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.

Coastal marine ecosystems worldwide are being altered rapidly by the invasion of nonindigenous species. Unlike terrestrial and freshwater systems, the impacts of an invading species have never been quantified on multiple trophic levels for a marine food web. We measured the impact of the nonindigenous green crab, Carcinus maenas, on a coastal marine food web in central California and found that this predator exerted strong ''top-down'' control, significantly reducing the abundances of several of the 20 invertebrate species monitored over a 9-yr period. Densities of native clams, Nutricola tantilla and Nutricola confusa, and native shore crabs, Hemigrapsus oregonensis, showed 5-fold to 10fold declines within 3 yr of the arrival of green crabs. Field and laboratory experiments indicated that green crab predation caused these declines. We also tested for indirect responses of invertebrates and vertebrates to green crab predation. There were significant increases in the abundances of two polychaete taxa, Lumbrineris sp. and Exogene sp., and tube-building tanaid crustaceans, Leptochelia dubia, most likely due to the removal of cooccurring green crab prey. However, we observed no changes in shorebird abundances (13 species) over a 9-yr period suggesting that green crabs have had no ''bottom-up'' effect on shorebirds, which subsist on benthic invertebrate prey. We predict that such bottom-up control will occur as the local effects and geographic range of green crabs increase. The 2-yr temporal scale of direct and indirect responses of the invertebrates in this low energy, soft-substrate system was also in agreement with the results of perturbation experiments by others on rocky shores, which showed that most direct and indirect responses also occur within a 2-yr time frame.

Long-term data are needed to assess spatial and temporal variability of communities and their resilience to natural and anthropogenic disturbances, particularly in climatic regions marked by high interannual variability (e.g. mediterraneanclimate). A long-term study at four sites on two streams in mediterranean-climate California (annual sampling over 20 yr) allowed us to quantify the influence of a 5-yr prolonged drought on stream invertebrate and fish communities. Invertebrate community composition did not show recovery following drought. The primary environmental factors influencing community composition, identified through principle components and multiple correspondence analyses were precipitation and flow permanence. Invertebrate taxon richness and abundance exhibited few responses (some site specific) and recovered quickly. Native fish abundance was lowest during the drought period and highest during the wet years that occurred at the end of the study period. Importantly, the prolonged drought facilitated the establishment and success of the invasive green sunfish (Centrarchidae: Lepomis cyanellus) that was then resilient to subsequent large flow events. There was high spatial synchrony in the temporal changes among all four sites, and three distinct periods were identified: early drought, late drought, and post-drought years. However, we still found differences among sites along the flow permanence gradient from temporary to perennial in the response to drought of both invertebrate and fish assemblages likely as a result of changes in substrate, vegetation, and other habitat characteristics. The observed lack of resilience and negative impacts to biodiversity due to prolonged drought associated with long-term habitat changes is important because hydroclimatic extremes are predicted to increase in frequency and magnitude with global climate change.

Sporadic patchy die-off of bush lupine, Lupinus arboreus, has long been known. We describe in detail a series of these incidents on the central California coast, based upon observational and comparative evidence. Stands of thousands of plants die, while nearby mature plants live on. In some sites, repeated die-off followed by regeneration from the seed bank has led to the cover and density of this woody, perennial plant fluctuating widely over the 40 year period for which records exist. Root damage by caterpillars of the ghost moth or "swift" Hepialus californicus (Lepidoptera, Hepialidae) is a major cause of individual bush death and a probable cause of die-off of stands of lupine. Hidden from view underground, a few of these insects readily kill a juvenile or young mature plant by girdling and reaming-out roots. The mass mortality of L. arboreus that we observed involved heavy root damage by these caterpillars in evenaged stands of plants in their first (1.5-year-old) or second (2.5-year-old) flowering season. The injured plants set seed before dying. Older, larger bush lupines better withstood root damage. In plants aged 3 or more years, damage and mortality were correlated with the intensity of ghost moth caterpillars in the roots. At the highest intensity (mean = 37.5, maximum = 62 caterpillars/root), a stand of large, old L. arboreus suffered 41% mortality; 45% of root cambium (median value) was destroyed by feeding caterpillars. Mass death of mature L. arboreus was not correlated with folivory, and leaf damage ranged from nil to moderate in instances of die-off. The western tussock moth, Orgyia vetusta, accounted for the highest levels of folivory, but this insect was rare when die-offs occurred. The lowest lupine mortality rates in our study occurred where tussock caterpillar intensities were high and where plants were repeatedly defoliated by this insect. However, experimental defoliation by high, but realistic, intensities of tussock moth caterpillars resulted in some mortality of mature bushes, and the combined effects of leaf and root herbivory have yet to be assessed. In its natural range on the California coast, bush lupine has several additional species of insect herbivores that can be locally abundant and injurious to the plant, although none is associated with die-off. Subterranean natural enemies of ghost moth caterpillars may play a role in the patchy waxing and waning of this shrub. Locally, a new species of entomophagous nematode (Heterorhabditis sp.) cause high mortality in the soil, before ghost moth caterpillars have entered the root. This natural enemy may thus afford lupines protection from heavy underground herbivory.

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