Wood Decomposition Following a Perennial Lupine Die-Off: A 3-Year Litterbag Study

Woody debris is a conspicuous feature of many ecosystems and can be a large pool of stored carbon and nutrients. In the California coastal prairie, yellow bush lupines (Lupinus arboreus) experience mass die-offs, producing large quantities of woody detritus. Live lupines are fed upon by the stem-boring caterpillars of the ghost moth, Hepialus californicus, and outbreaks of ghost moths are one factor contributing to lupine die-offs. A common detritivore, the terrestrial isopod Porcellio scaber, frequently inhabits ghost moth tunnels in lupine wood. We used a litterbag experiment to test the hypothesis that H. californicus increases decomposition of woody lupine detritus by facilitating its use by P. scaber. Isopod access to wood was crossed with simulated ghost moth boring to measure the independent and interactive effects of these two arthropods on total mass loss, as well as on carbon, nitrogen, and lignin dynamics. Isopods initially colonized litterbags but were not more abundant on L. arboreus logs that had simulated ghost moth boring than on logs without boring. They were rare in litterbags collected at 12 months or later and had no effect on wood decomposition. Simulated ghost moth boring increased wood decomposition (P = 0.0021), from 50.5 to 55.1% mass loss after 3 years. This effect was likely due to increased surface area for microbial utilization of the wood. Lupine wood had an initial lignin content of 14.70 ± 0.67%, but lignin did not appear to decompose during the 3 years of this study, and by the end of the experiment accounted for 32.6 ± 1.12% of the remaining wood. Neither ghost moth boring nor isopod access affected lignin loss. Lupine wood from a die-off in 2002 was estimated to have contained three times more nitrogen per unit area than the yearly input of annual grass litter. The slow decomposition of lupine wood, however, restricts the rate at which nitrogen is released into the soil and results in the storage of carbon and nutrients in lupine wood for several years following such die-offs.     

Lack of strong induced or maternal effects in tussock moths (Orgyia vetusta) on bush lupine (Lupinus arboreus)

Both induced and maternal effects may create delayed negative feedback on the population growth of herbivorous insects. I tested for these effects in a chronically dense population of tussock moths (Orgyia vetusta) feeding on bush lupines (Lupinus arboreus). Experimental bushes received different realistic levels of defoliation by tussock moths in the preceding year, and experimental moth larvae came from mothers that had experienced either high or low levels of crowding as larvae in the previous year. Weight of female moths at pupation was not affected by prior foliar damage, and showed only a weak (12%) effect of maternal crowding. Rates of early larval disappearance, from aerial dispersal (ballooning) or other causes, were unaffected by either foliar damage or maternal crowding, and were very low except when larvae were placed on dead bushes. These results help to explain why the tussock moth population has maintained high densities at the study site for >10 years.     

Entomopathogenic nematodes: natural enemies of root-feeding caterpillars on bush lupine

A new species of soil-dwelling entomopathogenic nematode Heterorhabditis hepialus killed up to 100% (mean=72%) of root-boring caterpillars of a ghost moth Hepialus californicus in coastal shrub lands. When unchecked, ghost moth caterpillars killed bush lupine, Lupinus arboreus. Here we describe this strange food chain. Although unappreciated by ecologists, entomopathogenic nematodes are widespread and probably one of the most important groups of natural enemies for underground insects. The free-living infective juvenile (IJ) of entomopathogenic nematodes searches for host insects in the soil. A single IJ can kill a host, although several often invade together. After entering the host through a spiracle or other orifice, the IJ regurgitates its symbiotic bacterium, Photorhabdus luminescens, which kills the host within 48 h. The bacteria digest the cadaver and provide food for the exponentially growing nematode population inside. The bacteria produce antibiotics and other noxious substances that protect the host cadaver from other microbes in the soil. When the cadaver is exhausted of resources, IJs break the host integument and can disperse. As many as 420,000 IJs can be produced within a large ghost moth caterpillar. Surface soil of the lupine rhizosphere is the primary habitat of IJs of H. hepialus. Attracted to waste gases emitted by insects, the 0.5-mm-long IJs can move 6 cm/day through moist soil. Prevalences of H. hepialus ranged from as high as 78% of rhizospheres in some lupine stands to almost zero in others, but it was absent from no stand at our study site. Field intensities ranged from 0.003 IJs/cm³ of soil to 7.5 IJs/cm³, and correlated roughly with prevalences among sites. Few ghost moth caterpillars (mean=6.7) succeeded in entering lupine roots where prevalence of H. hepialus was highest, and this stand had lowest mortality (0.02) of mature bush lupine. In the three stands with lowest prevalence (mean = 2%) of this nematode, many caterpillars (mean = 38.5) entered roots, and lupine mortality was high (range = 0.41–1.0). Old aerial photographs indicate that the stands with highest recent nematode prevalence have had little or no mass die-off of lupine over the past 40 years. The photos depict repeated die-offs of lupine during the past four decades in stands with lowest recent prevalence of the nematode. This pattern leads us to entertain the hypothesis that the nematode affects vegetation dynamics indirectly through a trophic cascade. Dispersal of entomopathogenic nematodes is little understood. We found that air drying of soil extirpates H. hepialus and speculate that this nematode is dispersed during the wet season in moist soil bits on the exterior of fossorial insects and mammals. H. hepialus colonized some previously unoccupied lupine rhizospheres during the wet winter-spring season and, obversely, became extinct from some rhizosperes as soil dried in summer. Root-feeding insects have only recently been recognized as a force in communities, and the regulation of these important herbivores is still largely an ecological terra incognita. All evidence indicates that entomopathogenic nematodes are found throughout terrestril ecosystems, and we propose that trophic chains similar to those described in this report should not be uncommon.     

Origin of an insect outbreak: escape in space or time from natural enemies?

Initiation of insect outbreaks is poorly understood, and may involve sporadic events that temporarily release insect populations from predation or parasitism. While studying a declining outbreak of the western tussock moth (Orgyia vetusta) on bush lupine (Lupinus arboreus), we witnessed the onset of a new tussock moth outbreak, separated by 1,000 m in space and 2 months in phenological timing from the original population. This new population underwent explosive growth for 2 years and then collapsed because of a massive die-off of lupines. We tested whether during its growth phase, this new outbreak benefited by escaping in either space or time from the natural enemies attacking the original population. In experimental populations on single bushes, we compared predation and parasitism at the sites of the new and the old outbreak. At the site of the old outbreak, we compared predation and parasitism early and late in the season. Parasitism was significantly lower and population growth significantly higher at the new outbreak site than the old one. Neither seasonal timing, predator exclusion, nor their interaction significantly affected survival at either site. Thus the new outbreak appeared to escape in space from parasitism. These results corroborate our previous experimental findings, which suggest that as predicted by theory, the interaction between the tussock moth and its parasitoids can produce large-scale spatial patterning in population densities.     

Evidence that predator satiation may restrict the spatial spread of a tussock moth (Orgyia vetusta) outbreak

Western tussock moths (Orgyia vetusta Bdv., Lymantriidae) infest one stand of bush lupine (Lupinus arboreus Sims, Fabaceae) heavily and several other stands very lightly at the Bodega Marine Reserve (Sonoma Co., Calif., USA). We found that the disappearance rates of experimentally placed larvae and pupae were consistently lower in the outbreak area than in non-outbreak areas. For pupae but not larvae, this difference was removed by using tanglefoot to repel nonflying predators. However, the major nonflying predator of pupae, the ant Formica lasioides, was no more abundant in non-outbreak areas than in the outbreak area. We found inverse density-dependence in the rate of attack by F. lasioides on experimental pupae, suggesting this generalist predator is satiated within the outbreak area, but preys more heavily on the moth where the moth is sparse.     

Impacts of defoliation by tussock moths (Orgyia vetusta) on the growth and reproduction of bush lupine (Lupinus arboreus)

Abstract.

• 1 Outbreaks of tussock moths (Orygia verusta Bdv.) on bush lupines (Lupinus arboreus Sims) may be very intense, although spatially localized, and may last >10 years. To understand better how such outbreaks may persist, we defoliated lupine bushes using variable realistic numbers of tussock moth larvae, and measured the immediate and delayed impacts of defoliation on the growth and seed production of lupine bushes.
• 2 Immediate effects of defoliation included the production of new leaves that were significantly smaller in diameter and biomass, and lower in water content, than the new leaves of undamaged bushes. Also, seed output was reduced by up to 80% on heavily damaged bushes.
• 3 In the longer term, surviving bushes recovered remarkably well, despite having received the full range of possible (survivable) insect densities. After 1 or 2 years’rapid growth, surviving bushes were not affected in terms of height, basal stem diameter or volume by their previous defoliation. However, there was a significant tendency for juvenile bushes to produce more seeds the more heavily they had been attacked the previous year.
• 4 The ability of the host plant to recover, in terms of its biomass, helps to explain the sustained nature of the insect outbreak.

     

A native nitrogen-fixing shrub facilitates weed invasion

Invasions by exotic weedy plants frequently occur in highly disturbed or otherwise anthropogenically altered habitats. Here we present evidence that, within California coastal prairie, invasion also can be facilitated by a native nitrogen-fixing shrub, bush lupine (Lupinus arboreus). Bush lupines fix nitrogen and grow rapidly, fertilizing the sandy soil with nitrogen-rich litter. The dense lupine canopy blocks light, restricting vegetative growth under bushes. Heavy insect herbivory kills lupines, opening exposed nitrogen-rich sites within the plant community. Eventual re-establishment of lupine occurs because of an abundant and long-lived seed bank. Lupine germination, rapid growth, shading and fertilization of sites, and then death after only a few years, results in a mosaic of nutrient-rich sites that are available to invading species. To determine the role of bush lupine death and nitrogen enrichment in community composition, we examined nutrient dynamics and plant community characteristics within a site only recently colonized by lupine, comparing patches where lupines had recently died or were experimentally killed with adjacent areas lacking lupine. In experimentally killed patches, instantaneous pool sizes of exchangeable ammonium and nitrate nitrogen were higher than in adjacent sites free of lupine. Seedlings of the introduced grass Bromus diandrus accumulated 48% greater root biomass and 93% more shoot biomass when grown in a greenhouse in soil collected under experimentally killed lupines compared to B. diandrus seedlings grown in soil collected at least 1 m away from lupines. At the end of the spring growing season, total above-ground live plant biomass was more than twice as great in dead lupine patches as in the adjacent lupine-free grassland, but dead lupine patches contained 47% fewer plant species and 57% fewer native species. Sites where lupines have repeatedly died and reestablished during recent decades support an interstitial grassland community high in productivity but low in diversity, composed of mostly weedy introduced annual plants. In contrast, at a site only recently colonized by bush lupines, the interstitial grassland consists of a less productive but more diverse set of native and introduced species. We suggest that repeated bouts of lupine germination, establishment, and death can convert a rich native plant community into a less diverse collection of introduced weeds.     

Interspecific competition and insect herbivory reduce bush lupine (Lupinus arboreus ) seedling survival

Seedlings suffer high mortality in most plant populations, with both competition and herbivory proposed as being important mechanisms causing seedling death. The relative strength of these factors, however, is often unknown. Here I ask how interspecific competition for light and insect herbivory jointly affect seedling survival of bush lupine (Lupinus arboreus), a native shrub common to coastal California. Bush lupine seedlings germinate in grasslands during winter, and throughout spring potentially compete for light with surrounding fast-growing annual grasses. By early summer, after grasses have died, seedlings can be defoliated by a locally abundant caterpillar, the western tussock moth (Orgyia vetusta). I examined the relative importance of competition and insect herbivory on seedling survival in two separate experiments. First, I compared seedling mortality in plots either exposed to or protected from tussock moth larvae. Plants were protected from herbivory by the judicious use of insecticide; control plants were sprayed with water. Tussock moth herbivory resulted in significantly greater (31%) seedling mortality. To determine the effects of interspecific competition for light on seedling survival, I manipulated the density of grass surrounding lupine seedlings. I removed all vegetation surrounding some individuals, and left intact vegetation surrounding others. Reducing competition resulted in a 32% increase in seedling survival from February to June, as well as changes in seedling growth. To determine whether there are interactive effects of competition and herbivory on seedling survival, I enclosed tussock moth larvae on half of all surviving seedlings within each of the two prior competition treatments, comparing growth and survival of defoliated and undefoliated seedlings. Defoliation in June led to an additional 50% mortality for individuals that had grown with competitors through spring, and a 53% additional mortality for seedlings that grew without competitors through spring. Thus, although competition and herbivory both caused substantial seedling mortality, there was no statistical interaction between these factors. Competition-free plants were not less vulnerable to herbivory than plants that previously grew with competitors. Taken together, these experiments indicate that competition and herbivory are both important sources of mortality for bush lupine seedlings. Received: 4 April 1996 / Accepted: 5 November 1996     

Resources and dispersal as factors limiting a population of the tussock moth (Orgyia vetusta), a flightless defoliator

The western tussock moth (Orgyia vetusta) is very abundant on one stand of bush lupine (Lupinus arboreus) at the Bodega Marine Reserve (Sonoma Co, Calif., USA), but is sparse or absent on nearby stands. To determine what controls its abundance, both within the outbreak area and more globally, I performed experimental manipulations of resource availability and dispersal. To measure resource limitation, I inoculated 30 caged and 30 uncaged bushes with a realistic range of numbers of tussock moth eggs. On caged bushes, starvation led to density-dependent reductions in survival, pupal weight and fecundity. Larvae on uncaged bushes experienced density-independent ant predation on early instars and density-dependent emigration by late instars. From the results of this experiment, I predicted the density of a resource-limited tussock moth population. The predictions agreed fairly well with data from the outbreak area in 1992. To measure dispersal by the moth, which has flightless adult females, I released 30,000 tussock moth eggs at a central point in each of two uninfested lupine stands, and censused larvae weekly in a circle of radius 16 m until pupation. Median displacement over one entire generation was only 2 m, or about 2 bush radii. Rearing experiments indicated that bushes outside the out-break area are fully nutritionally suitable for the moth. I conclude that two major factors limiting the population are resources (within the outbreak area) and inefficient dispersal (more globally).     

Facilitation of tiger moths by outbreaking tussock moths that share the same host plants

1.?Ecologists have argued about the commonness and strength of interspecific competition between insect herbivores, but facilitation between herbivores has received much less consideration. We previously found that when two species of folivorous caterpillars co-occurred on a shared host plant, feeding by early season tiger moth caterpillars reduced the growth and reproduction of later season tussock caterpillars. However, densities of tussock caterpillars in summer were positively correlated with densities of tiger moth caterpillars the following spring. 2.?In this study, we experimentally manipulated numbers of feeding tussock caterpillars and found that they facilitated tiger moth caterpillars. 3.?The depth of the litter layer beneath host lupine bushes was positively correlated with the number of tussock caterpillars feeding on each bush. Experimental additions of litter beneath lupine canopies during summer resulted in increased numbers of tiger moth caterpillars in the following spring, indicating a causal role of litter. Litter potentially provides food, habitat and protection from desiccation and predation. We failed to find evidence that tussock caterpillars facilitated tiger moth caterpillars by mechanisms independent of litter. 4.?Our study demonstrates that facilitation may operate between insect herbivores, across life-stages through indirect interactions that are non-trophic. Facilitation operated by a novel mechanism, the accumulation of litter which was a by-product of feeding by one species was valuable to a second species. Facilitation persisted in time and space far beyond the creation of litter by tussock caterpillars which should be considered important ecosystem engineers from the point of view of tiger moths. Facilitations that involve habitat modification may generally connect species that do not interact directly or trophically, and have not previously been considered to affect one another.     

Intraspecific competition and subterranean herbivory: individual and interactive effects on bush lupine

High mortality of plants growing in dense monospecific stands (i.e. self-thinning) usually results from intense intraspecific competition. However, inconspicuous below-ground insect herbivory might be a potent but overlooked source of mortality within dense stands of plants, particularly if crowding limits a plant's ability to compensate for herbivore damage. Here I ask how high conspecific density influences a plant's ability to cope with heavy below-ground insect herbivory.
I manipulated conspecific density and exposure to an abundant root-borer, the ghost moth ( Hepialus californicus ), and examined the impacts on the fecundity, growth, and survival of bush lupine ( Lupinus arboreus ), a fast-growing shrub that grows in dense monospecific stands in coastal grasslands. Both herbivory and intraspecific competition affected seed production, size, and mortality of bush lupine over the two years of the experiment. Plants consistently produced fewer seeds when growing at high versus low density and ghost moth herbivory also significantly reduced seed production. The negative effects of herbivory on plant fecundity were similar, regardless of plant density. In contrast, plant survival was affected by both competition, herbivory, and the interaction of these factors. In high density plots, plant survival was uniformly low (averaging 0.45–0.50); plants exposed to herbivores died from heavy herbivory, and plants protected from herbivores died due to intense intraspecific competition that compensated for losses due to herbivory. In low density plots, ghost moth herbivory similarly reduced lupine survival, from an average survival probability of 0.94 in plots protected from these herbivores to 0.55 in plots exposed to herbivory. Thus, results show that regardless of plant density, below-ground herbivory can be a potent source of mortality.     

Effects of an early-season folivorous moth on the success of a later-season species,mediated by a change in the quality of the shared host,Lupinus arboreus Sims

Summary Larvae of Platyprepia virginalis (Lep.: Arctiidae) and Orgyia vetusta (Lep.: Lymantriidae) feed on the foliage of bush lupine (Lupinus arboreus Sims) at Bodega Bay, California, USA, in February–April and May–July, respectively. Female O. vetusta attained lower pupal weights and produced fewer eggs on branches of L. arboreus which had experimentally received P. virginalis damage earlier in the same year, compared to those reared on control branches.Growth rates of first-instar O. vetusta were lower on undamaged leaves of previously damaged L. arboreus than on leaves of undamaged controls. This was true for lupine bushes damaged by P. virginalis in the field; and also for greenhouse-grown lupine plants damaged by Estigmene acrea (Lep.: Arctiidae) when excised foliage was used in the assay. However, fifth (last) instar female O. vetusta larvae showed no preference for damaged versus undamaged greenhouse-grown L. arboreus in their feeding or choice of a site for pupation.These experiments demonstrate the potential for competition-like interactions between herbivores mediated by changes in quality of the shared host.     

Effects of genetic structure of Lupinus arboreus and previous herbivory on Platyprepia virginalis caterpillars

Two leaf-feeding caterpillars, western tussock moth (Orgyia vetusta) and ranchman's tiger moth (Platyprepia virginalis) are abundant on Lupinus arboreus along the California coast. Previous experiments and observations suggested that feeding caused by either of these two folivores could reduce the performance and possibly the abundance and distribution of the other species. Previous common garden experiments also indicated that genetically determined characteristics of the host plants were important for O. vetusta. Here we examined the effects of familial origin of the host plant, and previous damage caused by O. vetusta on the abundance of P. virginalis. Plants with parents from one of three locations had higher numbers of P. virginalis than plants with parents from the other two locations. However, this effect of plant origin depended on the statistical analysis and was not as strong as the effect of prior damage by O. vetusta on numbers of P. virginalis. Counter to our expectation, bushes that supported higher levels of damage by O. vetusta in the previous summer had more P. virginalis caterpillars. This strong effect could result by both moth species selecting bushes with the same traits or as the result of herbivory by O. vetusta enhancing the susceptibility of bushes for P. virginalis. Received: 28 October 1998 / Accepted: 14 March 1999     

Diet mixing enhances the performance of a generalist caterpillar,Platyprepia virginalis

1. Platyprepia virginalis caterpillars are dietary generalists and feed on multiple host species within a single day. We conducted field experiments to evaluate their performance on diets consisting of only their primary food, Lupinus arboreus, or diets consisting of L. arboreus plus other acceptable host species. 2. We found that relative growth rates and rates of survival were higher when they fed on mixed diets compared to lupine only. These results were consistent with hypotheses that mixed diets provided balanced nutrition, diluted toxins, and/or allowed recovery from parasitoids, although our data did not allow us to separate these non‐exclusive explanations. 3. We assayed alkaloids in their host foliage, in the caterpillars themselves, in parasitoids within caterpillars, in food boluses passing through their guts, and in frass that they excreted. We consistently found positive assays for alkaloids in foliage and in frass but negative assays in caterpillars, parasitoids, and food boluses. This suggests that the alkaloids that they ingest are metabolised or rendered non‐reactive by unknown means during passage through the gut. We found no support for the hypothesis that mixed diets prevented caterpillars from exhausting food supplies or allowed them to sequester chemicals from their alkaloid‐containing hosts. 4. Behavioural observations revealed that previous experience influenced a caterpillar's likelihood of moving to a different host. Caterpillars that had previously fed on other hosts were more likely to move to lupine while caterpillars previously collected on lupine were equally likely to choose more lupine or a different host. 5. These results are unusual in providing a clear and consistent benefit of diet mixing in a natural field setting where multiple ecological factors act upon the caterpillars.     

Post-establishment assessment of host plant specificity of Arytainilla spartiophila (Hemiptera: Psyllidae), an adventive biological control agent of Scotch broom,Cytisus scoparius

Scotch broom, Cytisus scoparius (Fabaceae), is a shrub native to Europe that is invasive in the USA, New Zealand and Australia. The psyllid Arytainilla spartiophila has been purposely introduced to Australia and New Zealand as a biological control agent of C. scoparius, but is an accidental introduction to California. Lupines (Lupinus spp.) are the closest native taxon to Cytisus in North America, and are therefore considered to be at the highest risk for non-target damage. However, because no lupines are native to Australia or New Zealand, only one imported forage species was evaluated during prior host specificity testing. We conducted a laboratory nymphal transfer experiment, a field choice experiment and a field survey to assess risk to three lupine species (Lupinus albifrons, Lupinus bicolor and Lupinus formosus). In the laboratory, 20% of third-instar nymphs were able to develop to adulthood on L. formosus but not on the other lupine species, while 40% completed development on C. scoparius. In the field experiment, potted lupine and C. scoparius plants were placed beside large infested C. scoparius plants; oviposition occurred on all the potted C. scoparius plants, but on none of the lupines. In the field survey, no A. spartiophila eggs or nymphs were found on naturally occurring lupines growing adjacent to infested C. scoparius. The results indicate that A. spartiophila is not likely to damage or reproduce on lupines in the field. This study provides an example of how field studies can help clarify the host specificity of biological control agents.     

Potential interactions between invasive woody shrubs and the gypsy moth (Lymantria dispar), an invasive insect herbivore

As the range of the invasive and highly polyphagous gypsy moth (Lymantria dispar) expands, it increasingly overlaps with forest areas that have been subject to invasion by non-native shrubs. We explored the potential for interactions between these co-occurring invasions through a gypsy moth feeding trial using the following three highly invasive, exotic shrubs: honeysuckle (Lonicera maackii), privet (Ligustrum sinense) and burning bush (Euonymus alatus). We compared these with two native shrubs: spicebush (Lindera benzoin) and pawpaw (Asimina triloba). We fed gypsy moth caterpillars foliage exclusively from one of the five shrubs and measured their relative consumptive rate (RCR), relative growth rate (RGR), and development time (DT). The RCR of gypsy moth was strongly influenced by the species of foliage (F = 31.9; P < 0.0001) with little or no consumption of honeysuckle and privet. Caterpillar RGR was influenced by the shrub species (F = 66.2; P < 0.0001), and those caterpillars fed spicebush, honeysuckle or privet lost weight over the course of the assay. Caterpillar DT was also significantly (F = 11.79, P < 0.0001) influenced by the shrub species and those fed honeysuckle, privet and spicebush died prior to molting. Overall, our data suggest that honeysuckle, privet, and spicebush could benefit (indirectly) from the invasion of gypsy moth, while burning bush and pawpaw could be negatively impacted due to direct effects (herbivory). Similarly, invading gypsy moth populations could be sustained on a shrub layer of burning bush and pawpaw in the event of canopy defoliation. Further field and laboratory analysis is needed to clarify herbivore resistance of invasive shrubs, and to investigate the potential interactions among co-occurring insect and plant invasions.     

Climate affects predator control of an herbivore outbreak

Herbivore outbreaks and the accompanying devastation of plant biomass can have enormous ecological effects. Climate directly affects such outbreaks through plant stress or alterations in herbivore life-history traits. Large-scale variation in climate can indirectly affect outbreaks through trophic interactions, but the magnitude of such effects is unknown. On the California coast, rainfall in years during and immediately previous to mass lupine mortality was two-thirds that of years without such mortality. However, neither mature lupines nor their root-feeding herbivores are directly affected by annual variation in rainfall. By increasing soil moisture to levels characteristic of summers following El Ni?o/Southern Oscillation (ENSO) events, we increased persistence of a predator (the entomopathogenic nematode Heterorhabditis marelatus). This led to suppression of an outbreak of the herbivorous moth Hepialus californicus, indirectly protecting bush lupine (Lupinus arboreus). Our results are consistent with the marine-oriented Menge-Sutherland hypothesis (Menge and Sutherland 1987) that abiotic stress has greater effects on higher than on lower trophic levels. The mechanisms producing these results differ from those proposed by Menge-Sutherland, however, highlighting differences between trophic processes in underground and terrestrial/marine food webs. Our evidence suggests that herbivore outbreaks and mass lupine mortality are indirectly affected by ENSO's facilitation of top-down control in this food web.     

Variability of the grass Phragmites australis in relation to the behaviour and mortality of the gall-inducing midge Giraudiella inclusa (Diptera,Cecidomyiidae)

Summary Variability within and between shoots of the grass Phragmites australis proved to be important for both survival (successful gall induction) and reproduction (larval weight) of the gall-inducing midge Giraudiella inclusa. The ovipositing females and the migrating first instar larvae chose a predictable microhabitat within shoots and within internodes characterized by a high nutritional quality (nitrogen, mineral content, sugar, water) and reduced palnt defense properties (silicate). Clutch size increased with the shoot diameter, larval scramble competition could not be detected. Female shoot selection was random, although the chances of successful gall induction greatly differed between shoots. Thick and intact shoots (6 mm) led to a high larval mortality. But thick shoots destroyed apically by stem-mining caterpillars (of the noctuid moth Archanara geminipuncta) had on average large gall clusters. Accordingly, the adjustment of the clutch size to the shoot diameter resulted in an advantage for the gall midge females only when shoots were thin and short (i.e. stressed by water and nutrient deficiency) or heavily damage (i.e. stressed) by caterpillars. Thus, the monophagous gall maker G. inclusa did not compensate for all features of intershoot variation of P. australis, although grasses are well-known for their simple plant architecture and their low diversity and low concentration of secondary compounds.     

Yellow Bush Lupine Invasion in Northern California Coastal Dunes I. Ecological Impacts and Manual Restoration Techniques

We studied the ecological effects of the invasion of coastal dunes by Lupinus arboreus (yellow bush lupine), an introduced species, and used the results to develop manual restoration techniques on the North Spit of Humboldt Bay. Vegetation and soil data were collected in five vegetation types representing points along a continuum of bush lupine's invasive influence. We collected data on the number and size of shrubs, vegetation cover, and soil nutrients. One set of plots was subjected to two restoration treatments: removal of lupine shrubs only, or removal of all nonnative vegetation and removal of litter and duff. Treatments were repeated annually for four years, and emerging lupine seedlings were monitored for three years. Prior to treatment, ammonium and nitrate were found to increase along the lupine continuum, but organic matter decreased at the extreme lupine end. Yellow bush lupine was not the most significant variable affecting variation in soil nutrients. After four years, nonnative grasses, including Vulpia bromoides, Holcus lanatus (velvet grass), Bromus spp. (brome), and Aira spp. (European hairgrass), were significantly reduced in those restoration plots from which litter and duff was removed. Native species increased significantly in vegetation types that were less influenced by lupine. By the third year, soil variables differed among vegetation types but not by treatment. Bush lupine seedling emergence was higher, however, in plots receiving the litter and duff removal treatment. Based on these results, we conclude that bush lupine invasion results in both direct soil enrichment and indirect enrichment as a result of the associated encroachment of other nonnative species, particularly grasses. Although treatment did not affect soil nutrients during the period of this study, it did reduce establishment of nonnative grasses and recruitment of new bush lupine seedlings. Restoration should therefore include litter and duff removal. In areas that are heavily influenced by lupine and contain few native propagules, revegetation is also required.     

Ion-Exchange Properties of Cell Walls Isolated from Lupine Roots

Acid–base properties of cell walls isolated from various root tissues of 7-day-old lupine seedlings and 14-day-old lupine plants grown in various media were studied. The ion-exchange capacity of root cell walls was estimated at various pH values (from 2 to 12) and constant ionic strength (10 mM). The parameters determining the qualitative and quantitative composition of cell wall ionogenic groups along the root length and in its radial direction were estimated using Gregor's model. This model fits the experimental data reasonably well. Four types of ionogenic groups were found in the cell walls: an amino group (pK _a 3), two types of carboxylic groups (pK _a 5 and 7.3, the first being the carboxylic group of galacturonic acid), and a phenolic group (pK _a 10). The number of functional groups of each type was estimated, and the corresponding ionization constant values were calculated. It is shown that the chemical composition of the ionogenic groups was constant along the root length as well as in its radial direction and did not depend on either physiological state or root nutrition, while the number of different groups varied. The content of carboxylic groups of -D-polygalacturonic acid in the root cell walls of 14-day-old plants was shown to depend on the distance from the root tip, being maximal in the zone of lateral roots. The number of these groups was 10- and 2-fold less in the central cylinder compared to that of cortex for 14-day-old plants and 7-day-old seedlings, respectively.