Elements of sustainable agriculture

Abstract

A nearly infinite number of dimensions must be considered if productivity of an agroecosystem is to be sustained indefinitely. In many cases, the most limiting element may determine the future sustainability of the system and set the context in which management decisions, including those involving nematodes, must be made. Some examples of critical elements that may change over time include human population size, water, fossil fuel energy, nitrogen, carbon dioxide, salinity, economics, and amount of cropland. The implications of these factors in future nematode management strategies are discussed briefly. The multiple dimensions affecting the sustainability of an agroecosystem parallel the dimensions of the ecological niches of the organisms involved, but include a number of human-imposed dimensions (political, social, economic, and management practices) as well. A variety of complex problems, some with conflicting solutions, must be addressed in the planning and design of agroecosystems sustainable for future generations.

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PUNTO DE VISTA - VIEWPOINT
ELEMENTS OF SUSTAINABLE AGRICULTURE
†
R. McSorley, and D. L. Porazinska
Department of Entomology and Nematology, P.O. Box 110620, University of Florida, Gainesville, FL
32611-0620 and Natural Resource Ecology Lab, Colorado State University, Ft. Collins, CO 80523,
U.S.A.
ABSTRACT
McSorley, R., and D. L. Porazinska. 2001. Elements of sustainable agriculture. Nematropica 31:1-9.
A nearly infinite number of dimensions must be considered if productivity of an agroecosystem is
to be sustained indefinitely. In many cases, the most limiting element may determine the future sus-
tainability of the system and set the context in which management decisions, including those involv-
ing nematodes, must be made. Some examples of critical elements that may change over time include
human population size, water, fossil fuel energy, nitrogen, carbon dioxide, salinity, economics, and
amount of cropland. The implications of these factors in future nematode management strategies are
discussed briefly. The multiple dimensions affecting the sustainability of an agroecosystem parallel
the dimensions of the ecological niches of the organisms involved, but include a number of human-
imposed dimensions (political, social, economic, and management practices) as well. A variety of
complex problems, some with conflicting solutions, must be addressed in the planning and design of
agroecosystems sustainable for future generations.
Key words
: agroecosystem, ecological niche, nematode, nematode management, sustainability, sus-
tainable agriculture.
RESUMEN
McSorley, R., and D. L. Porazinska. 2001. Elementos de la agricultura sustentable. Nematrópica 31:1-9.
Un número casi infinito de medidas deben ser consideradas para que la productividad de un
agroecosistema se sostenga idefinidamente. En muchos casos, el elemento más limitante puede de-
terminar la futura sustentabilidad del sistema y establecer el contexto en el cual se deben tomar de-
cisiones sobre manejo incluyedo aquellas relacionadas con nematodos. Entre algunos ejemplos de
elementos criticos que pudieran cambiar a través del tiempo se encuentran: población humana,
agua, energia procedente de combustible fósil, nitrógeno, dioxido de carbono, salinidad, economía
y cantidad de tierra agrícola. Las implicaciones de estos factores en el futuro de las estrategias para
el control de nematodos se discuten brevemente. Las multiples dimensiones que afectan la sustenta-
bilidad de un agroecosistema van paralelas a las dimensiones de los nichos ecológicos de los organis-
mos que forman parte de él, también incluyen un número de medidas impuestas por el hombre
(politica, social, económica, y practicas de manejo). Una variedad de problemas complejos, algunos
con soluciones conflictivas que deben ser considerados en la planificación y diseño de agroecosiste-
mas sustentables para las generaciones futuras.
Palabras claves
: Agricultura sustentable, agroecosistema, manejo de nematodo, nicho ecológico, ne-
matodo, sustentabilidad.
†
Florida Agricultural Experiment Station Journal Series No. R-07677. Symposium presented at XXXII Annual
Meeting of ONTA, Auburn, AL, April 16-20, 2000.

2 NEMATROPICA Vol. 31, No. 1, 2001
INTRODUCTION
Although much emphasis has been
given to sustainable agriculture over the last
decade, the term has been variously defined
(Benbrook, 1990; 1991; Crews
et al.
, 1991;
Parr, 1991; Powers and McSorley, 2000).
Most current definitions emphasize mainte-
nance of ecosystem productivity and an ade-
quate food supply for all people,
preservation of environmental quality, and
conservation of nonrenewable resources
and biological diversity. Food safety, eco-
nomic, and social components are often
included. A time element is fundamental to
the concept of sustainability, due to the
dynamic nature of agroecosystems, and the
most credible systems are those which are
intergenerational or maintained for an even
longer time period (Christensen
et al.
, 1996;
Ellis and Wang, 1997). Systems cease to be
sustainable when a resource is depleted,
when unacceptable levels of pollution or
other environmental problems occur, or
when practices to maintain them are no
longer economically or socially acceptable.
While a sustainable agroecosystem is a
desirable goal, achieving this ideal can be
difficult, since so many elements affect the
function and integrity of agroecosystems. A
few of the more critical elements with
potential to affect many agroecosystems are
discussed below. Central to all sustainable
systems is the issue of time. Sustainability is
not a one-year, two-year, or short-term goal;
success is measured over generations or
even centuries. However, even within the
next few decades, several critical elements
may show important changes that may have
possible implications in agriculture and
therefore in nematode management.
SOME CRITICAL ELEMENTS
OF SUSTAINABILITY
World Population
. The human popula-
tion has been growing at an exponential
pace, passing 4 billion in the mid-1970s
and 6 billion in 1999. Various forecasts
project a population size of about 8.5 bil-
lion in 2025 (Powers and McSorley, 2000;
Spedding, 1996; World Resources Insti-
tute, 1994). A population size of 8 billion
represents an increase of 33% over the 6
billion present in 1999, and therefore
should demand a corresponding 33%
increase in food supply over current levels.
Note that sustainability does not imply
maintenance of present-day production lev-
els. For food supply per capita to remain
level, projected population increases over
the next two decades require that agricul-
tural production must increase greatly or
distribution of food must greatly improve.
One method of increasing production
is by limiting losses to nematodes, insects,
weeds, and plant pathogens. A world sur-
vey (Sasser and Freckman, 1987) estimated
annual losses of 10.7% to nematodes on 20
life-sustaining crops. Recent estimates
(Koenning et al., 1999) of losses to nema-
todes in the United States are somewhat
lower (<5% on most crops) but still repre-
sent a substantial amount of production.
CO
2
and Temperature
. The annual
increases in atmospheric CO
2
concentra-
tions have been well documented since the
late 1950s (Keeling and Whorf, 1994).
Trends in temperature increases antici-
pated from global warming are less clear,
with an increase of 0.5
°
C reported during
the 30 years prior to 1993 (Wilson and
Hansen, 1994). The warmest years of the
20th century occurred during its last two
decades (Brown, 1997).
Temperature increases of 1.0
°
C or
2.75
°
C would have predictable effects in
shortening the life cycles of nematodes
such as
Meloidogyne incognita
(Table 1).
Under these scenarios, a temperature
increase of 2.75
°
C would reduce the gener-
ation time by 15% (assuming 25
°
C initial
temperature) to 22% (assuming 20
°
C ini-

Elements of sustainable agriculture: McSorley, Porazinska 3
tial temperature). Such changes would
result in more generations per year as well
as more rapid development to reproduc-
tive stages. Increases in mean tempera-
tures could also be expected to result in
changes in the geographic ranges of some
plant-parasitic nematodes and their hosts.
In the Northern Hemisphere, the ranges
of species like
M. incognita
or
Radopholus
similis
may expand northward, while
ranges of nematodes preferring cooler cli-
mates (e.g.,
M. hapla, Globodera pallida
)
might contract. Based on proposed distri-
bution maps for root-knot nematodes
(Taylor and Sasser, 1978), an increase of
2.75
°
C would be predicted to result in a
northward shift of about 150 km for
M. incognita
and
M. javanica
in the eastern
United States. In addition to the increased
potential for nematode damage resulting
from range changes or more rapid genera-
tion times, higher temperatures may
reduce the expression of nematode resis-
tance in some crops (Dropkin, 1969;
Sydenham
et al.
, 1997). Increased potential
for nematode damage may be partially off-
set by increased plant growth rates and
increased activity of nematode antagonists.
A major concern is that climate change
may lead to disruption of weather and tem-
perature patterns, leading to problems in
crop production as well (Brown, 1997).
In addition, temperature change may
alter activity rates of nematodes and other
organisms involved in the decomposition
process, increasing decomposition rates
and the availability of nutrients from
organic sources. Carbon sequestration is
proposed as a strategy for moving some of
the atmospheric CO
2
into crop biomass
and soil organic matter (Lal
et al.
, 1999). If
this strategy is pursued, the increased soil
organic matter could be expected to stimu-
late soil food webs. Because of such trends,
future changes in soil organic matter and
soil food webs are difficult to anticipate;
organic matter may increase from carbon
sequestration, but its removal may be
accelerated by increased decomposition
rates.
Water.
The amount of land under irriga-
tion has increased greatly during the sec-
ond half of the 20th century (Brown,
1997). Agricultural demands for irrigation
water have resulted in lowering of water
tables, reduction in flow of rivers, reduc-
tion of size in lakes and ponds, and other
supply problems (Brown, 1997; Powers
and McSorley, 2000). Conservation of the
freshwater supply is essential for a sustain-
able agricultural system, and will likely
require a variety of practices such as recy-
cling of water, efficient irrigation practices,
optimum timing of irrigation, mulches,
conservation tillage, and optimum man-
agement of watersheds and recharge areas.
Nematodes, particularly those infecting
roots, can further increase water stress on
plants (Wilcox-Lee and Loria, 1987). On
the other hand, use of mulches or other
practices that retain soil moisture may help
improve crop tolerance to nematodes
(McSorley and Gallaher, 1995). Conserva-
tion tillage, while important for erosion
Table 1. Time required for
Meloidogyne incognita
to
reach second generation
y
under various temperature
change scenarios, based on initial temperatures of
20
°
C or 25
°
C.
Temperature
change
Time (days)
20
°
C initial
temperature
25
°
C initial
temperature
0
°
C 50.0
z
33.3
+1.0
°
C 45.4 31.2
+2.75
°
C 39.2 28.2
y
Second-stage juvenile to second-stage juvenile.
Z
Assume 500 degree days (above 10
°
C base) for com-
plete generation, based on data from Sydenham
et al.
(1997) for
M. incognita
on ‘Black Valentine’ bean.

4 NEMATROPICA Vol. 31, No. 1, 2001
and water management, does not appear
to impact plant-parasitic nematodes in a
consistent manner (McSorley, 1998; Minton,
1986). Conservation tillage and other prac-
tices that conserve or add soil organic mat-
ter may stimulate certain groups of nema-
todes involved in decomposition (Fu
et al.
,
2000).
Fossil Fuels
. Petroleum, coal, and natu-
ral gas are finite resources subject to deple-
tion. Although petroleum consumption
has escalated in recent decades, new dis-
coveries have kept reserves constant at a
40-50 year supply (World Resources Insti-
tute, 1996). However, assuming that a 33%
increase in world population corresponds
to a 33% increase in petroleum consump-
tion, a 50-year petroleum reserve would
become a 37.5-year reserve. An increase in
the per capita consumption of petroleum
or a failure in the ability of new discoveries
to keep pace with consumption would lead
to a more rapid depletion of known
reserves. Limitations in availability of
petroleum and other fossil fuels would
increase cost or limit many nematicides,
insecticides, herbicides, fungicides, or fer-
tilizers that are derived from fossil fuels or
require energy from fossil fuels for their
production or for their application.
Energy
. Currently, much of the energy
input to agricultural systems is derived from
fossil fuels. The production of many sys-
tems increases with the amount of energy
supplied (Pimentel and Dazhong, 1990;
Powers and McSorley, 2000; Tivy, 1992).
However, the efficiency of agricultural pro-
duction tends to decrease as energy inputs
increase. For instance, corn yield in the
United States in 1980 was nearly 3.5 times
that in 1920, when energy input was only
12.5% of the 1980 level (Pimentel and
Dazhong, 1990). While there was a great
increase in overall production during that
60-year period, it was accompanied by a
decrease in the efficiency of energy use.
One of the greatest challenges in establish-
ing sustainable systems is to maintain cur-
rent high production levels while increasing
energy use efficiency and using less energy
or renewable energy sources.
Nitrogen
. In many agricultural systems,
the greatest single energy input is for syn-
thetic nitrogen fertilizers (Pimentel and
Burgess, 1980). The high energy required
for manufacture of synthetic fertilizers is
currently obtained from fossil fuels, so any
future limitations in fossil fuels would
affect synthetic fertilizers as well. Under
such scenarios, nitrogen conservation is
essential, with increased emphasis on recy-
cling of nitrogen and reduction of losses,
and on the use of organic amendments
and nitrogen fixation by legumes and
green manures as primary nitrogen sources.
Overall, the effect of organic amend-
ments on plant-parasitic nematodes is vari-
able and inconsistent, but some materials
may have potential in this area (McSorley,
1998; McSorley and Duncan, 1995; Muller
and Gooch, 1982; Rodriguez-Kabana, 1986;
Stirling, 1991). Many of them help to
improve plant performance, regardless of
their effects on nematodes. Widespread
planting and use of legumes may be
required to meet future nitrogen demands.
However, research is needed to maximize
the efficiency of previous legume cover
crops or legume hays in meeting nitrogen
needs of subsequent crops in cropping sys-
tems (Giller and Wilson, 1991; Peoples
et al.
,
1995; Power 1990; Powers and McSorley,
2000). Whether or not the inclusion of more
legumes in cropping systems would increase
nematode problems depends on the nema-
todes and cropping systems involved. In the
southeastern United States,
Meloidogyne
spp.
often build up more readily on legumes
than on non-legume cover crops, and so
nematode-resistant legumes may be particu-
larly desirable for such systems (McSorley,
1999; Weaver
et al.
, 1993, 1998).

Elements of sustainable agriculture: McSorley, Porazinska 5
Economics
. For a practice to be sustain-
able, it must be adopted and used by grow-
ers. Economics often provides the incent-
ive for adoption. In some cases, economic
goals may conflict with other elements of
sustainability, as for example, if a practice
useful for conservation of water or nutri-
ents is not profitable for a farmer (Porazin-
ska
et al.
, 1998). Availability of inexpensive
synthetic nitrogen fertilizers is the main
reason that they are currently preferred
over organic fertilizers, and their use is
actually increasing for this reason (Robert-
son, 1997). This is in direct conflict with
the nitrogen and energy conservation
issues mentioned in previous sections, but
illustrates the overriding importance of eco-
nomics as a component of sustainability.
For a number of years, it has been rec-
ognized that the use of methyl bromide for
nematode management is not sustainable
due to environmental concerns and regula-
tion (McSorley
et al.
, 1985; Noling and
Becker, 1994). Yet in many systems, nemati-
cides derived from fossil fuels are preferred
over non-chemical alternatives because of
their efficacy and profitability. Non-chemi-
cal alternatives will be acceptable if effective
and profitable (Chellemi
et al.
, 1997), and
so an important challenge is to improve the
efficacy and profitability of sustainable
methods for nematode management.
Pollutants and Contaminants
. Systems
are not sustainable if hazardous materials
are produced or accumulated over time.
These may include objectionable concen-
trations of heavy metals, pesticide residues
or other toxic chemicals, fertilizer elements,
or other materials applied in excess,
although historically salts have probably
been the most destructive contaminants of
agricultural land. Salinization, a problem
since ancient times, continues to render
land unsuitable for production at a rate of
about 1.5-2.5 million ha per year, largely a
result of poor irrigation practices (Gard-
ner, 1997). Effects of salt and other con-
taminants on nematodes vary. For
example, salinity decreased population
densities of
Rotylenchulus reniformis
(Heald
and Heilman, 1971) but stimulated
Tylen-
chulus semipenetrans
under some conditions
(Mashela
et al.
, 1992).
Cropland
. If the world supply of crop-
land remained constant, the 0.25 ha per
person available to the 6 billion people
today would decline to 0.19 ha per person
with a population of 8 billion (Powers and
McSorley, 2000). Globally, cropland is ex-
panding slightly, with losses from erosion,
urbanization, and salinization more than
offset by conversion of forests, other natu-
ral areas, and grasslands (World Resources
Institute, 1996). Habitat destruction is the
main reason for loss of biodiversity (Collins
and Qualset, 1999), and negative effects
apply to soil ecosystems as well (Neher and
Barbercheck, 1999). However, fewer prob-
lems with plant-parasitic nematodes may
be expected on new land than on land
cropped continuously to food crops. Like-
wise, if pasture land is converted to food
crops, problems with some plant-parasitic
nematodes may be limited if the pasture
grass was a poor host. For example, it is
known that some pasture grasses, such as
bahiagrass (
Paspalum notatum
), are favor-
able rotation crops for managing
Meloido-
gyne
spp. and
Heterodera glycines
(Weaver
et
al.
, 1998).
ECOLOGICAL NICHE THEORY
AND SUSTAINABILITY
An agroecosystem requires many differ-
ent resource inputs that may result in many
potential environmental impacts. When
present in excessive or insufficient
amounts, any of these inputs or outputs has
the potential to render a system unsustain-
able. There is a useful parallel between the
components of agricultural sustainability

6 NEMATROPICA Vol. 31, No. 1, 2001
and those included in the concept of the
ecological niche. In a most general sense,
the ecological niche refers to all of those
factors that affect the occurrence of an
organism in a particular place (Begon
et al.
,
1990; Odum, 1983). Because these factors
are so numerous, the
n
-dimensional nature
(where
n
approaches infinity) of the niche
was recognized (Hutchinson, 1957).
Similarly, the components of agricul-
tural sustainability have
n
dimensions.
These include physical resources such as
water, light, land, or various nutrients,
which must be maintained at sufficient lev-
els to sustain a system. As with physical
niche dimensions in ecology (Begon
et al.
,
1990; Hutchinson, 1957), inadequate or
excessive levels of these elements would
limit the sustainability of a system (Fig. 1).
Excessive levels of pollutants may be partic-
ularly limiting. But the
n
dimensions of
agricultural sustainability encompass much
more than these physical factors. Some
dimensions of sustainable agriculture are
biological, such as population levels of a
non-target species (e.g., a beneficial insect,
free-living nematodes), or numbers of spe-
cies (biodiversity) in a region. Economic,
political, and social dimensions may be
included as well. Regardless of the magni-
tude of
n
, the great number of potential
interactions among so many factors limits
our ability to predict an outcome based
on information about individual factors
obtained under controlled conditions.
In ecological niche theory, the niche
dimension that is most limiting will dictate
the occurrence of an organism in a partic-
ular habitat. A similar law of the minimum
applies to the sustainability of agroecosys-
tems. Even if most dimensions are suffi-
cient, an agroecosystem may become
unsustainable if any one resource runs out
or if the level of some negative dimension
is reached or exceeded. Thus, defining sus-
tainability is a difficult goal because all ele-
ments (
n
dimensions) must be considered,
or the definition is limited! Operationally,
it may be convenient and necessary to limit
the definition, and therefore some of the
more critical elements were emphasized
here. But there is a possibility that some
unmeasured dimension or interaction may
eventually lead to an unsustainable system.
DEFINING A SUSTAINABLE SYSTEM
Due to the multitude of elements
involved and potential conflicts among
them, developing, or even defining, a sus-
tainable agroecosystem is not a simple task.
Much detailed specific information is
needed for the planning of sustainable sys-
tems. All agroecosystems are different, so
further clarification of objectives and defi-
nition of measurable tolerance limits are
needed to define a specific sustainable sys-
tem. Clear answers to the following ques-
tions are required:
What is to be sustained?
(People, endan-
gered species, specific resources, a spe-
cific level of production, a particular
lifestyle, or all of these?)
Fig. 1. Relationship between ecosystem health and nu
-
trient concentration, for a nutrient showing a relative
-
ly normal distribution. An optimum nutrien
t
concentration exists for which sustainability of the sys
-
tem is possible. If the nutrient level is inadequate, pro
-
duction will be insufficient to sustain the system, while
if the level is excessive, pollution may affect ecosystem
quality.

Elements of sustainable agriculture: McSorley, Porazinska 7
At what level?
(Population size, desired
resource level, how many species?)
For how long?
(One season, five years, a gen-
eration, 100 years?)
What is the size and location of the system?
(A
single field, a farm, a region, a country,
the world?)
What are the limits of each dimension?
(Desir-
able levels of each dimension, measur-
able definitions of pollution?)
How does the system fit into the landscape?
(Interactions with other ecosystems,
fate of inputs and outputs?)
Much research will be needed to define
the limits of the various dimensions and to
develop practices that maintain them. In
addition, the dynamic nature of the agro-
ecosystem and changes in dimensions over
a long time scale must always be consid-
ered. Economic factors and social trends
are particularly subject to market and tem-
poral fluctuations, and there is some ques-
tion about whether those should be
included as components of sustainability
(Crews
et al.
, 1991). When economic
dimensions are included, it is critical that
they reflect real and long-term values, not
short-term profits resulting from legisla-
tion, subsidies, or temporary oversupply
(Duncan and Noling, 1998). Even though
environmental sustainability is a prerequi-
site for social and economic sustainability
(Goodland and Daly, 1996), realistic and
accurate economic and social dimensions
should be included in agricultural sustain-
ability because exclusion of these factors
may result in an unrealistic system that will
not be adopted or sustained.
Definition and design of sustainable
agroecosystems are difficult issues because
of the many elements or dimensions
involved. Also, no agroecosystem by itself is
100% sustainable since some harvested
product is usually removed from the agroec-
osystem. Therefore, the sustainability of any
agroecosystem is interdependent with that
of urban and natural systems, and so true
sustainability may not be possible except in
a sustainable biosphere or universe. How-
ever, the recognition and definitions of the
elements of sustainable agriculture can pro-
vide goals to aspire to or approach in devel-
oping “more sustainable” systems.
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Received: Accepted for publication:
8.VIII.2000 13.II.2001
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