Peter G. Connors

THE IMPACTS OF A NONINDIGENOUS MARINE PREDATOR IN A CALIFORNIA BAY

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 10-fold 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, Lumbr...

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.

High mortality, fluctuation in numbers, and heavy subterranean insect herbivory in bush lupine, Lupinus arboreus

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.

Oceanic plastic particle pollution: Suspected effect on fat deposition in red phalaropes

In a sample of seven red phalaropes (Phalaropus fulicarius: Aves) collected from a flock of 6000 late spring migrants, six stomachs contained plastic particles. A negative correlation between amount of plastic and fat condition suggests a detrimental effect of a widespread oceanic pollutant on a marine bird.

Patterns of chlorinated hydrocarbon contamination in New Zealand sub-Antarctic and coastal marine birds

Abstract Eggs and tissues of seabirds breeding on the coast of South Island, New Zealand, the Snares Islands and the Auckland Islands in the sub-Antarctic were examined for chlorinated hydrocarbon contamination. Higher levels of both industrial and agricultural compounds in the birds from the New Zealand coastal area reflected local contamination sources. In the sub-Antarctic, DDE residues were several times lower and PCB levels were lower by 1–2 orders of magnitude than in species occupying comparable ecological niches at Amchitka on a comparable latitude in the North Pacific. Concentrations of DDE in an unhatched egg of the New Zealand falcon from the sub-Antarctic were sufficiently high, on the basis of comparison with related species, to be associated with significant shell thinning.

Genetic consequences of a single-founder population bottleneck in Trifolium amoenum (Fabaceae)

We investigated the genetic consequences of a single-founder bottleneck in a population of showy Indian clover (Trifolium amoenum), a species presumed to be extinct until rediscovered near Occidental, California, in 1993. Electrophoretic variation was evaluated in the bottlenecked population and in a larger population (Dillon Beach) discovered during the course of this study, as well as in populations of two closely related species, T. albopurpureum var. dichotomum and T. macraei. We found a surprisingly high amount of polymorphism in the single-founder T. amoenum population from Occidental (15% of loci polymorphic; an average of 1.1 alleles per locus). However, this represents a 53% reduction in number of polymorphic loci and a 20% reduction in average number of alleles per locus compared to three Trifolium populations with putatively similar mating systems (the Dillon Beach T. amoenum population and both populations of T. albopurpureum var. dichotomum). Expanding the genetic base of the Occidental T. amoenum population is a priority due to concerns about loss of evolutionary potential and the possibility of deleterious effects associated with inbreeding. However, using seed from the Dillon Beach T. amoenum population may not be beneficial due to distinct, presumably adaptive differences between plants from the two populations and concerns about outbreeding depression.

Top Down From Underground? The Underappreciated Influence of Subterranean Food Webs on Above-Ground Ecology

The similar, venerable theories of the green world (Hairston et al., 1960) and exploitation ecosystems (Oksanen, this vol.) are most clearly borne out in the aquatic trophic cascades. These food chains are linear with forceful, direct interspecific effects on population density of prey. Piscivorous fish suppress populations of planktivorous fish, which allows zooplankton to flourish and to suppress phytoplankton populations. In lakes lacking effective piscivore populations, planktivores suppress zooplankton and the phytoplankton flourish, sometimes to the degree of turning the water opaque green (Carpenter et al., 1993; Persson, this vol.). Similarly forceful food chains can form in streams, with sessile algae instead of phytoplankton as the basal plant species (Power, this vol.). In seeking generality, the ecologist asks if trophic cascades dominate most ecosystems as they do some lakes and streams (Hunter and Price, 1992; Strong, 1992)? Alternatively, how effective are indirect interactions, interspecific competition (Holt, 1984; Wooton, 1994), and tri-trophic interactions involving plant defenses in food webs (Price et al. 1983; Strong and Larsson, 1994). The greatest current challenge is to understand the diversity of trophic mechanisms within and among ecosystems (Sih et al., 1985; Paine, 1992; Polis, 1991; Hairston and Hairston, 1993). Only a few pioneering experimental programs have taken on the difficult task of addressing these questions through multiple trophic links from the plant to carnivores in terrestrial food webs (Schoener, 1989; Pastor and Naiman, 1992; Marquis and Whelan, 1994; McLaren and Peterson, 1994).