How to predict attachment potential of seeds to sheep and cattle coat from simple morphological seed traits

Dispersal is a process that determines many aspects in the life-history of plants. Up to now, however, it is difficult to quantify. Many studies rather assess it as a categorical trait, i.e. assuming a species is dispersed by a certain vector or not. Gradual differences in the dispersal potential between species are rarely considered.
In this paper we focus on the key process of epizoochory: the attachment potential of species to animal coats. We present two simple models (GLMs) how to quantify and to predict attachment potentials to sheep wool and to cattle hair from easily measurable seed traits. To calibrate the models, we investigated the attachment potential of seeds of 130 plant species on mechanically shaken coats of sheep and cattle and measured traits describing the seeds. We found that seed mass and seed morphology were correlated with attachment potentials. For sheep wool, the combination of the logarithm of seed mass and a constant for the different seed morphology types explained 85% of the variation of attachment potentials. For cattle hair, 71% were explained. To validate the models, they were applied to 36 additional species. Predicted and experimentally measured attachment potentials were correlated with r=0.84 for sheep wool and r=0.61 for cattle wool. Thus, the attachment potential of seeds to sheep or cattle coat can be assessed for a large set of species using the described models if seed mass and seed morphology are known.     

Why are small seeds dispersed through animal guts: large numbers or seed size per se?

Long-distance dispersal of seeds is an important process in metapopulation dynamics and in plant migrations, but at the same time extremely difficult to observe or quantify directly. If seed dispersal ability were related to attributes of seeds or motherplants, long-distance seed dispersal would be predictable by indirect approximation using easy-to-measure traits. Seed size has been suggested to be such a key trait in seed dispersal ability. However, having smaller seeds also implies having more numerous seeds per plant individual (given equal reproductive effort), and consequently increases the probability of seeds being ingested accidentally. The question is whether small-seeded species are more abundant in herbivore dung because smaller seed size increases survival rate during gut passage or because they are produced (and ingested) in greater numbers than larger seeds. We investigated endozoochorous seed dispersal via cattle grazing a meadow, and related seed abundance in dung samples to seed attributes. We found that seeds were ingested and passed through the bovine intestinal tract in proportion to the numbers produced per unit area in the grazed vegetation. In contrast, no relationship could be found between endozoochorous dispersal potential (measured as abundance of seeds in dung samples corrected for seed output in the grazed vegetation) and seed attributes such as seed mass, seed shape (roundness), and thickness of the seed coat. This finding underlines the importance of seed number in plant dispersal ability. In addition, it shows that grazing mammals may constitute an important dispersal vector for many plant species conventionally classified as 'unspecialised'.