Background

Bt crops modified by inserting and expressing the Cry toxin from Bacillus thuringiensis have raised environmental concerns over consequences for sustainability of soil biodiversity and ecosystems services in agricultural land. Part of this concern is related to the possible effects of the exposure to Cry toxins of non-target soil invertebrates as a result of Bt crops cultivation. Soil invertebrate members of microfauna, mesofauna and macrofauna play significant roles in nutrient cycling and energy flow and thus are crucial for soil ecological functions. In recent years, a number of studies have compared the population abundance and biomass of different members of soil biota in fields planted with genetically modified Bt crops and their conventional counterparts. In the present systematic review protocol, we describe the methodology and quality standards to perform a rigorous literature search and a quantitative synthesis of the evidence provided by these studies as required for conducting a Systematic Review.

Methods

The question that the systematic review will ask is whether populations of soil invertebrates differ under Bt crops and conventional crops. Relevant research literature will be collected systematically through a comprehensive search strategy. A scoping exercise was performed to identify search terms likely to capture appropriate studies and the results were verified using a list of relevant publications as references. The criteria against which studies will be included in the review are present, as well as the methodology for the quality assessment. To be included the study must contain relevant population abundances or biomass data on soil invertebrates exposed to characterised Bt proteins from field studies. The Review Protocol outlines the type of analyses that will be performed to assess bias of the selected studies and if covariables describing the heterogeneity of the studies introduce bias. Comparative effect sizes irrespective of statistical significance of effects will be calculated for individual studies and stored in publicly available databases ready for synthesis of all the studies. These treatment effects on population data will be compared across the studies in a meta-analysis using Hedges’ g.