agroecosystem diversity

One of the pillars of organic farming system practice is to increase total system biodiversity (Pimentel et al., 2005). This may be achieved by increasing the species and varieties or breeds of crops and livestock raised in a system (diversity in space), increasing crop rotation (diversity over time), removing biocides of all types, and maintaining wild areas. In paired studies of conventional and transitioning organic farms, trees and livestock were found to be more common on organic farms (de Jager and van der Werf, 1992). In a comparison to conventional farms, organic farms that had converted an average of 7.3 years earlier had larger areas of seminatural habitat and more diversity in arable fields (Gibson et al., 2007). Literature reviews examining the wildlife conservation value of organic farms find significantly greater abundance and species diversity of plants, birds, and bats than for conventional farms (Hole et al., 2005; Fuller et al., 2005). Another review of 66 papers found that organic farms, on average, had 50% greater abundance of wildlife and 30% more species than con-ventional farms (Bengtsson et al., 2005).

2.4.3.8 summary

An important point that emerges from the research reviewed above is that deliberate, planned increases in the biodiversity of agroecosystems tend to result in increases in associated biodiversity as well (Swift et al., 2004). In other words, increasing the

forms of agroecosystem diversity over which the farmer has direct control (crops, livestock, wild areas) results in increases in forms of diversity over which the farmer does not have direct control (soil microbe and macrofauna, terrestrial and aquatic wildlife, etc.). Additionally, since there appears to be a redundancy of function in the biological world, the diversity of functional groups (and perhaps diversity within certain trophic groups of nematodes), as opposed to species diversity, appears to be a key indicator of agroecosystem function.

The most useful parameters for ecological assessment of the conversion process may be the following: the amount of potentially mineralizable N and the ratio of ammonium to nitrate (as activity measures); the amount of microbial biomass, the extent of mycorrhizal fungus colonization, and counts of macrofauna (as abundance measures); and the taxonomic or trophic diversity of soil organisms (as a proxy for functional diversity).

2.4.4 p^ests

In the category of pests we include weeds, herbivorous insects, and diseases. All three types of pests can cause significant problems during the transition process, when the various biocides used in conventional systems have been withdrawn and biological control functions have not yet developed to their full potential. Therefore, studies involving measurement of pest damage and how it changes over time during the conversion process are important assessments of conversion.

2.4.4.1 Weeds

Although a few studies mention that weeds were not a problem during transition (e.g., Smukler et al., 2008), in most experiences during (and after) transition, weed com-petition appears to be a key factor reducing yield. In a study of cotton systems con-verted to organic, it was found that yield was significantly lower than conventional in all six years of transition studied; the authors conclude that weed pressure may best explain the yield difference (Swezey et al., 2007). The number of weeds and their biomass increased over a six-year transition period in a system converted to organic in Germany (Belde et al., 2000); in another study, Stinner et al. (2004) found that weeds impaired planting during wet springs and decreased organic yields during transition.

To assess the effect of weeds on yield in low-input systems compared to conven-tional, Posner et al. (2008) combined WICST data with that from other published reports on low-input systems (Table 2.1). Across the studies, the degree of weed control in the low-input systems correlated with yield. Closer examination of the data revealed an interaction between treatments and weather: in the 34% of site-years with low yields, wet weather made mechanical tillage difficult, preventing adequate weed control, and this resulted in yields averaging only 74% of conventional sys-tems. In the other 66% of the cases, where mechanical weed control was successful, the yield of the low-input crops was 99% of conventional systems.

The weed seed bank has generally been found to increase during and after the transition period (Riemens et al., 2007; Turner and Bond, 2004; Kummel et al., 2005). Including cereals in rotations, as is common, appears to increase viable weed seed numbers (Turner and Bond, 2004). In an 11-year study to determine the effect

of an increased weed seed bank during transition, a one-time pulse of wild oat seeds was found to have relatively few long-term agronomic effects (Maxwell et al., 2007), suggesting that increases in the weed seed bank during conversion can potentially be controlled over the long-term.

With respect to control, the extent of the increase in the weed seed bank can be limited by vigilance in preventing weeds from forming seeds (Riemans et al., 2007). Additionally, a level 3 conversion to a diversified farming system that includes a range of crop species of different heights that provide cover is likely to provide the greatest opportunity for weed seed destruction by seed-eating animals (Heggenstaller et al., 2006).

Not all weeds compete effectively with the crop, and type of fertilizer may affect weed competition. Organic versus agrochemical fertilization can affect the competi-tive ability of the weed to reduce the biomass of the crop (Davis and Liebman, 2001).

Although the abundances of various weeds increased with time during conversion to organic in a Finnish study, the abundance of only two of the species (Elymus repens TaBle 2.1

low-input versus Conventional Cropping system yields as influenced by Weed Control, from field Trials for row Crops and Nonrow Crops

study Citation

low-input yield as a Percent of Conventional system yield

Source: Data from Posner et al. 2008. Agronomy Journal 100:253–260. (With permission.)

a Weed control in the low-input system determined by visual ratings or biomass.

b Small grain: w = wheat and o = oat.

c Results given here are after three transition years. Authors presented forage yields for low-input sys-tems but without comparison to conventional yields.

d Number of site-years for corn and soybean, respectively.

e Number of site-years for wheat and forage crops, respectively.

and Circium arvense) was negatively correlated with crop dry weight (Riesinger and Hyvönen, 2006).

Weeds are not always detrimental to a system in their overall effect, as they can serve various positive functions ecologically (as insect “traps” or repellents, as habi-tat and food for beneficials, as components of higher biodiversity). In this context, it is important to note that weed diversity often increases during transition (de Jager and van der Werf, 1992; Turner and Bond, 2004). In a study to determine whether organic farming can restore weed diversity to preintensification levels, it was found that herbicide-sensitive nitrophilous species immediately increase and that peren-nials and species less responsive to high nitrogen apparently take longer to become established (Hyvönen, 2007).

2.4.4.2 insects

Pest pressure during the conversion process may be more important in fruits and vegetables than in grains. Insect pressure from corn borer (Ostrinia nubilalis) and bean leaf beetle (Cerotoma trifurcata) was below economic threshold levels and did not appear to be significant in a Midwestern grain system in the process of conver-sion (Delate et al., 2002). However, demonstrating the vulnerability of fruit crops, it was found in a three-year apple conversion in California that secondary lepidopteran pests (apple leafroller and orange tortrix, Argyrotaenia citrana) caused greater fruit scarring in the converted fields than in the conventional fields in the last year of con-version; similarly, apple leafhopper (Typhlocyba pomaria) had denser populations and caused more leaf damage in the converted fields than in the conventional fields in the second and third years of transition (Sweezey et al., 1998).

Like weeds, insects do not always function as pests. Many insects (and other arthropods) play important roles in biological control as parasitoids and predators of herbivorous insects. Increased populations of these beneficials during and after conversion have been consistently reported (Drinkwater et al., 1995). In a study of strawberry conversion in California, there was little economically important pest damage in the organic system over the three-year study period, while at the same time an increase in naturally occurring insect predators was observed (Gliessman et al., 1996). In all but one year of a six-year study, cotton fields converted to organic production in California had significantly greater insect predators than conven-tional fields (Swezey et al., 2007). When nine convenconven-tional and organic farms in California were compared using canonical discriminant analysis, it was determined that pest abundance did not differ significantly between the two types of farms, but the organic farms had higher natural enemy abundance and greater species richness of all functional groups of arthropods (herbivores, predators, parasitoids, and oth-ers); the authors concluded that natural enemies appeared to substitute for pesticides (Letourneau and Goldstein, 2001).

An increase in beneficial insects can be facilitated by raising and releasing them as needed. In Cuba, during the large-scale conversion of the country to organic-style management, the monitoring systems and biological control became important tools in the management of pests. Local production of biological control agents replaced many of the former imported insecticides (see Funes-Monzote, Chapter 10, this volume).

Increased populations of birds may be significant with respect to control of insect, rodent, and weed populations. Rachmann et al. (2006) found that aerial hunters and raptors significantly preferred converted organic farms over conven-tional; they also found that the organic farms supported significantly higher densi-ties of raptors during autumn and winter and more seed-eating and insect-eating birds in autumn.

2.4.4.3 diseases

A lack of disease can indicate that ecological interactions are keeping pathogenic organisms in check and maintaining a level of nutritional health that increases crop plants’ resistance to disease (Vaarst et al., 2004; Parrott et al., 2006); conversely, problems with disease during the conversion process may be the result of ecological interactions having not been fully reestablished. Crop rotations (a form of plant com-munity change over time) have long been known to suppress disease (Curl, 1963), and organic farming usually involves more complex and lengthy rotations than con-ventional, particularly when a soil-building phase is included.

Benítez et al. (2007) determined that a transition management strategy involving the planting of mixed hay increased levels of bacterial populations associated with disease-suppressive bacteria more than tilled fallowing or open-field vegetable treat-ments. Soil from the hay treatment consistently had the lowest incidence of damping off in greenhouse tests, and was correlated with specific gene sequences presumably indicative of certain microorganisms involved with disease suppression. In another study, higher propagule densities of the disease-suppressive fungus Trichoderma were found in the soil of organic farms than were found in conventional farm soil.

In addition, greater propagule densities of Trichoderma, thermophilic bacterial spe-cies, and enteric bacteria were detected in plots amended with organic matter than in plots fertilized with agrochemicals, and these higher densities were associated with lower densities of the propagules of the plant pathogens Pythium and Phytopthora (Bulluck et al., 2002).

In some cases, diseases can impact established, postconversion organic vegetable production. Mäder et al. (2002) found that yields in organic potato plots 21 years after conversion were 58 to 66% those of conventional equivalents due to potassium deficiency and late blight, Phytophtora infestans. With respect to fruit production in Europe, three diseases have been identified as key challenges to conversion of apple and pear due to lack of natural control: apple scab (Venturia sp.), sooty blotch (Glosodes pomigena), and fire blight (Erwinia amylovora) (Weibel et al., 2004). On the other hand, during large-scale, input substitution conversion of salad greens in California, 87 to 90% of visually inspected samples were without leaf or root disease (Smukler et al., 2008).

2.4.4.4 summary

If problems with pests (weeds, insects, or diseases) increase during the transition period, they usually decrease over time as biological interactions develop and the ecological robustness of the system is restored; however, pest problems may not always disappear entirely. Also, certain environmental conditions favorable to

dis-ease or unfavorable to pest control measures—particularly unusual periods of cold and wet—may cause pest problems to increase temporarily.

Judging from the research reviewed above, the pest-related parameters that are most useful for assessing conversion are the following: the number of interventions required over time for control of weeds, insects, or diseases; the degree to which the use of toxics is reduced (even those agents allowed by the Organic Materials Review Institute and consistent with U.S. organic standards); and changes in populations of biological control organisms, including parasites, parasitoids, and predators of poten-tial pests, as well as soil organisms known to suppress disease. When measuring par-asitoid and predator populations, it is important to consider the data in light of pest population levels, since populations of parasitoids may be dependent on the number of hosts present, which may change as the system transitions (BaoYu et al., 2007).

2.4.5 e^nergy

About 72% of energy use on conventional farms is due to the energy embodied in fertilizers and pesticides (USEPA, 2008). Fossil-fuel–based nitrogen fertilizers, in particular, use much energy in their production via the Haber process. In addition, as agrochemical fertilizers applied to soil partially denitrify, they contribute to green-house gases through the release of methane (CH₄)and nitrous oxide (N₂O), which are 21 and 310 times more powerful, respectively, at causing radiative forcing than CO₂ (USEPA, 2008). A decrease in the use of nonrenewable energy and a corresponding increase in the use of renewable energy are characteristic of conversion to organic production. Much of this reduction comes from abandoning fossil-fuel-based nitro-gen fertilizers.

When assessing changes in energy use during conversion, it is important to quan-tify both the energy use per hectare and the energy use per unit of production. For example, during the first three years of the WICST study, the conventional corn and corn-soy rotations used more energy than the organic, although the energy output to energy input ratios were lower in the organic plots due to lower yields (Posner et al., 1993). However, the WICST study found that the energy output to energy input ratio of the organic grain treatment postconversion was twice that of the conventional treat-ments (excluding human labor and the sun’s energy) (Posner et al., 1995). In a long-term (21-year) study in Switzerland, Mäder et al. (2002) found that the energy required to produce a crop dry matter unit in the organic treatment was 20 to 56% lower than in the conventional treatment, which corresponded to a 34 to 53% lower use of energy per unit of land area. During the nine-year postconversion, the Rodale study found that the animal- and legume-based grain systems required about 30% less fossil fuel energy input per hectare than the conventional system, while grain yields in the two were statistically similar (except in one year) (Pimentel et al., 2005).

Similar differences in energy use are seen in perennial crops. In a study that compared energy use in organic and conventional apricot production in Turkey, it was found that the total energy requirement under conventional apricot farming was 38% higher than organic on a per-hectare basis; in these systems the ratio of energy output to energy input was 2.22 for organic and 1.45 for conventional (Gündoğmus and Bayramoglu, 2006). In another study, organic apple systems were found to be

more energy efficient than conventional, with overall lower energy inputs and a 7%

greater output-to-input ratio (Reganold et al., 2001). In a study in Costa Rica that used cluster analysis to compare 39 coffee farms grouped into three models of small coffee production, it was determined that the organic model achieved the best results from the point of view of energy efficiency; in this system 0.51 MJ kg^–1 was invested to produce each 1 kg of coffee as cherry-like fruit, which was half of the energy required to produce 1 kg of coffee as cherry-like fruit in the conventional or mixed models of coffee production (Mora-Delgado et al., 2006).

A global-level analysis comparing the energy use of organic and conventional systems found that organic farming uses about 30% less energy than nonorganic per ton of cereal or vegetables produced and about 25% less for meat and dairy products (Azeez, 2007). In the United Kingdom, the Department of Environment, Food and Rural Affairs (DEFRA) carried out studies that found that the energy involved in organic production was significantly less per unit of production than conventional in 11 of the 15 crop or livestock operations examined. Only organic poultry, egg, potato, and long-season greenhouse tomato production systems used more energy than their conventional equivalents per unit of production, mainly due to lower yields. The study concluded that conversion to an organic diet would decrease energy use by 29% in comparison to conventional (DEFRA, 2008).

Often analyses of energy use appropriately place energy use calculations within a larger context of overall environmental impact, and include factors such as water use and greenhouse gas emissions. The authors of an Australian study that used life-cycle assessment* and included direct and indirect effects found that organic farming can reduce energy use, greenhouse gas emission, and the total water use involved with food production (Wood et al., 2006). A nine-year Michigan State University study concluded that the global warming potential of organic systems is only 43% that of conventional on a per-unit yield basis (Robertson et al., 2000). Another Australian study concluded that organic agriculture produces about half of the greenhouse gas intensity per unit that conventional farming produces (Wood et al., 2006).

In organic and conventional sugar cane production, calculated energy use and greenhouse gas emissions were similar per dry unit weight, due to the lower yield of the organic. The fossil fuel energy use avoided by not using synthetic fertilizers and chemicals is offset by the more intensive use of machinery and the transport of low-density nutrient sources in organic systems. However, growing organic cane may provide greenhouse gas benefits if the expected lower levels of denitrification with organic fertilization are taken into consideration. Additionally, the system enhances water and soil quality by eliminating inorganic fertilizers, herbicides, and pesti-cides, and it would be useful to quantify these benefits in economic terms (Renouf et al., 2005).

* “Life-Cycle Assessment is a technique for assessing the environmental aspects and potential impacts associated with a product, service or process. It compiles an inventory of a system’s inputs and outputs evaluating the potential environmental impacts associated with these and interpreting the results in order to determine relative performance and scope for improvement where appropriate. It is now being applied to the analysis of agricultural systems and technologies with special reference to farming”

(DEFRA, 2008).

In summary, although there are few studies of energy use during the conversion period per se, there are a number of postconversion energy analyses showing that organic farming uses 30 to 50% less energy than conventional, and that conversion to organic results in a 40 to 50% reduction in greenhouse gas emissions. Even though the energy use in organic systems is in general favorable compared to conventional, there is still in organic systems a great deal of energy use based on fossil fuels that has not been seriously addressed. Modeling studies in Denmark show that it is pos-sible, through on-farm production of biofuels and biogas production from grass and clover, to reduce this use significantly (Halberg et al., 2008).

Energy use indicators to follow during the transition period are fossil fuel use per unit of production; the ratio of output of the crop in kcal to total energy inputs in kcal, both per hectare and per crop unit; ratio of energy output to fossil fuel input;

Energy use indicators to follow during the transition period are fossil fuel use per unit of production; the ratio of output of the crop in kcal to total energy inputs in kcal, both per hectare and per crop unit; ratio of energy output to fossil fuel input;