Agriculture Reference
In-Depth Information
which leads to greater resource use efficiency, since interspecific or intervarietal com-
petition is often lower than intraspecific competition. For example, there may be less
root competition in mixtures, or less competition for light. Different crops or varieties
may have different resource requirements, such as needing water at different times. The
differing requirements of systems components can also mean that multiple components
provide resilience. A given stress or shock may not affect all constituents equally, and
those that survive can often compensate for those lost.
In addition to mechanisms related to competition and resource use efficiency,
there is a set of mechanisms pertaining to facilitation, in which one species provides
services to another through improved nutrient availability, water access, or pest pro-
tection. An obvious example of facilitation is nitrogen fixation by legumes, which
can benefit associated cereal crops (Peoples et al. 2009). In countrywide assessments
in Malawi, maize-legume rotations outperformed monoculture maize; pigeonpea (a
semi-perennial legume) did particularly well (Snapp et al. 2010). Polycultures can con-
tribute to improved nutrient use efficiency in other ways as well, for instance by contrib-
uting to increased P availability and to increased soil organic matter. More examples are
noted below in the section on soil and water management. An additional set of mecha-
nisms involves reduced losses due to insects, weeds, and pathogens (collectively consid-
ered “pests”). More on this range of mechanisms will be mentioned below, in the section
on pest management.
Perennials can contribute special roles in polycultures. As illustrated by many suc-
cessful examples of agroforestry systems, trees offer products and ecosystem services
that other species cannot. Wood is obviously valued for fuel and construction material,
and trees can provide fodder as well as protection from sun and wind. Trees and other
perennials and semi-perennials have deeper roots that may be able to tap water and
nutrients that are unavailable to other species (Cannell et al. 1996). This may smooth the
impacts of weather variations, hence reducing risk. For example, enset (“false banana,”
the starchy staple in risk-prone southwestern Ethiopia) can survive drought periods
that would kill a cereal crop. Trees can reduce runoff, transpiration, and erosion (Ong
et al. 2002). They can generate islands of beneficial soil biological activity (Pauli et al.
2010). On the down side, trees may compete with crops for nutrients and water. Trees
are long-lived and, as such, involve inflexibility. Investments in agroforestry can thus
take a long time to pay of. Because of the superior rooting systems that can be attained
by perennial crops, there is some effort being invested in developing perennial varieties
of certain annual crops, including rice, wheat, and maize (Cox et al. 2002).
Diversity and functional diversity are different things. That is, the number of spe-
cies is not as important as the number of distinct functional traits, and not all combina-
tions are created equal. In one study, for instance, fava bean responded well to mixtures
with maize, but not with wheat (Fan et al. 2006). Components may interact in positive
ways, but also in negative or neutral ways. Some interactions can be predicted, but oth-
ers may result from idiosyncratic features, such as the way that a secondary metabo-
lite from one species influences another. Successful systems may be developed based on
trial and error and/or the use of ecological and process understanding. Although a great
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