In contrast to top-down trophic cascades, few reviews have appeared of bottom-up trophic cascades. We review the recent development of research on bottom-up cascades in terrestrial food webs, focusing on tritrophic systems consisting of plants, herbivorous insects, and natural enemies, and attempt to integrate bottom-up cascade and material transfer among trophic levels. Bottom-up cascades are frequently reported in various tritrophic systems, and are important to determine community structure, population dynamics, and individual performance of higher trophic levels. In addition, we highlight several features of bottom-up cascades. Accumulation or dilution of plant nutritional and defensive materials by herbivorous insects provides a mechanistic base for several bottom-up cascades. Such a stoichiometric approach has the potential to improve our understanding of bottom-up cascading effects in terrestrial food webs. We suggest a future direction for research by integration of bottom-up cascades and material transfer among trophic levels.

Frass deposition to soil is an important pathway by which herbivorous insects impact decomposition and soil nutrient availability. However, little is known about how frass quality influences ecosystem properties. Here, we examined the effects of frass quality on the decomposition process, soil nitrogen (N) availability, and plant growth, using frass of Mamestra brassicae (L.) that fed on fertilized or unfertilized Brassica rapa L. var. perviridis Bailey. The frass quality was largely dependent on the host plant quality. Frass excreted by larvae that fed on the fertilized plants had higher N than that of larvae that fed on the unfertilized plants. The decomposition rate of the frass did not differ between N-rich and N-poor frass, except during the early decomposition period. The inorganic N concentration decreased during decomposition in both frass types. However, difference in the initial inorganic N concentration led to different consequences regarding soil N availability. Furthermore, addition of frass to the soil differently influenced the growth of B. rapa plants depending on the frass quality: plant biomass was increased by N-rich frass addition but decreased by N-poor frass addition, compared to the biomass without frass addition. These results indicate that frass quality is an important factor in determining the impact of herbivorous insects on nutrient dynamics, and that frass positively or negatively influences soil N availability and plant growth, depending on its quality.

T. 2006. Trunk cutting initiates bottom-up cascades in a tri-trophic system: sprouting increases biodiversity of herbivorous and predaceous arthropods on willows. Á/ Oikos 113: 259 Á/268.We examined the effects of trunk cutting on the regrowth responses of two dominant willow species, Salix eriocarpa and S. gilgiana , and the subsequent effects on the community structure (abundance and species richness) of herbivorous and predaceous arthropods. We studied ten randomly selected pairs of cut and uncut (control) trees of each willow species. Field observations showed that when the trunks were cut, shoots sprouted from the base and developed rapidly. These shoots continued to grow until July, producing lateral shoots and leaves, whereas current-year shoots on uncut trees essentially stopped growing by April. In July, the upper leaves of cut trees were less tough and had a greater water and nitrogen content than leaves of uncut trees. Leaf consumption and abundance and species richness of both herbivorous and predaceous arthropods were significantly greater on cut trees than on uncut trees in both willow species. Overall, trunk cutting resulted in at least a two-fold increase in both relative abundance and species richness within the arthropod community on the willow species. We concluded that the severe physical damage caused by trunk cutting greatly increased the biodiversity of herbivorous and predaceous arthropods through bottom-up cascading effects.

1. Plant quality can directly and indirectly affect the third trophic level. However, little attention has been paid to how changes in plant quality affect the performance of predators through trophic levels, and which herbivores or predators are affected more strongly by host-plant quality. The present study examined the effects of artificial cutting of willows on the performance of a willow leaf beetle (Plagiodera versicolora Laicharting) and its predatory ladybird beetle (Aiolocaria hexaspilota Hope).2. Laboratory experiments showed that performance (survival rate, developmental time, and adult mass) of the willow leaf beetle was higher when fed with leaves of cut willows than when fed with leaves of uncut willows. Performance (developmental time and adult mass) of the predatory ladybird was also improved when it was fed on the leaf beetle larvae that had been fed on leaves of cut willows, compared with those that had been fed on leaves of uncut willows. This indicates that a bottom-up cascade occurs in the tri-trophic system.3. In a comparison of improved performance parameters between the leaf beetle and the ladybird, regenerated willows shortened the developmental time of the willow leaf beetle more than that of the ladybird. This indicates that the impacts of willow cutting on insect performance differ between the second and third trophic levels.

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