Agriculture Reference
In-Depth Information
The
Simpson index
of diversity is the inverse of an
index of community dominance with the same name. It is
based on the principle that a system is most diverse when
none of its component species can be considered any more
dominant than any of the others. It is given by the follow-
ing formula:
is. In cropping systems where two or more crop species
are in close enough proximity to each other, various kinds
of between-species interference are possible (as described
in Chapter 11 and Chapter 13) that can provide clear ben-
efits in improved yield, nutrient cycling, and so on.
Despite the fact that researchers have accumulated a
great deal of evidence that intercropping can provide sub-
stantial yield advantages over monocropping, it is impor-
tant to remember that there can also be disadvantages to
intercropping. There may be practical difficulties in the
management of the intercrop, and yield decreases may
occur because of the effects of adverse interference. Such
cases should not be used as arguments against intercrop-
ping, but rather as a means of determining where research
needs to be focused to avoid such problems.
NN
nn
i
(
−
)
diversity =
∑
(
−
1
i
For the Simpson index, the minimum value is 1; for
the Shannon index it is 0. Both minimums indicate the
absence of diversity, the condition that exists in a mono-
culture. In theory, the maximum value for each index is
limited only by the number of species and how evenly
distributed they are in the ecosystem. Relatively diverse
natural ecosystems have Simpson indices of 5 or greater,
and Shannon indices of 3 to 4.
Calculations of Margalef, Simpson, and Shannon index
values for the hypothetical systems in Table 16.4 are given
in Table 16.5. The Shannon and Simpson values both show
that the even polyculture of two crops is more diverse than
the uneven polyculture of three crops, underscoring the
importance of species evenness in agroecosystem diversity.
More detailed descriptions of the Shannon and
Simpson indices, including the theory on which they are
based and the ways they can be applied, can be found in
the ecology texts cited in the recommended readings at
the end of the chapter.
The Land Equivalent Ratio
An important tool for the study and evaluation of inter-
cropping systems is the land equivalent ratio (LER). LER
provides an all-other-things-being-equal measure of the
yield advantage obtained by growing two or more crops
as an intercrop compared to growing the same crops as a
collection of separate monocultures. LER thus allows us
to go beyond a description of the pattern of diversity into
an analysis of the advantages of intercropping.
The LER is calculated using the formula
Yp
Ym
∑
i
LER =
i
where Yp is the yield of each crop in the intercrop or
polyculture, and Ym is the yield of each crop in the sole
crop or monoculture. For each crop (i) a ratio is calculated
to determine the partial LER for that crop, then the partial
LERs are summed to give the total LER for the intercrop.
An example of how the LER is calculated is given in
Table 16.6.
An LER value of 1.0 is the break-even point, indicat-
ing no difference in yield between the intercrop and the
A
SSESSING
THE
B
ENEFITS
OF
I
NTERCROP
D
IVERSITY
On a farm, a way of measuring the value gained from
greater diversity in the cropping system will be very useful
in helping the farmer evaluate the advantages or disadvan-
tages of different cropping arrangements. The diversity
indices described above can quantify diversity, but they do
not tell us how that diversity translates into performance,
or what the ecological basis of any improved performance
TABLE 16.5
Diversity Index Values for the Four Hypothetical
Agroecosystems in Table 16.4
TA B L E 1 6 . 6
Representative Data for Calculation of LER
Yield In
Polyculture
(Yp), kg/ha
Yield in
Monoculture
(Ym), kg/ha
Partial LER
Even
Polyculture
of Two
Crops
Even
Polyculture
of Three
Crops
Uneven
Polyculture
of Three
Crops
⎛
⎜
⎞
⎟
Yp
i
Ym
i
Mono-
culture
Margalef
diversity
0
0.4
0.81
0.81
Crop A
1000
1200
0.83
Crop B
800
1000
0.80
Shannon
diversity
0
0.69
1.10
0.57
Yp
Ym
i
∑=
163
.
Simpson
diversity
1.0
2.01
3.02
1.41
i
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