The ongoing debate about the ecological effects of Bt-crops
calls for thorough reviews about the impact
on soil biodiversity and their ecosystem services. Transgenic Bt-crops
have been genetically modified by inserting a Bacillus thuriengensis gene
so the plant expresses a Cry toxin
aimed for insect crop pests. Non-target soil invertebrates
are particularly recognized for
their contribution to plant nutrient availability and turnover of organic
matter and it is therefore relevant
to protect these invertebrate taxa. A
number of studies have compared the population abundance and biomass of soil
invertebrates in agricultural fields
planted with genetically modified Bt
crops and their conventional counterparts. Here, were review and analyze a selection of studies on Protista, nematodes, Collembola, mites, enchytraeids,
and earthworms systematically to empower the evi-
dence for asking the question whether population abundances and biomasses of
soil invertebrates are changed by Bt
crops compared to conventional crops. 6110 titles were captured, of which
38 studies passed our inclusion criteria, and a final number of 22 publications
were subject to data extraction. A database with 2046 records was compiled
covering 36 locations and the Bt types Cry1Ab, Cry1Ac, Cry3Bb1 and
Cry3Aa. Comparative effect sizes in
terms of Hedges’ g were calculated irrespectively of statistical significance of effects of
the source studies. Cry effects on populations were compared across the studies in a meta-analysis employing a
hierarchical Bayesian approach of weighted data according to the level of replication. The temporal
development of effect sizes was
modelled, thereby taking into account
the variable duration of the field experiments. There was considerable variation among soil invertebrate orders,
but the sample size was insufficient and the
sample heterogeneity too large to draw any credible conclusions on the
effect of Cry at the order level. However,
across orders there was no significant
effect of Cry on soil invertebrates.
