Current research suggests that the strength of top‐down forces in communities increases with resource availability to primary producers. We examined the relative impacts of top‐down and bottom‐up forces on oak–herbivore communities by factorial manipulation of predation pressure and plant growth and quality. Plant growth and quality (phenolics, nitrogen) and avian predation intensity were altered through the addition of fertilizer to and the exclusion of birds from Quercus prinus and Q. rubra saplings. We censused the densities of insect herbivore guilds and predaceous arthropods on experimental and control trees in the summers of 1996 and 1997. We assessed changes in oak foliage quality using chemical assays for nitrogen, gallotannins, proanthocyanidins, and foliar astringency (protein‐binding capacity). In general, leaf chewers, phloem feeders, and leaf miners were more frequent on fertilized than on unfertilized trees. Predaceous arthropods were also more frequent on fertilized trees and were positively correlated with herbivore densities on Q. prinus saplings. Bird exclusion by itself had no detectable effects on the presence or absence of herbivores or predaceous arthropods. However, avian predation intensity on phloem feeders on Q. rubra did increase in the presence of nutrient addition to the bottom trophic level in July 1997. Cumulative 1997 yearly densities of herbivores and predaceous arthropods showed similar patterns of increasing avian predation pressure on fertilized Q. rubra saplings. Nonetheless, fertilization and bird exclusion treatments did not alter seasonal defoliation levels. At this early stage in the development of our oak–herbivore community, bottom‐up forces appear to influence the impact of top‐down forces.

We used strong inference with Akaike's Information Criterion (AIC) to assess the processes capable of explaining long-term (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995) variation in the per capita rate of change of mottled sculpin (Cottus bairdi) populations in the Coweeta Creek drainage (USA). We sampled two fourth-and one fifth-order sites (BCA [uppermost], BCB, and CC [lowermost]) along a downstream gradient, and the study encompassed extensive flow variation. Physical habitat availability varied significantly both within and among the sites.Sculpin densities in all sites were highly stable (coefficients of variation ϭ 0.23-0.41) and sampling variability was low (coefficients of variation ϭ 0.11-0.15). Population stability was positively associated with habitat stability, and the only significant correlations of population parameters among sites involved juveniles. Sculpin densities were significantly higher in BCB than in CC. The data suggest that, despite their proximity, the dynamics of populations within the sites are being determined by small-scale (i.e., 30-50 m) rather than broad-scale spatial processes.Both AIC and Dennis and Taper analyses indicated that simple density dependence had the greatest ability to explain variation in r for all life-history classes in all sites (AIC, seven of nine cases; Dennis and Taper, nine of nine cases). Multiprocess models had little explanatory power. When adults were removed from two sites, juvenile sculpin shifted into microhabitats formerly occupied by adults. No shifts occurred in control sites. Consequently, it is likely that the patterns of density dependence observed in all three sites were a consequence of intraspecific competition for space. Our findings argue for a multitiered approach to the study of population variation, one that encompasses long-term monitoring, spatial variation, and experimental testing of potential mechanisms.

Increased nitrogen (N) mobilization and export from terrestrial forest ecosystems following canopy herbivory have been well documented, though the mechanism behind the loss is not clear. Because carbon (C) and N dynamics are closely linked, herbivore activity may also affect C distribution. We initiated a replicated mini-ecosystem experiment to test the hypothesis that insect frass (feces) influences soil C and N dynamics following insect defoliation. One hundred and sixty red oak (Quercus rubra) saplings were transplanted to seven-gallon (26.5-L) pots with soil and litter from the Coweeta Hydrologic Laboratory (CWT) (Otto, North Carolina, USA) and overwintered in experimental pot stands. During the 2002 growing season, trees were subjected to a 3 ϫ 2 factorial experimental design with three damage groups (herbivore, mechanical, ''undamaged'') and two frass depositions (frass, no frass).Frass deposition increased soil total C, total N, and the soil NH 4 ϩ pool. Leachate NO 3 Ϫ export also increased following frass additions. We suggest that herbivore frass mobilizes sufficient C and N to affect soil pools and N export, though abiotic factors may influence the ultimate fate of the nutrients in frass. In addition, herbivory increased soil respiration and decreased total soil N relative to ''undamaged'' controls independent of frass deposition. While we discuss four possible mechanisms for this observation, we hypothesize that the increased soil respiration results from enhanced root-exudate C and subsequent microbial oxidation. This mechanism has implications for C sequestration and N retention in forest soils. In addition, the effects of mechanical damage consistently did not match those of real herbivory, suggesting that differential responses of Q. rubra to damage types also may affect soil nutrient dynamics. Our results demonstrate that the feeding activity of insect herbivores can have direct and indirect effects on the cycling of C and N within the season of defoliation.

Facultative diapause is a strategy that allows insects to initiate an additional generation when conditions are favorable, or to enter diapause when they are not. The occurrence of additional generations within one year can have a profound effect on the ecology and evolution of a species. Most previous studies of the cues that induce facultative diapause in insects have concentrated on photoperiod and temperature. In contrast, we studied the effects of host-plant quality on the induction of larval diapause and subsequent voltinism in a polyphagous insect herbivore, Choristoneura rosaceana (Lepidoptera: Tortricidae). Field experiments demonstrated that the proportion of larvae entering diapause differed among four host-plant species. Larvae feeding on chokecherry were more likely to continue development and produce a second generation than larvae feeding on red maple or black ash. Paper birch was intermediate in diapause induction. Laboratory experiments with excised foliage supported the results of the field experiments and demonstrated that the induction of diapause depended on host species under constant conditions of photoperiod and temperature. A further study with low-and high-quality artificial diet demonstrated that the nutritional quality of larval diet has a direct effect on diapause induction: lowquality diet favored diapause induction. Dispersal of neonate larvae from host foliage did not necessarily reflect host quality and the potential for bivoltinism. We argue that the effect of plant quality on diapause induction in natural populations of insect herbivores merits more attention.