Effects on biodiversity in semi-natural pastures of giving the grazing animals access to additional nutrient sources: a systematic review

Traditionally managed semi-natural pastures are recognised for their high biodiversity. One drawback is that these pastures are often low in fodder production and hence rather unprofitable, which may lead to abandonment. Two ways to increase production and profitability and maintain grazing are to (i) offer the grazers supplementary feed, or (ii) co-enclose the semi-natural pasture with an improved pasture. Both practices may transfer nutrients to the semi-natural pasture, with potential negative effects on biodiversity. This systematic review aimed to analyse the available evidence concerning the following primary question: “What is the effect of giving grazers access to additional nutrient sources on biodiversity in semi-natural pastures?” (Q1). We also used two supporting questions: “What is the effect of giving grazers access to additional nutrient sources on nutrient status of the soils of semi-natural pastures?” (Q2) and “How do the grazers of semi-natural pastures behave while having access to additional nutrient sources?” (Q3). Searches for peer-reviewed and grey literature were made using bibliographic databases, search engines, specialist websites, and stakeholder contacts. Literature was screened for relevance according to predefined eligibility criteria, and critical appraisal was performed using the tool CEECAT. A database of the relevant studies was compiled. Descriptive information about the evidence base is presented in tables and an interactive evidence atlas. Because of absent study setup replication, Q1 and Q2 were not analysed quantitatively. However, sample size allowed the use of mixed modelling to quantitatively analyse Q3 regarding the effects of (i) co-enclosing an improved pasture on grazers’ electivity for the improved area, and (ii) supplementary feed on the forage intake of grazers. A total of 12 articles on the effects of supplementary feeding and 19 on the effects of co-enclosing an improved pasture were included, of which some targeted multiple review questions. Because of the limited literature, it is not possible to draw any conclusions concerning the effects on biodiversity (Q1) or nutritional status (Q2) in semi-natural pastures. For Q3, 28 studies fulfilled our criteria, of which 18 investigated the behaviour of grazers related to co-enclosing an improved pasture, and 10 investigated their forage intake while having access to supplementary fodder. The results show that all grazer species except goats preferred grazing in the improved areas regardless of whether they were grazing together with other grazer species or not. We found no effect of supplementary feeding on forage intake of the grazers. We detected a knowledge gap concerning the effects of the two additional nutrient sources on semi-natural pasture biodiversity (Q1) and nutrient status (Q2), which points toward further research needs. Analysis of Q3 showed that grazers prefer to graze improved compared to semi-natural pasture areas. However, how this behaviour subsequently affects nutrient transport and biodiversity is unclear and cannot be translated into management recommendations. To gain better knowledge about the primary question of our review, research focusing specifically on this question is needed. We provide suggestions for how such studies could be designed, including spatio-temporal setup, and key management and environmental conditions to consider.