Horse dung germinable seed content in relation to plant species abundance, diet composition and seed characteristics

Abstract

Abstract The abundance and species richness of the germinable seed content of dung of free ranging horses was quantified and related to seven variables: plant species abundance in the field (cover) and in the horse diet, seed sizes (length, width and mass), seed longevity index and seed shape which was expressed as variance in seed dimensions. Analyses were run both across species, with species treated as independent data points, and for 40 ‘phylogenetically independent contrasts’. From 56 dung samples, 2.5 l each, 53,493 seedlings emerged, representing 106 different plant species, i.e. 21.4% of all species recorded in the established vegetation of the study sites. A majority of plant species (50%) was only found in less than 10% of the dung samples while 68% of the germinating species emerged with on average less than 1 seedling per dung sample. Overall dung seed density ranged on average between 280.4 and 525.2 seedlings per litre. Mean species richness of dung samples varied between 19 and 34 plant species. Urtica dioica , Veronica spp. and several graminoid species were among the most abundant and frequently out of dung emerging plant taxa. Dung seed content was positively correlated with plant species abundance in the field and in the diet. Relative dung seed density (i.e. per unit cover) was positively correlated with seed longevity index and negatively correlated with all three seed size variables. There was no significant relationship with seed shape. These relationships were not affected by phylogenetic constraints. As far as the possible role of endozoochory for conservation of plant diversity in grassland is concerned, the results emphasise the importance of large herbivores as potentially strong seed dispersal vectors.