The Environmental Complex - Agroecology: The Ecology of Sustainable Food Systems - page 164

the “safe site.” Factors immediately surrounding the seed
are what influence the seed most directly. Factors around
the outside perimeter of the diagram are factors and vari-
ables that influence the effect, degree, or presence of the
direct factors.

Solar radiation

Relative
humidity

Temperature

Humans

Atmospheric
composition

Associated
animals

HETEROGENEITY OF THE ENVIRONMENT

Wind

The environment of any individual organism varies
not only in space but also in time. The intensity of each
factor in Figure 12.1 shows variation from place to place
through time, with an average for each factor setting the
parameters of the habitat within which each organism is
adapted. When variation in a factor exceeds the limits of
tolerance of an organism, the effects can be very damag-
ing. Farming systems that take this variation into account
are much more likely to have a positive outcome for the
farmer.

Crop
organism

Associated
plants

Rainfall,
irrigation

Soil
nutrients

Fire

Soil

Topography,
latitude

Parent
material

Gravity

FIGURE 12.1 Representation of the environmental complex.
The environment of an individual crop plant is made up of many
interacting factors. Although the environment's level of
complexity is high, most of the factors that make it up can be
managed. Recognizing factor interactions and the overall
complexity of the environment is the first step toward sustainable
management. (Adapted from Billings, W. D. 1952. Quarterly
Review of Biology 27: 251-265.)

S PATIAL H ETEROGENEITY

The habitat in which an organism occurs is the space
characterized by particular combinations of factor inten-
sities that vary both horizontally and vertically. Even in a
field planted to a single variety of grain crop, for example,
each plant will encounter slightly different conditions
because of spatial variation in factors such as soil, mois-
ture, temperature, and nutrient levels. The amount of vari-
ation in these factors will depend upon the extent to which
the farmer tries to create uniformity in that field with
equipment, irrigation, fertilizers, or other inputs. Regard-
less of these attempts, however, there will be slight vari-
ation in topography, exposure, soil cover, and so on that
will create microenvironmental differences across the
space of the field. Very small variations in microhabitat,
in turn, can bring about shifts in crop response.
In a wet tropical lowland environment, for example,
where soils are poorly drained and rainfall is high, slight
topographic variation can make a big difference in soil
moisture and drainage. In such an area, the lower lying
areas of a field may be subject to much more waterlogging
than the rest of the field, and crop plants growing there
may experience arrested root development and poorer per-
formance, as illustrated in Figure 9.3. Some farmers in the
region of Tabasco, Mexico, where the photograph in
Figure 9.3 was taken, plant waterlogging-tolerant crops,
such as rice or local varieties of taro ( Colocasia spp. or
Xanthosoma spp.), in the lower lying areas of their farms
as a way of making a better match between crop require-
ments and field conditions. Finding ways to take advan-
tage of the spatial heterogeneity of conditions by adjusting
crop types and arrangements is often more ecologically
efficient than trying to enforce homogeneity or ignore
heterogeneity.

management, in contrast, begins with the farm system as
a whole and designs interventions according to how they
will impact the whole system, not just crop yield. Inter-
ventions may be intended to modify single factors, but the
potential impact on other factors is always considered
as well.

C OMPLEXITY OF I NTERACTION

The way in which a complex of factors interacts to impact
a plant can be illustrated by seed germination and the “safe
site” concept of Harper (1977). We know from ecophy-
siological studies that an individual seed germinates in
response to a precise set of conditions it encounters in its
immediate environment (Naylor, 1984). The locality at the
scale of the seed that provides these conditions has been
termed the safe site . A safe site provides the exact require-
ments of an individual seed for the breaking of dormancy,
and for the processes of germination to take place. In
addition, there must be freedom from hazards such as
diseases, predators, or toxic substances. The conditions of
the safe site must endure until the seedling becomes inde-
pendent of the original seed reserves. The requirements
of the seed during this time change, and so the limits of
what constitutes a safe site must also change.
Figure 12.2 describes some of the environmental fac-
tors that influence the germination of a seed and make up