Relationships between parasite diversity and host diversity

The diversity of organisms is affected by a variety of biotic and abiotic factors. One of the most important forces that affect the diversity of a community is the relationship between this community and communities of higher and/or lower trophic levels. Indeed, a strong link between the diversity of consumers and that of resources is a general characteristic of natural food webs (Polis & Strong, 1996). Top-down effects occur when the diversity of communities at a higher trophic level influences the diversity of communities at lower trophic levels (e.g. Jakobsen et al., 2004), while bottom-up effects occur when the diversity at lower trophic levels controls the diversity at higher levels (e.g. Siemann, 1998; Brandle et al., 2001; Haddad et al., 2009). Moreover, top-down and bottom-up forces can act on communities simultaneously (Hunter & Price, 1992).