Cultivation of GM crops with herbicide tolerance is mostly associated with changes in crop and weed management, like simplified rotations, simplified tillage and change from selective herbicides to the broadband glyphosate. All of these management practices decrease weed densities in crops and change their composition. Although weeds cause yield loss and harvest problems, they are an important part of arable biodiversity and offer food and habitat for different groups of animals. A simulation study was conducted to analyse indirect effects of intensified cropping in GM maize on biodiversity. The dynamic crop: weed model FlorSys was used to simulate weed densities and composition of common cropping systems and probable (GM) cropping systems in two regions of France and Spain. Results were translated into a set of biodiversity and harmfulness indicators. The presentation will show that the change from selective herbicides to glyphosate (which GM maize is tolerant to) has only smaller effects on biodiversity compared to simplified rotations. Change from a soybean-maize-wheat-maize rotation to a continuous maize cropping decreases weed species richness as well as food resources for birds, beetles and pollinators strongly. The total abandonement of tillage strongly increased biodiversity indicators in our simulation, but was associated also with high damage, and can therefore not be recommended to farmers.