No other human activity has greater impact on the earth’s biodiversity than agriculture. From its origins some 12,000 years ago, the goal of agriculturists has been to enhance production of desired species over competing species. Expansion of human agricultural activity around the globe historically has resulted in significant impacts on global biodiversity in four major ways:
1) loss of wild biodiversity and species
shifts resulting from conversion of native ecosystems by agroecosystems;
2) influence of agroecosystems’ structure and
function on agrobiodiversity;
3) off-site
impacts of agricultural practices; and
4) loss of
genetic diversity among and within agricultural species. Although agriculture
and biodiversity often are inversely related, biodiversity enhancement can be a
key organizing principle in sustainable agroecosystems
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| Agriculture and Biodiversity |
AGRICULTURE’S IMPACT ON WILD BIODIVERSITY GLOBALLY
Historically,
the earliest subsistence farmers and pastoralists had low population densities
and limited technology and their small-scale patchworks of fields, pastures, and
home gardens had little net effect on global bio-diversity In some ecosystems,
agricultural activity may have actually increased biodiversity because more
diverse habitats and ecotones were created—a pattern that may still exist in
some areas. However, as surplus agricultural production allowed human
populations to increase and with the development of civilizations, the impacts
of agriculture on wild biodiversity increased, even to the point that
biodiversity loss may have contributed to the decline of some ancient
civilizations. Since 1650, there has been at least a 600% increase in the
worldwide de- forestation of native ecosystems for agriculture and wood extraction
that have resulted in radical changes to wild biodiversity globally.
Wild
biodiversity is more threatened now than at any time since the extinction of
the dinosaurs, with nearly 24% of all mammals, 12% of birds, and almost 14% of
plants threatened with extinction. If current trends continue, it is estimated
that at least 25% of the earth’s species could become extinct or drastically
reduced by the middle of this century. Conversion of natural ecosystems to
agroeco systems is a primary cause of these alarming trends.
At least
28% of the earth’s land area currently is devoted to agriculture to some degree.
Intensive agriculture dominates 10% of the earth’s total land area and is part
of the landscape mosaic on another 17%, while extensive grazing covers an
additional 10%–20%.[2] Nearly half of the global temperate broadleaf and
mixed-forest and tropical and subtropical dry and monsoon broadleaf forest
ecosystems are converted to agricultural use (45.8% and 43.4%, respectively).
However, agriculture’s greatest impact has been on grassland ecosystems,
including temperate grasslands, savannas, and shrublands (34.2%); flooded grasslands
and savannas (20.2%); and montane grasslands and shrublands (9.8%).Combined,
64.2% of the earth’s grassland ecosystems have been converted to agriculture, primarily
for production of cereal grasses—maize, rice, and wheat. In the past 20 years,
net expansion of agricultural land has claimed approximately 130,000 km2/yr
globally, mostly at the expense of forest and grassland ecosystems, but also
from wetlands and deserts.
The native
ecosystems that agriculture has replaced typically had high biodiversity. A
hectare of tropical rain forest may contain over 100 species of trees and at
least 10 to 30 animal species for every plant species, leading to estimates of
200,000 or more total species In contrast, the world’s agroecosystems are
dominated by some 12 species of grains, 23 vegetable crops, and about 35 fruit and
nut crop species Furthermore, conversion of native ecosystems to agriculture
causes dramatic shifts in ecosystem structure and function that affect
ecosystem processes above and below ground including energy flow, nutrient
cycling, water cycling, food web dynamics, and biodiversity at all trophic
levels The amount of wild biodiversity loss depends on the degree of
fragmentation of the native landscape. Whereas some species require vast
continuous areas of native habitat, many can survive as long as the appropriate
size and number of patches with connecting corridors of native habitat are left
intact and provided that barriers to species movement—such as road and
irrigation networks—are limited. However, when conversion leads to critical
levels of native landscape fragmentation, chain reactions of biodiversity loss
have been observed as interdependent species loose the resources they need to
survive Loss of wild biodiversity
at this
level leads to loss of numerous ecosystem benefits that are essential to
agriculture, e.g.,
1) drought
and flood mitigation;
2) soil
erosion control and soil quality regeneration;
3) pollination
of crops and natural vegetation;
4) nutrient
cycling; and 5) control of most agricultural pests.
STRUCTURE AND FUNCTION OF AGROECOSYSTEMS AND BIODIVERISTY
The
structure and function of agroecosystems are largely determined by local
context, including interaction of eco-logical conditions (including bio-, geo-,
and chemical) with social factors, including farmers’ economic needs, cultural and
spiritual values, and social structure and technology.
Two types
of agrobiodiversity have been defined:
Planned
biodiversity is the specific crops and/or livestock that are planted and
managed; associated biodiversity is nonagricultural species that find the
environment created by the production system compatible (e.g., weeds, insect and
disease pests, predators and parasites of pest organisms, and symbiotic and
mutualistic species). Plannedand associated biodiversity can enhance stability
and predictability of agroecosystems Traditional forms of agriculture—such as
home gardens and shade coffee farms in the New and Old World tropics and
traditional Amish dairy farms in North America—have a complex and diverse
spatial and vertical structure and high planned and associated biodiversity.
For example, traditional neotropical agroforestry systems commonly contain over
100 annual and perennial plant species per field.
Traditional
agroecosystems create landscape patterns of small-scale diverse patches with
many edges, habitat patches, and corridors for wild biodiversity. In contrast
to traditional agroecosystems, the vertical and horizontal structure of modern
industrial agroecosystems is simplified into monocultures on a large scale that
create landscape patterns of widespread extreme genetic uniformity with few
edges, habitat patches, and corridors for dispersal For example, in the United
States 60–70% of the total soybean area is planted with 2–3 varieties, 72% of
the potato area with four varieties, and 53% of the cotton area with three
varieties. The structure and function of industrial livestock agriculture
impose similar negative impacts on biodiversity worldwide.Livestock operations
for all major species—particularly swine, poultry, beef, and dairy—are becoming
increasingly concentrated, with feed produced in monocultures and brought to the
animals in feedlots. Even in more extensive grazing operations, although good
management can increase plant biodiversity these systems replace native forests
and/or grasslands that once supported highly diverse com-plexes of coadapted
plants and migratory grazing and browsing ungulates and their predators.
OFFSITE IMPACTS OF AGRICULTURAL ACTIVITIES
The third
major way that agriculture impacts global bio-diversity is through the direct
and indirect off-site effects of the various managements used to maintain their
structure and function. Growing annual species in large mono-cultures goes
against ecological forces of plant community succession; therefore, a great
deal of intervention is required to maintain high levels of production.
Fertilizers
applied to maximize production of crop plants create favorable habitat for
other plant species that are adapted to nutrient enriched conditions, including
alien invasive species. Tillage, herbicides, and genetic engineering may prevent
competition between crop plants and annual and perennial weeds. Widespread
monocultures of nutrient enriched plants create an easily exploited resource
for in-sect pests and disease organisms. Insecticides, fungicides, and genetic
engineering may protect crops from these competitors. Furthermore, conventional
cropping agro- ecosystems are notoriously leaky (i.e., the sheer volume of external
inputs being applied in combination with soil disturbance and decreased soil
quality often exceeds the capacity of the agroecosystem to absorb and process
the inputs). Concentrated livestock agriculture also can be a major source of
chemical and biological pollution. As a result of these many factors, sediment,
excess fertilizer, manure, and pesticides run off into streams and down into groundwater.
Hydrological alterations to land and natural streams in combination with
chemical and biological pollution cause considerable reductions in aquatic
biodiversity that can extend throughout whole watershed systems. The Hypoxia in
the Gulf of Mexico is a dead zone that covers 18,000 km2 where aquatic
biodiversity has been drastically reduced by impacts of agriculture in the Mississippi
River watershed. A new concern regarding off-site impacts of modern agriculture
on biodiversity is the genetic pollution that can result as genetically modified
crops expand worldwide. Possible transfer of genes for resistance to weeds,
insects, fungi, and viruses could overwhelm wild populations and communities.
LOSS OF DIVERSITY WITHIN AGRICULTURAL SPECIES
Of 7000
crop species, less than 2% are currently important, only 30 of which provide an
estimated 90% of the world’s calorie intake—with wheat, rice, and maize alone providing
more than half of plant-derived calories.
Some 30–40
animal species have been used extensively for agriculture worldwide, but fewer
than 14 account for over 90% of global livestock production, whereas some 30%
of international domesticated breeds are threatened with extinction. There are
additional trends of decreased varietal and landrace diversity within crop
species as more farmers adopt modern high-yielding varieties. These alarming
trends have prompted government policy recommendations whose purpose is to:
1) ensure that current agricultural genetic
diversity in plants is preserved in seed banks and plant and germplasm
collections (ex situ) or as growing crops (in situ), particularly wild
relatives of major crops and livestock breeds in their centers of origin;
2) ensure that wild crop and livestock
relatives are conserved in carefully identified natural systems.
CONCLUSION
Biodiversity
as a Principle of Agroecosystem Management Although industrial agriculture is
generally inversely related to biodiversity, there are promising examples of alternative
agroecosystems that protect and enhance biodiversity and are also highly
productive Some ofthese include:
1) organic
agriculture;
2)
sustainable agriculture;
3)
permaculture;
4) natural
system agriculture;
5) holistic
management; and
6) eco_agriculture.
These models are based on ecological principles and the assumption that
biodiversity can contribute significantly to sustainable agricultural
production. Within ecoagriculture, the following strategies are proposed to
protect and enhance wild biodiversity:
1) create biodiversity reserves that also
benefit local farming communities;
2) develop habitat networks in nonfarmed
areas;
3) reduce (or reverse)
conversion
of wild lands to agriculture by increasing farm productivity;
4) minimize agricultural pollution;
5) modify management of soil, water, and
vegetation resources; and
6) modify
farming systems to mimic natural ecosystems.
Examples of
specific management practices that sustain or enhance biodiversity include:
1) hedgerows;
2) dykes with wild herbage;
3) polyculture;
4) agroforestry;
5) rotation with legumes;
6) dead and living mulches;
7) strip crops, ribbon cropping, and alley
cropping; 8) minimum tillage, no-tillage, and ridge tillage;
9) mosaic landscape porosity;
10) organic
farming;
11) biological pest control and integrated
pest management;
12) plant resistance; and
13)germplasm diversity.
New
research, particularly if conducted in a participatory mode with farmers,
should lead to many more ways to protect and enhance biodiversity, including:
rotational grazing of high-diversity grasslands for dairy and beef cattle
production, timber and pulp production systems that use perennial plants,
high-diversity mixtures of single annual crops and/or rotational diversity, and
precision agriculture that closely matches small-scale soil conditions with
optimal crop genotypes.
Although
specific management practices are helpful, also needed are whole-farm planning
approaches and decision- making processes that encompass farmers’ values and economic
needs, in addition to environmental concerns for biodiversity (e.g., holistic
management).
More research and education and policies that
encourage farmers and consumers to appreciate the ecological, economic, and quality-of-life
values of biodiversity are needed to thwart current threats to global
biodiversity and agrobiodiversity, and to address the many challenges and
opportunities of global sustainable food security
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