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Flora and Fauna Changes During Conversion from Conventional to Organic Farming

6 General discussion and conclusions (Esbjerg, P., Petersen, B.S., Navntoft, S., Jensen, A.-M.M. & Johnsen, I.)

6.1 Summary of results

The overall aim of the present project was to quantify changes in flora and fauna biodiversity during conversion from conventional to organic farming. The study was designed to reveal possible changes at each of the three main trophic levels as well as the interaction between these levels. The project took place at one farm over two seasons with organic practice immediately following previous conventional practice. The study crops were spring barley and winter wheat.

Flora:

• In wheat, the density of weeds increased to a level significantly higher (more than 100%) than during the preceding conventional period with normal pesticide dosage, however, it was not different from the density found at reduced dosages in the conventional period. In spring barley, no difference between periods was found.
  
• Carry-over effects were absent. The weed density in the organic period did not reflect the difference in dosages as was the case during the conventional period.
  
• The density of Cruciferae increased significantly in both crops after conversion.
  
• As to the number of weed species, the overall picture was parallel to that of density, since species number was strongly correlated to weed density.
  
• The flora composition did not change with conversion.
  
• Flowering improved from 30% in the conventional period to 70% in the organic period, the improvement was most pronounced in wheat.
  
• Bird available seeds on the soil surface increased from an average of 0.026 g/m² in the conventional phase to 0.95 g/ m²after the conversion, however the seed mass showed very high variation between years.
  
• During the organic period the weed biomass accounted for 2% of crop + catch crop + weed biomass in barley but only for 0.2% in wheat.

Arthropods:

• There was no significant period effect across crops. The total insect dry mass was higher in barley after conversion to organic farming but in wheat the dry mass was lower after conversion.
  
• In the organic period the insect fauna showed no significant response to the dosages of the previous conventional period.
  
• Skylark prey in barley increased significantly in the organic phase while it decreased significantly in wheat.
  
• In barley all analyzed herbivorous groups (e.g. Auchenorrhyncha, Crysomelidae adults, Curculionidae adults and larvae of Lepidoptera (Fig. 3.3)) increased in the organic phase. In wheat only larvae of Symphyta increased.
  
• In organic barley a significant increase took place of all the specific aphid predators while this was only the case for larvae of Syrphidae in wheat.
  
• The proportion and density of polyphagous predators, mainly Carabidae and Staphylinidae, dropped considerably in wheat during the organic phase.
  
• There was a clear relation between density of flowering plants and abundance of adult Lepidoptera, relevant for both barley and wheat.

Birds:

• Skylark abundance was influenced by the conversion of agricultural practice: averaged over the breeding season, Skylark numbers were 55% higher after conversion than before.
  
• The number of Skylarks in early May was not different from the numbers in the preceding conventional period but soon increased and stayed high until the end of July.
  
• Skylark abundance during the organic period was inversely related to crop biomass while no correlation between Skylark prey and Skylark abundance was found.
  
• The composition of arthropod remnants in faecal pellets from Skylarks differed significantly from the composition in suction samples.
  
• The composition of arthropod remains in Skylark faecal pellets changed significantly during the breeding season. There was statistical difference between contents from wheat and barley and also between years.

Yield:

• The crop yield decreased with 42 and 41% in winter wheat and spring barley, respectively.
  

6.2 Discussion

Interestingly the apparent benefit, in terms of density and species number of weeds, of conversion is not more than the flora improvement of reducing the herbicide dosages with 75%. In spring barley, such flora improvement was not found, therefore the mechanical weeding associated with organic practice appears as immediately rather effective in killing/controlling weed plants. However, the biologically attractive, more than doubled proportion of flowering plants in the organic period with its total absence of herbicides underlines the difference between the two agricultural methods. Mechanical weeding has an immediate lethal effect as opposed to the sustained sublethal effect of herbicide application, which resulted in a delayed development of the surviving weed plants. This is underlined by the 36-fold increase in bird available weed seeds, which constitutes a potential support to bird survival during winter but also may be a subject of agricultural concern. Of importance for biodiversity is the improved flowering and the probably related increased occurrence of butterflies and moths. The improved flowering, however, also indicates a higher mean biomass per specimen and results in a more complex spatial vegetation structure, which may imply a better food quality for herbivorous insects (Kjær & Elmegaard 1996) and stabilise the microclima, respectively.

Given the lack of carry-over effects on the weed flora from previous dosage levels, the lack of reflectance of these dosage levels in the distribution and densities of insects is not surprising. Many of the species included have life stages with high mobility, as e.g. adult butterflies and beetles with wings, and these will from season to season redistribute depending upon a whole array of factors including the crop. Even many of the non-flying beetles, as e.g. some of the large Carabidae, have a fairly high mobility (Pterostichus melanarius 9 m/h (Frampton et al. 1995), P. cupreus range over an area of 6-29 ha in a season (Bommarco 1997)).

Most remarkable among the insect results is the clear increase of dry mass and more specifically Skylark prey in barley, an increase that did not take place in wheat. This may seem surprising as the weed density increased in wheat during the organic period. Despite this increase, however, the weed biomass in organic barley was 10 times higher than the weed biomass in organic wheat. This may point at the relieved insecticide pressure as the determining factor, and possibly with the highest effect in barley, which has a more open structure than wheat. Interestingly all the herbivores (e.g. larvae of Lepidoptera and adults of Chrysomelidae and Curculionidae) in barley did increase, and so did also aphid predators such as larvae of Chrysopidae, Syrphidae and Cocinellidae. For the latter the possible effect of large amounts of aphids in the 2000 season (Table 3.5) cannot be neglected. However, both the positive effect on these aphid predators of reducing insecticide dosages in the preceding conventional period (Navntoft & Esbjerg 2000) and their high sensitivity to most insecticides (Hassan et al. 1987) suggest that some of the increase can be ascribed to the omission of insecticides. Expressed in another way: aphids are a prerequisite for a good amount of the larvae of these specific predators but with the exception of Pirimicarb even a limited insecticide dosage would presumably have removed a considerable proportion. How much the influence of a non-insecticide situation means is further underlined by the strongly increased amount of Symphyta larvae in wheat during the organic period. The absolute majority of these belong to the genus Dolerus, which live high in the crop and potentially is strongly exposed to insecticide treatment.

Among the results on arthropods the significant decrease in the organic wheat of the surface dwelling polyphagous predators Araneae, Carabidae and Staphylinidae is surprising. A similar decrease did not take place in barley where the weed biomass as mentioned earlier was 10 times higher, and in wheat the real decrease occurred in 2001 (cf. Fig. 3.7). A possible reason could be the effect of wheat with both a low level of weeds and mechanical weeding repeated in two successive years. The effect may become especially strong if e.g. the genus Trechus, accounting for nearly 80% of the sampled Carabids, in some way is sensitive to this situation. Taking into account the potential lack of weed plants as shelter and the soil surface condition just after mechanical weeding it might be relevant to pay some future attention to possible side effects of intensive mechanical weeding.

The result of the Skylark counts, an increase from 8.6 individuals per 6 ha in the conventional period to 13.3 per 6 ha in the organic period automatically calls for attention on the food situation, but no general correlation between Skylark numbers and prey biomass could be established. However, the relative increase of soft-bodied larvae, easy to catch and digest, belonging to Lepidoptera and Chrysopidae in barley and Tenthredionidae and Syrphidae in wheat is suggestive. Only the variation in crop biomass appeared as a significant predictor of Skylark densities, highlighting the importance of an open crop structure for the Skylarks’ feeding possibilities. Hence it is not possible to obtain a coherent tri-trophic-level explanation with Skylark as the top. On the other hand the improvement of food availability should on a longer time scale be a possible support to breeding success and survival, which could lead to increasing populations of Skylark if such “organic fields” are available within a reasonable distance. However, the relative importance of productivity and survival in determining population densities of Skylarks is not well understood and probably varies. Available data indicate that yearly mortality of adult Skylarks may vary between 11 and 71 percent (Schläpfer 1988, Jenny 1990c).

The average decrease in crop yield of 41-42% was in this case higher than the average biomass decrease of 25%, which was found in a comparison of old organic and conventional cereal fields (Hald 1999) and just outside the range of 21-37% yield decrease expected in cereals by converting to organic dairy farming in Denmark (Halberg & Kristensen 1997).

The strong yield decrease is difficult to explain. The level of available nitrogen in the soil might be a limiting factor for the cereal production. The weed density did not increase so much that competition may be the cause and the wheat varieties grown in the organic period were not less competitive than the varieties grown in the conventional period (P.K. Hansen pers. comm.). Only one of the fields had one year a high aphid population, thus the omission of insecticides cannot explain the yield decrease either. In the organic period the cereals were sown in double rows, this may result in a higher intra-specific competition and may explain some but not all of the yield decrease. No fungicides were used in the organic period, which may decrease the yield, if fungi are present.

6.3 Conclusions

While the effect of differences in pesticide dosage were clear at all trophic levels studied in the conventional period (Esbjerg & Petersen 2002) no carryover effect of the three different dosage levels to the organic period could be stated neither for plants nor for arthropods and Skylarks.

On the plant side, diversity of species and the density of weeds increased after conversion to organic practice, however the highest density found was at the same level as obtained by reducing herbicide dosages with 75%. Flowering and seed production increased strongly with the organic practice, and using flowering as an indicator of mean biomass per specimen, it should be safe to conclude that the average dry mass per plant increased and thus also biodiversity including a biomass parameter.

The insect life reacted more markedly in several ways. E.g. the diversity within the experimental area increased as a result of more flowers, particularly attractive to Lepidoptera. Also dry mass and relative abundance of many taxonomic groups and accordingly biodiversity improved.

An interesting conclusion is that increased performance of weeds and the improvement of insect life, which can mainly be ascribed to absence of pesticides, could hardly have been reached through an agriculturally acceptable dosage reduction. Apart from its own value this forms a potentially high value resource of bird food.

Despite this food aspect it cannot be concluded that the strong increase of Skylark numbers in the organic phase is due to improved food resources. It can only be confirmed that the immediate presence of this bird depends much on vegetation structure as evidenced by the strong negative correlation with crop biomass. However, adequate food amounts are a prerequisite for the beneficial effects of changes in crop structure.

In total the biodiversity has been changed by conversion to organic practice but with the mixed picture it has little meaning to express this in biodiversity indexes. However, it can be stated that with the exception of epigaeic polyphagous predators conversion improved biodiversity. The exception calls for attention on the possible effect of mechanical weeding under some conditions.

6.4 Perspectives

As an outcome of the studies from the conventional period it could be suggested to use 75% reduction of pesticide dosages as a route to improving clearly and safely the “nature content” of arable fields. This would create some agricultural problems which can most likely be solved in the near future.

The conversion to organic practice is slightly more delicate to evaluate. First the change in flora which is obvious at a certain age (Hald & Reddersen 1990) can of course not be obtained if conversion is used only as a short-sighted environmental tool. However, the improvement of flowering is a most desirable change to the better automatically including improvements of insect life in response to flowers (nectar feeders) and to a better plant biomass/quality (herbivores). From that perspective it is recommendable to select single fields in some mosaic way and use organic practice, or at least some key elements of organic practice, for a season. As this is also an agricultural weed challenge because of a strongly increased seed production it has to be dealt with very carefully and certainly it will be recommendable not to include “weed hot spots” in such an environmental short term action.

On the insect side the results are clear enough to recommend untreated fields as a support to nature. The decision not to use insecticides could in the larger picture be dropped in case of a very unusual threat, e.g. extreme aphid attacks with a prospect of 10% losses or more. The results, however, also tell that one of the better investments for nature would be a generally increased effort to avoid insecticide treatments which are not absolutely necessary, also including fields which are not part of a particular environmental effort.

In order to increase the number of Skylarks using a field the importance of the crop structure is beyond dispute but in this light the food aspect should not be forgotten. For the Skylark population there is little doubt that a network of fields with improved food availability will be of importance. Parallel with this and leaning on analyses of long term effects of agricultural practice on farmland birds in England and Wales (Chamberlain et al. 1999) and Scotland (Benton et al. 2002) there is hardly any doubt about the environmental value of using both the reduced dosages elucidated in the conventional period (Esbjerg & Petersen 2002) and a strategy of short organic or zero-pesticide periods at single field level. This can ensure a network of nature-improved fields in the somewhat poor agricultural landscape.  

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