Animal activity is one of the primary agents bringing seeds to the surface of the soil and burying them. It is, therefore, a profound factor that affects the seed bank contribution to plant recruitment. Also, Earthworms, among animal species are recognized as crucial predators and dispersers of seeds.
Seed dispersal is the motion of seeds away from the parent plant. Plants rely on a variety of vectors (living or nonliving) for dispersal (Murray, 2011). The type of the vector responsible for the dispersal of a particular seed is suited to by the general seed characteristics. Common modes of dispersal include gravity, wind, animal, water and ballistic. A particular seed may be acted upon by more than one vector, depending on the conditions and seed type. Exposed seeds on surfaces of soils are vulnerable to extreme contents of moisture and temperature, thus compromising the viability of seeds. In addition, they are easily detected by predators (Hirsch, 2012)
Researchers have established that secondary dispersal is a critical characteristic that influences seed survival (Lal, 2002). Secondary dispersal can be defined as the movement of seeds across, and into, the soil (Forget, 2005). When seeds are placed on soil columns, some will penetrate to deeper levels than others, and some will disperse throughout the soil column. In addition to morphology, the role a facilitator species plays affects dispersal. In this case, an earthworm is the facilitator species. The facilitator species activity has a substantial impact on the soil seed bank dynamics and hence on the possibility of plant recruitment in an ecosystem at large (Allessio, Thomas, & Simpson, 2012).
Milkweed, buckthorn, sumac and burdock seeds have different morphological features. Milkweed (Asclepias spp.) seeds are shiny, silky seeds which have snow- white fluffs.