5.6单元 — 农业中的可持续性挑战

(a) Subsistence agriculture is farming carried out primarily to feed the farmer's own household or local community, with little or no surplus produced for sale in a market.

(b) One accepted hearth of the First Agricultural Revolution is the Fertile Crescent in Southwest Asia (the region including parts of modern Iraq, Syria, and Turkey), where wheat and barley were domesticated (other accepted hearths: Mesoamerica — maize/beans/squash; the Andes — potatoes/quinoa; East Asia — rice/millet; Sub-Saharan Africa — sorghum/yams).

(c) Shifting cultivation involves clearing a plot of land, often by cutting and burning vegetation, farming it for a few growing seasons until soil fertility declines, then abandoning the plot to let it lie fallow and regenerate while the farmer clears a new plot elsewhere. It is typically practiced in tropical rainforest environments and supports only low population densities.

(d) Pastoral nomadism involves herding domesticated livestock (such as cattle, sheep, goats, or camels) over seasonal, non-fixed routes in search of pasture and water, rather than cultivating crops on fixed plots of land. It is typically practiced in arid, semi-arid, or high-altitude environments unsuitable for crop farming.

(e) Relocation diffusion occurs when people who carry a practice physically migrate from one location to another, taking the practice with them and establishing it in the new location while it may cease to spread from the origin — for example, European colonists carrying wheat cultivation techniques to the Americas as they settled there. Expansion diffusion occurs when a practice spreads outward from its origin through contact between people, without the originators relocating, and the practice keeps expanding within the source region as well — for example, contour plowing techniques spreading farmer-to-farmer across a agricultural region as neighboring farmers observe and adopt the method.

(f) The Second Agricultural Revolution, associated with the Industrial Revolution, increased productivity through innovations such as mechanized farm equipment (e.g., the seed drill and, later, mechanized reapers and tractors), improved crop rotation systems, and selective breeding of livestock and crops, which increased yields per unit of land and reduced the labor required per farm worker.

(g) Rural settlement patterns reflect the agricultural system practiced: for example, a dispersed (scattered) settlement pattern is common in areas of extensive commercial agriculture (such as U.S. Midwest grain farming) where large individual landholdings require farmhouses to be spaced far apart, while a clustered settlement pattern is common in areas of intensive subsistence rice farming (such as parts of Southeast or South Asia) where farmers live together in a central village and walk out daily to small, nearby fields, allowing shared labor and efficient use of limited arable land.

(a) The model shown is von Thünen's model of agricultural land use, developed by Johann Heinrich von Thünen.

(b) Bid-rent refers to the rent a farmer is willing and able to pay for a parcel of land, which reflects the value of the land for a given agricultural use; bid-rent decreases as distance from the central market increases because transportation costs to move goods to market rise with distance, reducing the profit available to pay as rent.

(c) Land closer to Meadowbrook commands higher rent because transportation costs to move goods to the central market are lower for nearby land, leaving farmers more profit to bid for that land; competition among farmers who want to minimize transport costs and time (especially for perishable goods) also drives up rent for the closest parcels.

(d) Dairying and market gardening are located nearest the town because they produce perishable goods (milk, fresh produce) that must reach market quickly before spoiling and are also costly to transport per unit of value, so farmers of these goods can afford, and need, to outbid other land uses for the most accessible land.

(e) In von Thünen's original model, wood was a bulky, heavy commodity that was expensive and difficult to transport over land before mechanized transportation existed, and it was in constant demand in town for fuel and building material, so it needed to be located close enough to the market to keep transport costs manageable — closer than lower-value, less bulky grain, which could be transported profitably over greater distances.

(f) One valid limitation: von Thünen's model assumes an isotropic plain (uniform, featureless terrain, transportation cost equal in all directions, a single central market, and no external trade), conditions that do not hold in the present-day United States, which has variable terrain, multiple transportation modes and networks (highways, rail) that create transport corridors rather than uniform rings, multiple competing markets, and global trade that allows perishable or bulky goods to be shipped long distances profitably using modern refrigeration and rapid transport.

(g) Refrigerated trucking would reduce the spoilage risk and effective transportation cost of moving dairy products long distances, weakening the locational advantage that land near Meadowbrook currently holds for dairying; as a result, dairy production could shift farther from the town center where land rent is lower, and the steep rent gradient favoring the innermost ring for dairying and market gardening would likely flatten, since farmers producing perishables would no longer need to be located immediately adjacent to the market to remain profitable.

(a) The Green Revolution (also called the Third Agricultural Revolution) refers to the period beginning in the mid-twentieth century in which the development and spread of high-yield hybrid seed varieties, synthetic fertilizers, irrigation, and mechanization dramatically increased agricultural output, especially of grain crops such as wheat and rice, in many developing countries.

(b) Source 1 shows that both wheat and rice yields per hectare increased substantially and consistently from 1965 to 1985 — wheat yields roughly tripled (from 850 to 2,300 kg/hectare) and rice yields more than doubled (from 1,100 to 2,450 kg/hectare) over the twenty-year period.

(c) Two valid inputs: high-yield variety (HYV) hybrid seeds (developed through selective breeding to produce more grain per plant) and synthetic chemical fertilizers (which supply nutrients that allow the HYV seeds to reach their higher yield potential). Other acceptable inputs: expanded irrigation and mechanized equipment (tractors, mechanized harvesting).

(d) Source 2 illustrates vertical integration / contract farming, a characteristic of agribusiness in which a single large firm controls or coordinates multiple stages of the agricultural supply chain — in this case supplying the seed input, and also purchasing and processing the entire output — so that independent farmers effectively grow a single standardized crop for one buyer rather than making independent planting and marketing decisions.

(e) One valid risk: because the farmers sell exclusively to one firm and plant only the variety it supplies, they have no alternative buyer and little bargaining power over the price they receive, leaving them economically vulnerable if the firm lowers the price it pays, changes contract terms, or ends the contract; because they no longer save their own seed, they must also purchase new seed from the company each season, increasing their costs and dependency.

(f) One valid concern: intensive Green Revolution farming relying on heavy irrigation and synthetic fertilizer/pesticide application can degrade soil quality over time, deplete or contaminate groundwater and surface water supplies through runoff, and encourage monocropping of a narrow set of HYV seeds, which reduces agricultural biodiversity and can increase vulnerability to pests or disease that affect a single genetically similar crop.

(g) Vertical integration by agribusiness firms has benefited rural farming communities by providing farmers with guaranteed access to improved seed inputs, a reliable buyer for their harvest, and often technical support, which can raise and stabilize household income compared to selling on an open, price-volatile market. At the same time, it has threatened these communities by concentrating economic power and decision-making in a single outside firm, eroding farmers' independence (loss of seed-saving and choice of what to grow), and making the community's livelihood dependent on the continued presence and pricing decisions of one company, with little recourse if the firm withdraws or changes terms.