Diversifying crop rotations with temporary grasslands - Université de ...

and higher production costs or eventually reduced crop yields may be compensated by higher
marked prizes of the labelled products.
A.III.3 Combining high weed diversity with low weed abundance?
One strategy for alleviating the trade-off between the harms and functions of weeds might be
the increase of weed species diversity without increasing the total weed abundance, thus
increasing the evenness of the plant community. Higher plant diversity may increase the
diversity of other trophic levels (animals and micro-organisms) as detailed in section A.II.3
above. However, weed abundance and weed diversity are generally positively related as both
depend on the intensity of weed control. Hyvonen et al. (2003) observed always positive
linear relationships when plotting the weed species richness against the number of weed
individuals (both on logarithmic scales). However, the intercepts of these regressions were
higher for organic cropping systems compared to conventional systems indicating that the
abundance-diversity relationship may differ between cropping systems. At equal abundances,
organic systems had about 2 more species compared to conventional systems. Such an
improvement might be caused by the replacement of one dominating weed control technique
that is very efficient on most weed species (e.g. herbicides with broad ranges) by several less
efficient techniques and principles (IWM, see section A.III.2 above) or due to the rotation of
dissimilar crops (see section A.III.8 below).
A.III.4 ‘Good’ vs. ‘bad’ weeds?
Another approach goes beyond the former approach in considering differences between the
weed species according to (i) their potential harm for crop production, (ii) their difficulty to be
controlled and (iii) their role for biodiversity (habitat and trophic resource) and ecosystem
functioning. Fig. 4 shows that weed species may considerably differ in ‘harmfulness’
(competitive ability) and ‘biodiversity value’ (number of associated insect species). But when
combining these two criteria, many weed species are neither ‘very good’ nor ‘very bad’ but
somehow intermediate.
12