Flora and Fauna Changes During Conversion from Conventional to Organic Farming

Flora and Fauna Changes During Conversion from Conventional to Organic Farming

Summary and conclusions

The present project involved two years of investigations of wild flora, arthropods and Skylarks immediately after conversion of large fields from conventional to organic farming on one estate. A prerequisite for this project was a detailed knowledge of flora and fauna stemming from another project on effects of reduced pesticide dosage levels during the preceding three years of conventional farming on the present estate and four others. Both projects, which are related to the Danish Pesticide Action Plans I and II, have been financed by the Danish Ministry of the Environment.

In the organic period of two years only two cereal crops, spring barley and winter wheat could be included. As in the preceding project the number of species, plant density, flowering and seed production were the flora variables studied, and in addition the plant biomass was assessed. Insects were suction sampled as before, and Skylarks were counted by a combination of point and transect counts. Also sampling of Skylark faecal pellets for investigation of insect remnants was included in order to identify important Skylark prey.

Both the plant density and the number of species were higher than at normal dosage during the conventional phase but at the same level as at 75% dosage reduction. The flowering increased from only 30% when exposed to herbicides to 70% under organic practice. This change presumably increased the abundance of adult Lepidoptera which showed a linear correlation to the occurrence of flowers. Naturally also seed production increased strongly leaving much more potential winter food for birds but also agricultural concerns. This food resource is only accessible as long as the field is left with stubble.

A group of ground dwelling predatory arthropods decreased considerably (50% or more) in wheat, maybe partly as a side effect of frequent mechanical weeding. In barley, however, there was a significant increase in abundance and biomass of all analysed herbivores (50% or more) as well as of aphid specific predators and spiders (more than 50%). This led to significant increases of both insect dry mass and Skylark prey.

The abundance of Skylarks increased by 55% in comparison with the conventional period. It would be natural to link this occurrence to the better food supply but there was no significant correlation. The main factor determining the presence of Skylarks was the crop biomass (negative correlation), indicating that the structure of the vegetation is very important.

The contents of faecal pellets gave an interesting picture of Skylark food choice but also involved some problems concerning the representative value of the different samples. Arthropod contents in the faecal pellets changed from May to July, reflecting seasonal changes in the availability of different arthropod prey groups. However, the composition of the faecal samples always differed significantly from the composition of the suction samples.

At the all over level the finding of no carry over effect from the previous dosage levels during the conventional period was important. This might have occurred but was questionable from the beginning due to the existing large seed reservoir in the soil and the high or relatively high mobility of most of the insects.

In conclusion the changes in biodiversity were not pronounced but still the enrichment of nature in agricultural fields was noticeable. The improved flowering and subsequent seed production is remarkable. Further adding of plant species will require much more time but already at the present level the positive impact of improved plant status (biomass, flowering) on non-pest insect life is important. Part of the insect enhancement is most likely due to removal of insecticides, particularly benefiting herbivores and aphid specific predators in the canopy which are very exposed to treatment. This indicates how important it is to avoid insecticides as much as possible, not only as part of a several component environmental approach as e.g. a short organic period, but also in the broad common agricultural practice. Despite the lack of a significant correlation between arthropod prey abundance and Skylarks, the notable increase in Skylark numbers leaves little doubt about the potential improvement of population densities of Skylark (and maybe other birds) if such individual fields with improved prey availability are available scattered in between other fields. Noticing that the present results are mainly valid during conversion to organic practice in crop husbandry dominated by cereals, a sort of environmentally improving mosaic strategy might be worth considering if the agricultural drawbacks can be handled without economical problems of importance.