Environmental Engineering Reference
In-Depth Information
productivity (McNaughton, 1985; Walker, 1988). For instance, the eco-
nomic value of a change in diversity can be evaluated from the change in
livestock liveweight gain associated with a decline in forage biomass as a
result of a decline in grassland diversity.
A number of studies in the applied economics literature have used the
stochastic production function approach suggested by Just and Pope
(1978) to capture the value of crop diversity. These studies indicate that
genetic variability within and between crop species confers the potential
to resist stress, provide shelter from adverse conditions and increase the
resilience and sustainability of agro-ecosystems. Plot studies indicate that
intercropping can reduce the probability of absolute crop failure and that
crop diversii cation increases crop income stability (Walker et al., 1983).
Therefore, the greater the diversity between and/or within species and
functional groups, the greater is the tolerance to pests. This is because
pests easily spread through crops with the same genetic base (Sumner et
al., 1981; Altieri and Liebman, 1986).
Crop diversity may enhance farm productivity, stabilize farm income
and reduce the risk of outright crop failure (Long et al., 2000). The exist-
ence of a limited number of crops grown in an area makes these crops
more vulnerable to diseases and pests. By maintaining proper crop rota-
tions diversity can improve soil productivity and reduce the need for agro-
chemical applications. Land managers also recognize that soil and climatic
conditions can vary considerably. In such circumstances, growing dif er-
ent crops and crop varieties can lead to more ei cient use of resources.
Some crops can be grown on fertile land while others can utilize marginal
areas. Therefore, the greater the variability of soil and climatic conditions,
the greater the impact biodiversity will have on improved agricultural
production.
For example, Smale et al. (1998), report that crop diversity is positively
related to the mean of yields and negatively correlated with the variance of
yields in rain-fed districts of the Punjab in Pakistan. Di Falco and Perrings
(2003, 2005) found cereal diversity to be positively correlated with yields
and negatively correlated with revenue variability in two studies in south-
ern Italy. Di Falco and Chavas (2006) point out increased crop diversity
may also reduce the likelihood of complete crop failure. Diversity is impor-
tant also for commercial farmers, since they are dependent on diversity in
the breeding pool, regardless of whether it is provided on or of farm.
Other examples of indirect values associated with diversity include
mycorrhiza, which are important for the functioning of ecosystems and
can be considered as a complementary input to timber production. They
represent an indirect use value. Silvicultural practices that eliminate myc-
orrhiza from the system will involve the loss of timber revenue. Although
