In many human cultures, agriculture is practiced as an extractive industry, in which the nutrition in the soil is sucked out by the repeated growing of crops, and soils are routinely degraded throughout the world.
The conversion of land to agricultural purposes alters the entire ecosystem, and the resulting impact on soil structure and fertility, quality and quantity of both surface and groundwater, and the biodiversity of both terrestrial and aquatic communities diminishes present and future productivity. Crop rotation and the application of livestock manure help to reduce soil erosion, but in some parts of the world the land is robbed of even cow dung by the need of growing populations for something to burn as fuel for cooking and staying warm.
The eroded soil has to go somewhere, of course, and its usual destinations are riverbeds and lake bottoms, where it often causes secondary problems by choking reservoirs.
The conversion of land to agricultural purposes indirectly:
The passage says converting land to agriculture alters the ecosystem and diminishes present and future productivity, so it reduces prospective productivity.
The impact falls on soil structure and fertility, on surface and groundwater, and on biodiversity.
By degrading all these, the change lowers the land's future, or prospective, output.
This is an indirect effect, working through damage to the wider ecosystem rather than an immediate loss.
Options claiming higher fertility or greater biodiversity directly contradict the passage.
A lower livestock death rate is not discussed as an effect of the land conversion.
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