Introduction

Seed dispersal by animals

Plants vary in the mechanisms by which they disperse their seeds. Plant dispersal mechanisms are either abiotic (wind, water and ballistic dispersal), or biotic (endozoochory and epizoochory) (Howe and Smallwood, 1982). Epizoochory refers to dispersal of seeds via adhesion to animal fur, while endozoochory refers to the consumption of fleshy fruits and seeds by frugivores, and their subsequent deposition, through either spitting or defecation (Couvreur et al., 2005). While the latter mechanism is relatively rare in temperate forests, up to 90% of tropical tree species depend on endozoochory, relying on animals as dispersal vectors (Howe and Smallwood, 1982). Different animal dispersers affect seed dispersal and seed viability via seed handling (Lambert, 1999), gut passage (Knogge et al., 2003; Valenta and Fedigan, 2009) and the spatial location of seed deposition (Valenta and Fedigan, 2010).

The effect of gut passage on seeds is a potentially crucial determinant of plant reproductive success. In some cases, studies have shown that gut passage neither helps nor hinders a seed's chance of germinating (Smith, 2004). However, the majority of research on the effect of gut passage suggests that it increases the potential for germination for numerous species (Knogge et al., 2003). For some plant species, passage through a specific animal dispersal vector is necessary for successful seed germination (Temple, 1977; Chapman et al., 1992).

Other processes influencing the success of seed dispersal are secondary dispersal and predation (Nathan and Mueller-Landau, 2000; Valenta and Fedigan, 2009). The deposition of a seed is not necessarily the final step in the plant–animal interaction, because the forest floor is home to invertebrates and vertebrates that variously prey on, or safely redeposit, dispersed seeds. One study of primate-dispersed seeds in Costa Rica found that approximately 80% (N = 793) of seeds were removed from animal feces a mere week after initial deposition (Chapman, 1989). A study of howler monkey (Alouatta palliata)-dispersed seeds in Mexico found that a majority of the dung beetle species in the area were overwhelmingly attracted to howler dung, steering clear of the feces of other, more folivorous, herbivores, resulting in the massive redeposition of defecated seeds (Estrada and Coates-Estrada, 1991).