The impact of dung on inter- and intraspecific competition of temperate grassland seeds

Abstract

Questions: In temperate grasslands, seeds of numerous dry-fruited plant species are dispersed via ingestion and subsequent defecation by grazing animals. Depending on the herbivore species and season, dung pats may contain a large assemblage of conspecific or heterospecific seeds competing for space, light and nutrients in the space-limited environment of an individual dung pat. In an environment rich in nutrients, such as herbivore dung, the outcome of inter- and intraspecific competition might differ from situations where nutrients are limiting. Additionally, dung pats being small and spatially isolated habitats with very specific conditions may also impact competitive interactions. Besides the plant-soil interactions on competition known from literature, the specific quality and structure of dung pats might provoke more complex interactions between different seed densities and species combinations. Methods: We conducted a greenhouse competition experiment using three common perennial grassland species. Agrostis stolonifera, Trifolium pratense and Trifolium repens were used in two-species combinations with different proportions of each species and in monocultures. Seeds were sown in three seed densities (50, 150 and 250 seeds) and the effects of cattle and horse dung on establishment, growth and flowering were tested. Results: Interactions, most probably attributable to interspecific competition, differed between species mixtures. Seeds sown in polycultures generally emerged sooner, but the resulting seedlings had lower relative growth rates compared with seeds sown in monocultures. Increased biomass was measured for each species when growing in polycultures while evidence for intraspecific competition was found in monocultures. T.pratense developed relatively more flowers when plants were growing in polycultures compared with monocultures. Few effects of seed densities were found, although higher seed densities led to lower establishment success in both monocultures and polycultures. Adding dung generally increased the time needed to emerge, relative growth rates and flowering, but decreased establishment success in monocultures. Conclusions: Both seed density and the presence of dung shape the post-dispersal fate of seeds. While high seed densities imply a cost due to lower germinability, the nutritive environment of dung acts as compensation, resulting in faster growth and an increased investment in reproductive tissues.