What forces structure food web interactions? Theoretical, experimental, and mathematical approaches

The question of how food webs operate, and what forces are responsible for their regulation, is central to understanding natural systems. This thesis uses an array of methods to address this question, and is divided into three sections: theoretical (chapters one and two), experimental (chapters three and four), and mathematical (chapters five and six) approaches.; Chapter one examines whether the strength of the herbivore-plant interaction in food webs can explain trophic 'cascades' versus 'trickles'. We argue that root-feeding and seed-eating herbivores have disproportionately large effects on plant fitness relative to their abundance or biomass. Leaf-chewing and sap-feeding herbivore guilds are less likely to affect plant fitness; predators of these herbivore are thus likely to cause 'trickles' rather than 'cascades'.; Chapter two is a meta-analysis of the relative impact of density- versus trait-mediated interactions on predator-prey dynamics. We found that trait-mediated interactions affect prey resources more than the prey themselves, and that these interactions are more important in aquatic than terrestrial systems.; Chapter three documents a terrestrial trophic cascade in the California coastal prairie, in which the entomopathogenic nematode Heterorhabditis marelatus suppresses populations of the ghost moth Hepialus californicus, thereby increasing growth and seedset in the bush lupine Lupinus arboreus.; Chapter four shows that this trophic cascade is affected by seasonal fluctuations in moisture associated with wet winters caused by the El Nino/Southern Oscillation (ENSO) climatic pattern. We demonstrate that nematode predators are capable of suppressing a ghost moth outbreak, but only under ENSO conditions.; Chapter five models the persistence of the nematode-ghost moth association, and shows that it should collapse in isolated areas. Field surveys, however, reveal that this association does occasionally persist. We highlight the sensitivity of this interaction to fluctuations in nematode mortality, and provide experimental data resolving this apparent contradiction.; Chapter six documents the importance of soil moisture in determining long-term nematode population persistence. We present experimental data revealing that nematode survival is affected by vegetation and seasonality, but not local population density. We use this data to derive mortality rates for nematode populations in the field, and compare these with published estimates.