Agriculture and Biodiversity
INTRODUCTION
No other human activity has a 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 the 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 the 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.
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 a little net effect on global biodiversity. 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 have been at least a 600% increase in the worldwide deforestation of native ecosystems for agriculture and wood extraction which has 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. The conversion of natural ecosystems to agroecosystems 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%. 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 the 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.[1] The native ecosystems that agriculture has replaced typically had high biodiversity. A hectare of tropical rainforest 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, the 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 lose the resources they need to survive. Loss of wild biodiversity at this level leads to the 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 BIODIVERSITY
The structure and function of agroecosystems are largely determined by local context, including the interaction of ecological 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, insects, and disease pests, predators and parasites of pest organisms, and symbiotic and mutualistic species).
Planned and associated biodiversity can enhance the 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 creates 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 complexes 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 biodiversity 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 monocultures goes against the ecological forces of plant community succession; therefore, a great deal of intervention is required to maintain high levels of production.
Fertilizers applied to maximize the production of crop plants create favorable habitats 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 insect pests and disease organisms. Insecticides, fungicides, and genetic engineering may protect crops from these competitors. Furthermore, conventional cropping agroecosystems 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 the impacts of agriculture in the Mississippi River watershed. A new concern regarding the off-site impacts of modern agriculture on biodiversity is the genetic pollution that can result as genetically modified crops expand worldwide. The 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; and
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 of these 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 eco-agriculture, 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) the conversion of wild lands to agriculture by increasing farm productivity;
4) minimize agricultural pollution;
5) modify the 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 for global sustainable food security.