The Environmental Complex - Agroecology: The Ecology of Sustainable Food Systems - page 166

take such variability into account. High standard devia-
tions do not necessarily mean that something was wrong
with the research methodology. It may just mean that the
sample area was extremely variable!

organisms. The interaction of factors can have both positive
and negative consequences in agroecosystems.

C OMPENSATING F ACTORS

When one factor overcomes or eliminates the impact of
another, then it is referred to as a compensating factor .
When a crop is growing under conditions that would
otherwise be limiting for its successful growth or devel-
opment, one or more factors may be compensating for the
limiting factor.
The effect of a compensating factor is commonly
seen in fertilization trials, when a particular soil nutrient
(e.g., nitrogen) is limiting as determined by the plant
response. Reduced growth and lower yields are signs of
the deficiency. But rather than simply adding more of
the deficient nutrient, it is sometimes possible to alter
some other factor of the environment that renders more
of the “limited” nutrient available to plants. In the case
of nitrogen deficiency, it may be that poor soil drainage
is restricting nitrogen uptake by roots, so that once soil
drainage is improved, the lack of nitrogen uptake is
compensated for.
Another case of compensation for a limiting factor
occurs when a farmer counters the negative impact of a
leaf-eating herbivore by stimulating more luxurious or
rapid growth of the affected crop through an intervention
such as adding compost to the soil or applying a foliar
fertilizer. The added biomass can allow the crop to carry
the herbivore load and still produce a successful harvest.
The added plant growth compensates for herbivory.
In coastal regions where fog is common during the
dry summer season (e.g., the Mediterranean maritime
region of coastal California), the fog can compensate for
the lack of rainfall. This occurs through the reduction in
transpirational water loss, and the lower evaporative
stress due to less direct sunlight and lower temperatures.
The leafy vegetable crops common in the lower Salinas
and Pajaro Valleys of California could probably not be
grown profitably during the middle of the summer with-
out such compensation, because these crops are subject
to considerable water loss through transpiration on
hot days.

D YNAMIC C HANGE

Since the combination of factors in any environment is
constantly changing through time, a farmer must also take
into account temporal heterogeneity. Changes take place
hourly, daily, seasonally, yearly, and even as part of longer-
term climatic shifts. Some of this change is cumulative
and some of it is cyclic. For any particular factor, there is
a need to be aware of how rapidly its intensity can change
over time, and how the changes can affect a particular
organism, based on its length of exposure and its limits
of tolerance. At the same time, each organism, as it goes
through its life cycle, will undergo shifts both in the way
it responds to different factor intensities and in its toler-
ance for those intensities.
A crop plant, for example, experiences a continually
changing environment as it progresses through its life
cycle. If a factor or combination of factors reach some
critical level at the same time the plant reaches some
particularly sensitive stage in its life cycle, suppression
of further development can occur and result in crop fail-
ure. Germination, initial seedling growth, flowering, and
fruiting are the stages during which extreme or unusual
variation in environmental factors is most likely to
impact crop performance. As was seen in Figure 9.4, for
example, a period of waterlogging during the growth of
cowpeas had a negative effect on yield, but the nature
and extent of this effect depended on when the water-
logging occurred.
Because of dynamic change, interventions in the
field often need to be carefully timed. For example, a
farmer wanting to use a propane-fired burner (described
in Chapter 10) to kill weed seedlings is limited to a small
window of time in the early stages of development of
the crop. If the crop is too small and delicate, flaming
can kill the crop seedlings along with the weed seedlings.
If the crop is too tall, it might be difficult to avoid
damaging the plants with the flaming apparatus itself.
The effective window for using flame weeders might be
as short as 4 or 5 d in delicate crops such as carrots or
onions, both of which have little ability to deal with
interference from weeds on their own.

12.3.2 M ULTIPLICITY OF F ACTORS

When several factors are closely related, it may be partic-
ularly difficult to separate the effect of one factor from
another. The factors can act as a functional unit, either
simultaneously or in a chainlike manner. One factor influ-
ences or accentuates another, which then affects a third;
but in terms of crop response, where one factor stops and
another takes over is impossible to determine. The factors
of temperature, light, and soil moisture often function in
such a closely interrelated manner. For a corn crop in an

INTERACTION OF ENVIRONMENTAL
FACTORS

Each of the many factors that make up the environmen-
tal complex has the potential to interact with other factors
and thereby modify, accentuate, or mitigate their affects on