Pathways for the effects of increased nitrogen deposition on fauna

Effects of increased N deposition, caused by agricultural practices and combustion of fossil fuels in traffic and industry, have been studied in detail for soil and water chemistry as well as for vegetation and ecosystem functioning. Knowledge on fauna is limited to descriptive and correlative data for a small number of species or communities. Therefore, mechanisms behind effects of N deposition on animal species and diversity remain unclear, which hampers optimisation of nature restoration and conservation measures. The aim of this review is to identify and structure possible different pathways in which fauna is affected by high N deposition. We identify ten pathways leading to six basic potentially negative bottlenecks: (1) chemical stress, (2) a levelled and humid microclimate, (3) decrease in reproductive habitat, (4) changes in food plant quantity, (5) changes in nutritional quality of food plants and (6) changes in availability of prey or host species due to cumulative effects in the food web. Depending on species and habitat type, different pathways play a dominant role and interference between different pathways can strengthen or weaken the net effect of N deposition. Although all identified pathways and bottlenecks are supported by peer reviewed literature, we conclude that scientific evidence on the causal relationship between increased N deposition and effects on fauna in the complete causal chain is still insufficient. We recommend that future research should aim to clarify the causal mechanisms underlying the observed changes in species composition attributed to N deposition. The most severe gaps in knowledge concern subtle changes in plant chemistry and changes in availability of prey and host species to higher trophic levels.