Agriculture Reference
In-Depth Information
requires more knowledge. Ultimately, however, under-
standing the ecological basis for how diversity operates in
agroecosystems, and taking advantage of complexity
rather than striving to eliminate it, is the only strategy
leading to sustainability.
TABLE 16.4
Diversity Measures of Four Hypothetical
Agroecosystems
Even
Polyculture
of Two
Crops
Even
Polyculture
of Three
Crops
Uneven
Polyculture
of Three
Crops
EVALUATING AGROECOSYSTEM DIVERSITY
AND ITS BENEFITS
Mono-
culture
Corn plants
300
150
100
250
To manage diversity most effectively, we need means of
measuring diversity and evaluating how increases in
diversity actually impact the performance and functioning
of an agroecosystem. We need to be able to recognize the
presence of diversity and the patterns of its distribution on
the landscape, and we need to know if, and to what extent,
the presence of that diversity is of benefit to the perfor-
mance of the agroecosystem, especially from the farmer's
point of view. Several approaches can be taken to analyze
and research the presence and impacts of diversity.
Squash plants
0
150
100
25
Bean plants
0
0
100
25
Number of
species (
s
)
1
2
3
3
Number of
individuals (
N
)
300
300
300
300
Relative species
richness
Low
Medium
High
High
Relative species
evenness
High
High
High
Low
Instead of using the number of individuals of each
species as a basis for measuring a system's species diver-
sity, it is possible to use some other species characteristic,
such as biomass or productivity. This may be more appro-
priate, for example, when the biomass of a typical
individual of one species is very different from the bio-
masses of the individuals of the other species. Number of
individuals, biomass, and productivity are all examples of
importance values
for a particular species.
Ecology offers various ways of quantifying the species
diversity of a system. The simplest method is to ignore
species evenness, and to measure the number of species
in terms of the number of individuals. Such a measure is
provided by Margalef's index of diversity:
I
NDICES
OF
S
PECIES
D
IVERSITY
It is obvious that any kind of intercrop is more diverse
than a monoculture. Comparing the diversity of two dif-
ferent intercropping systems, however — varying in both
species numbers and planting ratios — requires that we
measure the diversity of each. To do so, we can borrow
tools and concepts developed by ecologists for natural
ecosystems.
Ecologists recognize that the diversity of an ecosystem
or community is determined by more than just the number
of species. A community made up of 50 redwood trees,
50 tanbark oaks, and 50 Douglas firs is more diverse than
one made up of 130 redwood trees, 10 tanbark oaks, and
10 Douglas firs. Both have the same number of species
and total individuals, but the individuals in the first
community are distributed more evenly among the species
than those in the second community, where redwood trees
dominate.
This example demonstrates that there are two com-
ponents of species diversity: the number of species,
called
species richness
, and the evenness of the distri-
bution of the individuals in the system among the
different species, called
species evenness.
Both compo-
nents must be considered in any comprehensive mea-
surement of diversity, in both natural ecosystems and
agroecosystems.
How these concepts can be applied in analyzing the
diversity of agroecosystems is demonstrated in Table 16.4,
where four different hypothetical systems, each with the
same number of individual crop plants, are compared.
Among these systems, the even polyculture of three crops
is the most diverse, since it is the only one in which both
species richness and species evenness are high in relation
to the other systems.
diversity = (
s
- 1)/ log
N
where
s
is the number species and
N
is the number of
individuals. The usefulness of Margalef's index is limited
because it cannot distinguish the varying diversity of sys-
tems with the same
s
and
N
, such as the even and uneven
three-crop polycultures in Table 16.4.
There are two other diversity indices that do take
species evenness into account, and are therefore more
useful. The
Shannon index
is an application of informa-
tion theory, based on the idea that greater diversity
corresponds to greater uncertainty in picking at random
an individual of a particular species. It is given by the
following formula:
S
⎛
⎜
⎞
⎟
⎛
⎜
⎞
⎟
n
N
n
N
∑
H
=−
i
log
i
e
i
=
1
where
n
i
is the number of individuals in the system (or
sample) belonging to the
i
th species.
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