The Properties of Agroecosystems 125A ubiquitous input is the subsidy, often in the form of a fertilizer application,
intended to counter the stress of repeated harvesting. Sustainability is maintained
only by renewed fertilizer application. Another common form of input is a control
agent; for example, a pesticide to counter pest or disease attack. Again, sustainability
may necessitate repeated pesticide applications, but an alternative strategy may be
the introduction of a biological control agent, such as a parasitic wasp, which may so
permanently alter the intrinsic sustainability characteristics of the agroecosystem as
to obviate the need for further intervention. In some situations inputs may become
part of the problem because, directly or indirectly, they generate stresses and shocks.
Frequent pesticide applications, for example, may elicit pesticide resistance and hence
growing pest attack. The number of applications may have to be increased to sustain
productivity, but in the end productivity may still collapse (Figure 6.3).
Figure 6.3 The effect of pest control on sustainability