Thus, the population trends in the skylark do not necessarily indicate crop type, but rather other aspects of the agricultural scheme like the use of pesticides. Other factors may apply to varying extent, but the predominant factor remains the application of non-natural substances to the fields. In general, structural differences are small and may not have major effects on the biodiversity. As discussed elsewhere none single bird species indicator will in itself yield a clear and unique indication of the farming type as they are affected by a few or a number of factors of which the effect of non-natural substances like pesticides is just one. Plant species may demonstrate a clearer one-stringed relation with their environment and as such turn out as providing stronger indication of the ecological quality of their environment.