Predators are more eflective in controlling herbivores
in low habitat stability ecosystems than in highly stable
ones (Southwood and Comins 1976). As positive
relationship been found between plant diversity and
habitat stability (Tilman et a|.2006), plant diversity
then would likely affects the relationship between
herbivorous insects and predators. In addition, according
to the resource concentration hypothesis by Root (1973),
herbivores are more likely to find and remain on hosts in
monocultures. Reduced plant diversity therefore increases
the potential damage to vegetation by pest species, while
simultaneously reducing overall insect diversity. Supported
by experiments, it has been predicted that herbivorous
insect diversity is positively correlated with plant species
diversity (Tilman 1986; Niemela et al. 1996; Chey et al.
1997; Siemann et al.1998; Intachat et al.1999l' Beck et
al.2002; Ghazoul 2002;L6winsohn and Roslin 200S).
Increasing diversity in herbivores could further enhance
the diversity of predators and parasites (Root 1973).
However, the relationship between plant diversity and
insect diversity is not always positive, and some studies
investigating natural habitats have found an opposite trend.
For example, the diversity of geometrid moths on Mount
Kilimanjaro decreases with increasing vascular plant
diversity (Axmacher et al. 2004; 2009).