Microbial community composition in the dung of five sympatric European herbivore species.

The dung microbiome is a complex system that is highly influenced by species and diet. This study characterized the dung bacterial and fungal communities of five herbivore species inhabiting the National Park Zuid-Kennemerland, the Netherlands. The five selected herbivore species were rabbit (Oryctolagus cuniculus L.), cow (Bos taurus L.), horse (Equus ferus caballus L.), fallow deer (Dama dama L.), and European bison (Bison bonasus L.). We explored the effects of distinct digestive physiology (ruminants vs. non-ruminants) and diverse dietary preferences on the microbial community composition of herbivore dung. Firmicutes and Bacteroidetes were dominant bacterial phyla in the dung of all five herbivore species, and Ascomycota was the predominant fungal phylum. Verrucomicrobiota and Mucoromycota were more present in horse dung and Proteobacteria were more abundant in rabbit dung than the three ruminant dung types. There were few significant differences in the microbial community structure among the three ruminant dung types. The alpha and beta diversity of dung microbial communities significantly differed between ruminants and non-ruminants, especially in bacterial communities. Based on MetaCyc pathways, we found that the primary functions of bacteria in herbivore dung were focused on biosynthesis, various super pathways, and degradation, with a few differences between ruminant and non-ruminant dung. FUNGuild analysis showed that horse dung had more saprotrophic fungi, while the fungi in fallow deer dung had more symbiotrophic properties, with the fungal functions of bison, cow, and rabbit dung somewhere in between. There was also a correlation between microbial community and nutrient composition of the substrate in herbivore dung. Understanding the dung microbial community composition of these herbivore species can enrich the database of mammalian gut microbiomes for studying the mechanisms of microbial community variation while preparing for exploring a new perspective to study the impact of herbivores on ecosystems through dung deposition.

Evolutionary history of grazing and resources determine herbivore exclusion effects on plant diversity.

Ecological models predict that the effects of mammalian herbivore exclusion on plant diversity depend on resource availability and plant exposure to ungulate grazing over evolutionary time. Using an experiment replicated in 57 grasslands on six continents, with contrasting evolutionary history of grazing, we tested how resources (mean annual precipitation and soil nutrients) determine herbivore exclusion effects on plant diversity, richness and evenness. Here we show that at sites with a long history of ungulate grazing, herbivore exclusion reduced plant diversity by reducing both richness and evenness and the responses of richness and diversity to herbivore exclusion decreased with mean annual precipitation. At sites with a short history of grazing, the effects of herbivore exclusion were not related to precipitation but differed for native and exotic plant richness. Thus, plant species' evolutionary history of grazing continues to shape the response of the world's grasslands to changing mammalian herbivory.

Fertilized graminoids intensify negative drought effects on grassland productivity.

Droughts can strongly affect grassland productivity and biodiversity, but responses differ widely. Nutrient availability may be a critical factor explaining this variation, but is often ignored in analyses of drought responses. Here, we used a standardized nutrient addition experiment covering 10 European grasslands to test if full-factorial nitrogen, phosphorus, and potassium addition affected plant community responses to inter-annual variation in drought stress and to the extreme summer drought of 2018 in Europe. We found that nutrient addition amplified detrimental drought effects on community aboveground biomass production. Drought effects also differed between functional groups, with a negative effect on graminoid but not forb biomass production. Our results imply that eutrophication in grasslands, which promotes dominance of drought-sensitive graminoids over forbs, amplifies detrimental drought effects. In terms of climate change adaptation, agricultural management would benefit from taking into account differential drought impacts on fertilized versus unfertilized grasslands, which differ in ecosystem services they provide to society.

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands.

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature - herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local-scale herbivory, and its interaction with nutrient enrichment and climate, within global-scale models to better predict land-atmosphere interactions under future climate change.

Herbivore dung quality affects plant community diversity.

Nutrient availability is important for plant community composition and diversity, but most studies focus on inorganic nutrients. Far less is known about the impact of nutrients in organic forms such as herbivore dung. Here we show that dung of 11 European herbivore species varies widely in nitrogen (N) and phosphorus (P) concentrations, as well as in C:N:P ratios. We demonstrate that variation in dung quality of five herbivore species influences the diversity and composition of a mesocosm plant community. The impact of dung quality was at least as strong as, or stronger than, the effect of manipulating the quantity of dung by a factor six. Our study supports the hypothesis that both nutrient quantity and nutrient imbalances are important controlling factors for plant species diversity, and stresses the important role of herbivores on plant communities, not only via selective foraging, but also via stoichiometric variation of nutrients in their dung.

A stoichiometric perspective of the effect of herbivore dung on ecosystem functioning.

Ungulate herbivores play a prominent role in maintaining the tree-grass balance in African savannas. Their top-down role through selective feeding on either trees or grasses is well studied, but their bottom-up role through deposition of nutrients in dung and urine has been overlooked. Here, we propose a novel concept of savanna ecosystem functioning in which the balance between trees and grasses is maintained through stoichiometric differences in dung of herbivores that feed on them. We describe a framework in which N2-fixing trees and grasses, as well as ungulate browsing and grazing herbivores, occupy opposite positions in an interconnected cycle of processes. The framework makes the testable assumption that the differences in dung N:P ratio among browsers and grazers are large enough to influence competitive interactions between N2-fixing trees and grasses. Other key elements of our concept are supported with field data from a Kenyan savanna.