirs for irrigation purposes can alter downstream
riverflow, to the detriment of many organisms. The trapping of sediment behind
dams is causing a reduction in the size of deltas such as those of the Mississippi and
the Nile.
Today, farming is utterly dependent upon fossil fuels. For instance, the Green
Revolution, which after World War II spread modern agriculture to poor countries,
entails the use of gasoline-powered machines such as tractors, artificial N fertilizer
manufactured using natural gas, irrigation pumped with motors powered by fossil
fuel, and herbicides and pesticides largely manufactured from hydrocarbons. As
such, sustainability is inherently doubtful.
These transformations reflect, and have caused, a change from subsistence to
market oriented farming, with the resulting disadvantaging of relatively infertile or
otherwise marginal arable lands. For instance, much of the Northeastern US, which
was given over to subsistence agriculture in the 19th and into the 20th century, is
now abandoned and has reverted to forest because both soils and climate are not
conducive to high yields, while improving transportation technology has enabled
farmers in more fertile climes to ship their produce over increasing distances and
undersell local farmers. On the other hand, genetic engineering combined with
massive applications of phosphorus and other fertilizer is transforming some
formerly infertile regions into arable. For instance, the cerrado of Mato Grosso, an
extremely diverse, infertile shrub savannah formerly utilized for extensive cattle
grazing, is increasingly given over to soy cultivation. Legumes such as soy have a
high phosphorus requirement, but once that is supplied can add the nitrogen that
such soils lack.
Now, the increasing use of ever-larger machinery combined with herbicideresistant, GMO crops, is causing fields to become ever larger also, and also leading to
the elimination of hedgerows and other vegetated field boundary areas. The
biodiversity of these fields is extremely low. Among many other impacts is the
recent decline in the abundance of monarch butterflies, which depend on a food
plant, milkweed, that formerly inhabited field edges and other open areas but has
been greatly reduced by spraying and plowing. Many birds and other animals that
depend on weeds for food, and/or use field edges for migration corridors, are now
declining rapidly towards possible extinction (Stoate, 2011).
Theoretical debates over the causes of biodiversity remain unresolved. For
instance, many ecologists believe that high productivity is associated with high
diversity, some believe that the relationship is humped (unimodal), and still others
point to the low fertility of soils in western Australia and the South African Cape,
which have remarkably high plant species diversity. The best evidence is that the
enormous quantities of fertilizer now being applied are causing a major loss of plant
biodiversity due to competition for light. The role of disturbance also is somewhat
controversial, with perhaps the consensus being that up to a point it is beneficial,
but that diversity declines if disturbance becomes massive. If so, farming might
originally have fostered diversity, and indeed, the evidence from traditional farming
suggests that may have happened. However, the enormous scale of the current
industrialized system is clearly inimical, as it reduces landscape heterogeneity, a
known associate of diversity. Impacts also are likely to depend on the type of
natural ecosystem within which farming is practiced. Farming fragments forests, for
instance, which has the effect of favoring “edge” or ecotone species, while
disfavoring species that live in the interior.
Today, there is increasing concern about diversity of the crops themselves.
Globally, the human diet is becoming ever more restricted to a very few staples,
though in the supermarkets of developed countries the food diversity is spectacular.
In part, this reflects the conversion of agriculture in much of the underdeveloped
world to cash cropping, spurred on by Western institutions such as World Bank.
The spread of Green Revolution genotypes has raised concern for the genetic
diversity of crops, since much of that diversity resides in traditional varieties grown
in often-marginal environments that are now subject to abandonment due to
inability to compete on the international market. Attempts are being made to
preserve diversity in gene banks, but these suffer from introgression, and
adaptation to the environment in which the gene bank is located.
SEE ALSO: Agricultural geography, Agriculture, Agrobiodiversity, Biogeography,
Desertification, Disturbance, Invasive Species, Mediterranean-type Ecosystems, Soil
erosion and conservation, Soil fertility and management
References and Further Readings
Diamond, Jared. 1997. Guns, Germs and Steel. New York: Norton.
Harlan, Jack. 1975. Crops & Man. Madison WI: Society of Agronomy.
Sauer, Carl. 1952. Agricultural origins and dispersals. New York: American
Geographical Society.
Stoate, Chris. 2011. Biogeography of agricultural environments. In Handbook of
Biogeography, edited by Andrew Millington, Mark Blumler, and Udo Schickoff , 338356. London: Sage.
Thomas, David S. 1993. “Sandstorm in a teacup? Understanding desertification.”
Geographical Journal 159:318-331.
Key Words
Agriculture, sustainability, biodiversity