Peter G. Connors

Bio: Peter G. Connors is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Plover & Pluvialis. The author has an hindex of 14, co-authored 24 publications receiving 1495 citations.

The impacts of a nonindigenous marine predator in a california bay

Abstract: 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

Abstract: 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.

Territory Size in Wintering Sanderlings: The Effects of Prey Abundance and Intruder Density

Abstract: Winter territory size in the Sanderling (Calidris alba) on marine beaches varies inversely with prey density. Multivariate analyses suggest that the inverse correlation results indirectly because more intruders are attracted to areas of higher prey density, and increased intruder frequency makes territorial defense more costly. Once the interaction between prey density and intruder density is controlled statistically, prey density has no effect on territory size.

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

Abstract: 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 (15-year-old) or second (25-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 = 375, 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

Interhabitat Movements by Sanderlings in Relation to Foraging Profitability and the Tidal Cycle

Abstract: --Sanderlings (Calidris alba) wintering near Bodega Bay, California move between outer coast sandy beaches and nearby harbor sandflats on a regular tidal schedule. Birds forage on outer beaches at high and mid-level tides, switching to the protected sandflats as the tide recedes; Sanderling density fluctuations measured along transects were complementary in these two habitats. The habitat time budget of the local Sanderling population varied between days with different tidal regimes, averaging 45% of daylight hours spent on beaches during November 1976. Some Sanderlings defended territories on the beaches, while others foraged in flocks; both groups moved to the lagoon tidal flats at low tide. The density of Sanderling prey in the two habitats changed through the tidal cycle in foraging sites used by Sanderlings. On harbor sandflats, energy density increased sharply with falling tide level. On outer beaches, energy density was highest at middle and upper tidal levels, decreasing at low and at very high tides. These results suggest hat Sanderlings switch feeding sites on a tidal schedule to maximize foraging efficiency. We offer a simple graphical model based on tide-related changes in foraging efficiency to explain the shifts in habitat use. Sanderling behavior under seasonally varying prey conditions and behavioral comparisons of territorial and nonterritorial birds are consistent with the model. Received 16 April 1980, accepted 29 July 1980. \"There are birds everywhere, but not always.\"---Edward Howe Forbush (1921) SHOREBIRD populations wintering in coastal areas exploit a habitat mosaic dominated by the tidal cycle. Numerous studies document shorebirds' choices of foraging habitats (see papers in Pitelka 1979 and references therein). Furthermore, within habitats shorebirds respond to spatial variations in prey density and the relative profitability of using different foraging sites (Goss-Custard 1970, 1979; Goss-Custard et al. 1977; Myers et al. 1979a). Few studies, however, address the temporal effects of tides directly. This is unfortunate because of the clear importance of tides for wader feeding (Ehlert 1964, Heppleston 1971, Prater 1972, Burger et al. 1977). In this paper we examine movements by Sanderlings (Calidris alba) in response to the strong and complex tidal cycle at Bodega Bay, California. More specifically, we consider the proportion of the day spent by local Sanderlings on sandy beaches and harbor sandflats and their timing of movement between these habitats. We then develop a simple model based on changing profitabilities of foraging that summarizes the patterns of movement. Finally, we explore two predictions based on changes in the model's parameters.

The Population Biology of Invasive Species

Abstract: ■ Abstract Contributions from the field of population biology hold promise for understanding and managing invasiveness; invasive species also offer excellent opportunities to study basic processes in population biology. Life history studies and demographic models may be valuable for examining the introduction of invasive species and identifying life history stages where management will be most effective. Evolutionary processes may be key features in determining whether invasive species establish and spread. Studies of genetic diversity and evolutionary changes should be useful for

Fluctuating resources in plant communities: a general theory of invasibility

Abstract: Summary 1 The invasion of habitats by non-native plant and animal species is a global phenomenon with potentially grave consequences for ecological, economic, and social systems. Unfortunately, to date, the study of invasions has been primarily anecdotal and resistant to generalization. 2 Here, we use insights from experiments and from long-term monitoring studies of vegetation to propose a new theory in which fluctuation in resource availability is identified as the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions. 3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.

Mechanisms underlying the impacts of exotic plant invasions

Abstract: Although the impacts of exotic plant invasions on community structure and ecosystem processes are well appreciated, the pathways or mechanisms that underlie these impacts are poorly understood. Better exploration of these processes is essential to understanding why exotic plants impact only certain systems, and why only some invaders have large impacts. Here, we review over 150 studies to evaluate the mechanisms underlying the impacts of exotic plant invasions on plant and animal community structure, nutrient cycling, hydrology and fire regimes. We find that, while numerous studies have examined the impacts of invasions on plant diversity and composition, less than 5% test whether these effects arise through competition, allelopathy, alteration of ecosystem variables or other processes. Nonetheless, competition was often hypothesized, and nearly all studies competing native and alien plants against each other found strong competitive effects of exotic species. In contrast to studies of the impacts on plant community structure and higher trophic levels, research examining impacts on nitrogen cycling, hydrology and fire regimes is generally highly mechanistic, often motivated by specific invader traits. We encourage future studies that link impacts on community structure to ecosystem processes, and relate the controls over invasibility to the controls over impact.

Does global change increase the success of biological invaders

Abstract: Biological invasions are gaining attention as a major threat to biodiversity and an important element of global change. Recent research indicates that other components of global change, such as increases in nitrogen deposition and atmospheric CO2 concentration, favor groups of species that share certain physiological or life history traits. New evidence suggests that many invasive species share traits that will allow them to capitalize on the various elements of global change. Increases in the prevalence of some of these biological invaders would alter basic ecosystem properties in ways that feed back to affect many components of global change.