Content Objectives
This one-week unit, developed for AP Environmental Science with elements that can be adapted for introductory and intermediate level students, focuses first on the environmental consequences of the Green Revolution and the industrialization of agriculture. Students then learn about more sustainable alternatives, including the overarching concept of agroecosystems, and more specific examples of polycultures, integrated pest management, and Indigenous agricultural practices. Finally, students are tasked with investigating the agroecosystems that produce coffee, cacao, and yuca and other locally relevant crops, and identifying specific sustainable practices within them. Students’ ultimate goal is to better understand Wes Jackson’s quote about farming the way nature farms.
The Environmental Impacts of the (Not So) Green Revolution
The Green Revolution is not the focus of this unit, as it is covered as part of a larger period of instruction on agriculture. But, the environmental impacts of the Green Revolution and the alternative methods with lower environmental impacts are the focus of this unit, so it is important to at least define and provide context for this term. The Green Revolution is most simply defined as the large increase in crop production across the world, but mostly in developing countries.3 Historically it is considered the result of the following developments: the hybridization of corn, the nine-fold increase in fertilizer use and the tripling of irrigation post World War II, the rapid spread of new high-yield wheat and rice varieties in developing countries, and the adoption of chemical insecticides, herbicides, rodenticides, and fungicides as the primary tool for pest management.4 The negative environmental consequences most related to this unit are the increase in fertilizer use, irrigation, and chemical pesticide use. Each of these are discussed in limited detail below. Other environmental consequences include biodiversity loss due to the clearing of forests and conversion of prairies into farmland, and the extermination of predator species to protect livestock.5
The Green Revolution and the resulting intensification of agriculture exponentially increased the productive capacity of the world’s farms, in part due to the development of synthetic fertilizer. This helped farmers overcome the natural limits of the biogeochemical cycles within their soil. Natural processes only fix so much nitrogen and release limited amounts of available phosphorous for plants to take up, acting as a major limitation on yields. Additionally, the removal of biomass at harvest time limits the amount of carbon that gets recycled back into the soil. But the development of synthetic fertilizers ushered in a new era of nutrient management. Natural fluctuations in nutrient loads lead to inconsistent yields, which are not tolerated in modern agricultural systems. As such, modern management strategies deliver soluble and inorganic nutrients directly to crops in a way that leaves them in a permanent state of nutrient saturation. In addition to this, the previously coupled biogeochemical cycles of nutrients have been uncoupled over both space and time. And while it dramatically increased yields, it also led to the degradation of soil and water resources and the alteration of biogeochemical cycles.6 The sustainable alternatives presented below aim to reduce the reliance on this practice to restore natural biogeochemical cycles.
Worldwide, agriculture accounts for roughly two-thirds of all water use.7 Irrigation has been used to turn many low-rainfall areas into agricultural powerhouses; about thirty percent of all the food grown worldwide comes from irrigated land. In several regions, agricultural water use is depleting groundwater at rates hundreds to thousands of times faster than it can be recharged. This excessive water use has severe environmental consequences, including waterlogging and salinization. Waterlogging occurs when soil is saturated to the point where gas exchange between soil and atmosphere is inhibited. This depletes oxygen in the soil and leads to nutrient deficiencies, root death, and reduced growth or even death in plants.8 Salinization occurs as irrigation water evaporates, leaving behind salts that slowly accumulate over time, diminishing soil productivity.9 Irrigation is also incredibly inefficient: less than half the water delivered in irrigated systems gets used by plants. The rest runs off, infiltrates, or evaporates. Irrigation isn’t just a problem in crop agriculture; in fact, water usage in livestock raising is even more problematic because of wild inefficiencies. Beef production requires about 100 times more water for an equivalent amount of grain.10
The impacts of chemical pesticides (which includes herbicides, insecticides, fungicides, and rodenticides)11 have been well known since at least the 1970s, yet their use has only continued to increase. These environmental impacts include die offs of both target and non-target species which is especially concerning for pollinating and other beneficial insects, contamination of drinking water, the bioaccumulation and biomagnification of persistent organic pollutants through various ecosystems, and links to several types of cancers. Perhaps the most (in)famous environmental legacy of chemical pesticide use was the decline in bird populations caused by DDT chronicled by Rachel Carson in Silent Spring.12 Another lesser-known impact is the development of pesticide resistant target species that leads to what has been termed a pesticide treadmill. More technically speaking, the “exposure to pesticides has led to genetic selection for individuals with the biochemistry or behavior necessary to nullify their toxic effects.”13 The modern industrial agriculture industry has responded not by rethinking the approach to pest management, but by increasing pesticide doses and developing novel pesticides that only continue the selection for genetically resistant pest populations. This leads to further negative environmental impacts but also threatens the economic security of smaller farmers whose margins don’t allow for a never-ending increase to their pest management costs, which accelerates the shift toward industrial-scale agriculture that favors this method of pest control over more environmentally friendly approaches.14
Sustainable Alternatives to Modern Industrial Agriculture
The primary benefit of sustainable agriculture practices is that instead of working against nature, they use features of nature to their benefit. A shift towards more sustainable agricultural systems should emphasize the restoration of natural ecosystem functioning, including biogeochemical cycling and pest control/management. Each of the following practices favors and fosters these natural ecological processes over human intervention, though such measures are often incorporated as part of the system. Agroecosystems are perhaps the best examples of such sustainable practices, since they often involve one or more of the other examples relevant to this unit.
Agroecosystems
One way of improving the sustainability of agricultural systems is to view them not as a series of inputs (seeds, soil, fertilizer, pesticides) and outputs (crops), but as ecosystems that happen to produce food, fiber, and/or other agricultural products, called agroecosystems.15 16 Such systems function somewhere between highly managed modern industrial agriculture (crops, pests, weeds, etc.) and natural ecosystems (a wide variety of wildlife).17 Figure 1 demonstrates the basic differences between modern industrial agricultural systems and agroecosystems.
Figure 1: Modern agricultural systems versus agroecosystems.
An easy way of explaining the differences between these two systems is that “one is the expression of imposed will, the other the expression of the land’s will.”18 Expressing the land’s will in a way that produces food or timber or other raw materials is not an easy task, but through careful design and management, such systems can be productive, stable, and sustainable. Productivity is best expressed in terms of agricultural yield (weight or volume per acre or hectare). Stability in this application is best defined as the consistency of productivity and is subject to disturbance by fluctuations and cycles in the supporting and surrounding environments, such as changes in climate or market demand. Sustainability is the ability of the system to maintain productivity when subjected to a major disturbing force, such as a rare drought, flood, or the emergence of a new pest. A key feature of productive, stable, and sustainable agroecosystems is the ability to withstand stresses or shocks in ways that modern industrial agricultural systems cannot.19 For example, an agricultural system that relies heavily on synthetic fertilizer cannot maintain its productivity if the price of that fertilizer skyrockets without passing along costs to the consumer. In contrast, a well-designed agroecosystem that relies more on natural biogeochemical cycling and the thoughtful planting of crops with different nutrient demands will be relatively unaffected by more expensive fertilizer. Like their more natural counterparts, agroecosystems are more resilient than industrial agriculture systems.
A critical aspect of the sustainability feature of agroecosystems lies within their nutrient management systems. This is done very differently than modern industrial agricultural systems; instead of oversaturating crops with a synthetic cocktail of soluble inorganic nutrients, an agroecosystem emphasizes organic and mineral nutrient reservoirs that rely on natural microbial and plant-mediated processes. Agroecosystems don’t abandon the use of fertilizers all-together, they just get used more strategically. Fertilizer sources are varied and paired with a diverse system of plants that expands the functional roles of plants within the ecosystem.20 For example, a nitrogen fertilizer might be used to supplement nitrogen demands in a crop system that includes a high-N crop and a N-fixing plant that may or may not be harvested.
In practice, an agroecosystem approach to nutrient management should have the following parameters: the strategic use of a variety of nutrient resources, the ability to maintain reservoirs with long residence times that can be accessed by both microbes and plants, promote the exchange of carbon (C), nitrogen (N), and phosphorous (P) between producers and decomposers, mix inorganic and organic N and P, maximize C-fixation and N and P assimilation over space and time through plant diversity, reduce the overall size of nutrient pools, and promote natural mediation of cycles through plant and microbe activity.21 This natural approach to nutrient management has shown promise: in systems where natural biogeochemical cycling was restored, yields were maintained, runoff of excess nutrients was decreased, and nutrient uptake by plants and storage in soil microbial pools was more efficient than modern industrial agriculture.22
Most of this is not novel – farmers were planting crops in specific rotations or in pairs long before modern industrial agriculture opted for efficiency and increasing yields over ecosystem function. However, much has been learned about the interplay between certain species, and the successful restoration of ecosystem functioning will require more than planting crops in their traditional rotations. In this way, the same push for technological innovation that led to the Green Revolution and all its negative environmental consequences can be applied to develop advances in technology that support the agroecosystem approach.
The concept of agroecosystems is intentionally broad and vague, in part because the development of such systems requires local/regional concerns regarding crop selection. These include economic considerations of demand for products, and environmental considerations of sunlight, water, nutrient availability, soil, and more. As such, the effective design of agroecosystems must consider smaller, more concrete examples of sustainable agriculture, such as integrated pest management, multiple cropping/intercropping, crop-livestock polycultures, agroforestry, and communal resource use.23 But when designed properly, agroecosystems are able to remain resilient in the face of disturbances, yielding generously and without the depletion and degradation typical of modern industrial agriculture. 24
Polycultures
Polycultures are simply mixtures of multiple crops planted in the same space at the same time.25 This is in direct conflict with modern agriculture, which emphasizes monocultures (one-crop systems such as corn, soy, or wheat) for their efficiencies of planting and harvesting. In general, polycultures align well with the core premise of agroecology, and thus have significant environmental benefits, including enhancing soil fertility, soil retention and accretion, climate regulation, natural pollination, pest and weed control, and better aesthetic appeal.26 Through the intentional planting of crops with different nutrient requirements and the promotion of soil health and natural biogeochemical cycling, polycultures reduce the need for synthetic fertilizer applications. The dense root structures and shading provided by above-ground biomass reduce the growth of nutrient-sapping and light-grabbing weeds, therefore reducing the need for herbicide applications. Those same root structures physically bind soil, holding it in place against wind and water. Rain is more likely to fall on plants’ leaves than exposed soil, promoting infiltration and reducing runoff, even in intense rainfall events. Polycultures are more resistant to pests than their modern industrial agricultural systems for two reasons: first, they offer pests more than one plant to eat, and second, the involve the intentional planting of non-harvested species to attract pests would otherwise feed on the desired crop species.27 In this way, several different crops may experience some damage, but no single crop will be decimated.
Like with agroecosystems, this practice must be tailored to the local environment. The types of crops grown in polyculture in Delaware must differ from those grown in Kansas and from those grown in Costa Rica or Peru. Polycultures must be place-based: the species grown in polyculture must fit the biotic and abiotic conditions of their geography. Though this is a challenge (especially for the economic side of agriculture), it is a feature not a bug. Careful observation of natural processes within each of those locations provides the necessary information for effective polyculture design.
For example, a polyculture in Cape Cod includes this use of fish ponds, ground cover provided by vegetable and forage crops, livestock that graze on the groundcover, and several tree species that produce fruit, nuts, timber, and fodder to supplement the livestock’s diet. In Costa Rica, some polycultures intentionally change over time. Following a natural clearing of rainforest, ecological succession takes over to begin restoring ecosystem function. Farmers here have closely observed the different stages and began using this knowledge to design crop systems that mimic natural succession. They start with grasses and legumes that grow well in full sun, then add in shrubs and eventually fruit and nut trees. The process starts again once another natural clearing occurs. Meanwhile, the maturing plot can be used to grow shade tolerant crops in polyculture with timber-producing hardwoods. A similar model has been developed for New England, which prior to contact, colonization, and clearing by Europeans starting the 1600s, was a hardwood forest. Successful polycultures here make use of the well-known succession models, and include a tree crop in the overstory, a stable understory that helps retain soil and regulate nutrient loads, a nitrogen source (typically a legume), and livestock to graze/browse and promote nutrient cycling.28 These specific polycultures reflect the natural environment in which they are located, and the amount of human input necessary to sustain yields is minimized.
Integrated Pest Management
Integrated pest management (IPM) is the “careful consideration of all available pest control techniques and subsequent integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment…[It] promotes the growth of a healthy crop with the least possible disruption to agroecosystems and encourages natural pest control mechanisms.”29 This emphasis on natural processes makes it an excellent example of the agroecosystem approach.
A good IPM plan has the following goals: reduce pest status, conserve environmental quality, accept tolerable pest densities, and improve net profits from production. In practice, this is done by reducing pest numbers, reducing the susceptibility of the host plant, or some combination of the two. Tactics for achieving these outcomes include preventive practices as a first line of offense and therapeutic practices as a last line of defense. Figure 2 provides a basic visual for an IPM plan.
Figure 2: Basic IPM Plan
Preventive practices include biological control, crop rotation, staggered planting dates, trap cropping, spatial arrangement of crops and non-harvested plant species, and the planting of insect-resistant crop varieties. Therapeutic practices include the use of pesticides to kill or disrupt the mating/reproductive cycles. Preventive practices are the foundation of effective IPM plans, and since they don’t rely on the use of chemical pest control measures, are what makes IPM sustainable.30 Therapeutic practices should be used only when preventive practices have failed to reduce pest density or the susceptibility of the host species. However, in practice, most IPM plans have become overly reliant on the use of chemical control measures and thus fail to live up to their sustainable promises.31 A more thorough investigation into this trend is beyond the scope of this unit, but it is a trend that is worth discussing with students.
An example of an IPM strategy relevant to Delaware is in the management of aphids and hornworms, commonly found in homeowners’ backyard gardens. While these pests can be eliminated with chemical pesticides, they can also be managed using biological controls. Planting flowering native plants in proximity provides resources for beneficial insects, including wasps or flies that help control the population of aphids and hornworms. The use of chemical pesticides then becomes a supplementary step if biological control fails to reduce the pest population to acceptable levels.32 On a larger scale, corn farmers can reduce the impact of aphids by creating edge habitats along their fields where native plants flourish and provide habitat for those same beneficial insects, and by reducing their insecticide use since their use also kills the aphids’ natural enemies and contributes to the selection for pesticide resistance within the aphid population that survives.33
Case Studies of Sustainable Agriculture
The technical information presented above means very little to students without context. Therefore, it is appropriate to provide examples of how these practices function in the real world. Connecting these practices to agroecosystems that students have interest in (coffee and cacao) is an excellent way of increasing student engagement and showing them that Indigenous knowledge that makes chacras productive agroecosystems can be a powerful learning experience because it demonstrates that knowledge exists outside Western institutions and experiences. My passion and interest in this area will hopefully inspire students to learn about them more deeply and promote long-term knowledge retention.
Coffee
Coffee is an excellent polyculture crop because of its unique abiotic and biotic growth conditions. The basics of how coffee functions in such an agroecosystem are represented in an oversimplified manner in Figure 3 and are discussed more deeply below.
Figure 3: Coffee/cacao polyculture with coffee bushes, cacao trees, and miscellaneous hardwood/citrus trees and regulating services.
Coffee bushes require shade from the intense tropical sun. The growing of fruit and timber species as part of an agroforestry system provides the coffee bush the shade it needs. Additionally, coffee polycultures also benefit from pollination, pest control, and nutrients from decaying leaf litter. Research has shown that the presence of bird species in coffee agroecosystems reduces the incidence of yield-reducing pests. Other complex interactions within the coffee agroecosystem include the suppression of coffee rust, a serious fungal infection that resembles rust on the leaves of the coffee bush. This happens via a mutualistic relationship between ants and other insects.34 These natural pest control measures highlight the resiliency of well-developed and managed coffee agroecosystems that emphasize natural ecosystem function and regulation over human intervention.
Additionally, coffee agroecosystems benefit from the presence of nitrogen-fixing plants, including Cordia alliodora (commonly known as Spanish elm or Ecuador laurel) and several Erythrina species (a genus of plants in the pea family). The consistent litterfall provided by these trees and symbioses with soil-dwelling bacteria provide natural sources of nitrogen. This reduces the amount of nitrogen losses to leaching. The soil organic material that develops over time provides nutrients, including carbon, nitrogen, potassium, calcium, and magnesium, in the correct proportion. In addition to providing shade and nutrients, the presence of these tree species also provides a potential income source through the sale of timber products.35 Successful coffee polycultures integrate traditional knowledge and biodiversity conservation initiatives and protect local farmers’ autonomy.36
Cacao
Cacao is an excellent crop to grow in polyculture as a part of a larger agroecosystem, as it is an under-story tree that requires shade and shelter from the intense rainfall typical of the tropics, as well as nutrients from leaf litter on the forest floor. This is represented in Figure 3 above. Growing cacao in this way also helps attract beneficial insects (for pollination and pest control) and provides a diverse habitat for many different species. This also minimizes traditional agricultural inputs, including synthetic fertilizers and fossil fuels. Other crops that can be grown in this agroecosystem include fruits, spices, root crops, timber, and medicinal plants, which sustains cash flow for cacao farmers.37 Like coffee, cacao agroecosystems benefit from the presence of C. alliodora and Erythrina species in numerous ways, including the shade they provide and their delivery of balanced nutrient loads.38 Research has shown that cacao agroecosystems that rely on natural ecosystem functioning perform on par with those that are intensively managed with pesticides and fertilizers but are more resilient in the face of ecological pressures like pest infestations.39 As with coffee agroecosystems, the success of cacao-producing systems based on natural ecological function should serve as a model for the effective design of other agroecosystems.
Chacras
Farming in the Amazon rainforest has a well-studied legacy of environmental damage and disruption. At the forefront of this legacy are shifting cultivation and slash-and burn agriculture, both of which threaten the preservation of the rainforest and the health of the local environment.40 Although there is an abundance of plant and microbial life in the rainforest ecosystems, their soil is quite nutrient-deficient. Because of this, farming in the region often involves the clearing of forest, burning the remaining vegetation, raising crops or livestock until the soil is too depleted to continue, and then moving on to the next plot of land and repeating. These practices cause and accelerate deforestation, biodiversity loss, soil erosion, changes to local hydrology, and decreased food security for local populations.41
However, Indigenous people have been successfully farming in this ecosystem for thousands of years without causing such widespread environmental damage. This is in part due to the small scale of their operations, but also because they embrace the agroecosystem approach (and were doing so long before this was a term). In northeastern Peru in the Amazon Rainforest, Maijuna people have been successfully cultivating yuca, sugarcane, banana, plantain, and even cacao in small clearings in the forest called “chacras,” an Andean term meaning farm or agricultural field. These chacras are even better embodiments of agroecosystems than cacao or coffee because they operate on smaller scales and are typically family operated. They are most often located near primary or secondary forests, forming a productive and ecologically friendly landscape that works within the biodiversity of the area. Chacras are true polycultures, as they combine the cultivation of local staple crops with medicinal plants and timber species. This agroecosystem requires no additional inputs – just sunlight and rainfall. It functions as part of the forest ecosystem, not separate from it.42
These chacras are essential to Indigenous people because they provide food security, medicine, and building materials to the community, and in some cases allow for trading and export (as is the case with cacao).43, 44 This example of sustainable agriculture holds special importance to me because I was lucky enough to visit a chacra in the Sucusari region of the Amazon rainforest in Peru, where I learned from members of the local Maijuna community. Sharing and elevating Indigenous knowledge is a goal of mine as I continue to develop the environmental science program at my school, especially in the introductory and intermediate courses.
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