Going with the flow : plant vascular systems mediate indirect interactions between plants, insect herbivores, and hemi-parasitic plants

There is increasing interest in the consequences of indirect interactions for community structure and function (Wootton 1994 ). Herbivory by one phytopha- gous species has the potential to affect other herbivores exploiting the same plant, hence plants are able to mediate indirect interactions between organisms that exploit them, even if these organisms are spatially or temporally separated (Masters and Brown 1997 ). For example, root-feeding herbivores may impact on the perform- ance of foliar feeding insects (Gange and Brown 1989 , Masters and Brown 1992 ), while herbivores feeding early in the season affect the growth and development of those feeding later (West 1985 , Harrison and Karban 1986 ). Many such inter- actions are mediated by damage-induced changes in the chemical composition of the shared host plant (Hartley and Jones 1997 , Karban and Baldwin 1997 ), par- ticularly increases in secondary compounds (Hartley and Lawton 1987 , Haukioja et al. 1990 ), but there are also cases where alterations in the nutrient levels within the host explain the impact of one insect herbivore on another (McClure 1980 , Denno et al. 2000 ). Thus both changes in nutrient and in secondary compounds have been associated with detrimental effects on other phytophagous insects and may underpin competitive indirect interactions between herbivores (Denno et al. 1995 ).