Robert L. Jefferies

Destruction of wetland habitats by lesser snow geese: a keystone species on the west coast of Hudson Bay.

SUMMARY (1) Large numbers of lesser snow geese breed in colonies in coastal brackish and freshwater marshes in the lowlands along the west coast of Hudson Bay. Where colonies are present, the geese determine the structure and species composition of the coastal plant communities. (2) During recent decades, substantial increases in the number of birds have occurred, resulting in the outward spread of the colonies into new areas which are used for nesting, feeding and moulting. (3) Patterns of foraging vary in space and time. In spring at the nesting sites, before the onset of above-ground growth of vegetation, adult geese grub for roots and rhizomes of graminoid plants in relatively dry areas, and in wet habitats they eat the swollen bases of shoots of sedges, particularly Carex aquatilis. In summer, adults and goslings graze intensively on leaves of grasses and sedges over wide areas. Swards dominated by Carex subspathacea are produced in brackish marshes, but the leaves of graminoids of the freshwater sedge meadows are also clipped extensively. (4) Grubbing of vegetation by geese each spring creates bare areas (1-5 mi2) of peat and sediment. The increased numbers of birds have increased the scale of disturbance, with large areas now stripped of vegetation, particularly by the McConnell River. At some sites, erosion of peat has exposed the underlying glacial gravels. There is little likelihood that the vegetation which re-establishes will closely resemble the original. Further expansion of goose colonies in this region may be limited by available food resources.

A POSITIVE FEEDBACK : HERBIVORY, PLANT GROWTH, SALINITY, AND THE DESERTIFICATION OF AN ARCTIC SALT-MARSH

1 A 2-year study is described which suggests that a positive feedback process results in the destruction of salt-marsh swards and the exposure of bare sediments at La Perouse Bay, Manitoba, Canada. Lesser snow geese initiate the process by grubbing for roots and rhizomes of salt-marsh graminoids (Puccinellia phryganodes and Carex subspathacea) in spring. The increased rates of evaporation from sediments beneath disturbed or destroyed swards in summer result in high soil salinities that adversely affect the growth of the remaining grazed plants. 2 Above-ground biomass and soil salinity differed between sites in the salt marsh. Soil salinity was inversely related to above-ground biomass and shoot density of Puccinellia phryganodes. Increased biomass led to reduced soil salinity at sites where exclosures were erected. 3 Plant growth, measured as the rate of leaf births on Puccinellia shoots, was reduced by high soil salinities at sites where exclosures were erected. 4 Leaf demography of transplanted experimental plants of Puccinellia differed in 1992, but not 1991, between plants transplanted into sites with different amounts of above-ground biomass. Leaf births and deaths were highest for plants grown in sites where above-ground biomass was high and lowest for plants transplanted into bare sites. Grazing had no effect on leaf demography in 1991 and only marginally increased the rate of leaf deaths in 1992. 5 Growth of transplanted individuals of Carex subspathacea was similarly highest at sites where the standing crop of Puccinellia and Carex was high and was lowest in bare sites. 6 Algal crusts, which formed on bare or poorly vegetated sites, also reduced the growth of Puccinellia plants. 7 The effects of this deleterious positive feedback on plant growth are discussed in relation to changes occurring in the lesser snow goose colonies at La Perouse Bay and elsewhere.

RESTORING ENRICHED GRASSLANDS: EFFECTS OF MOWING ON SPECIES RICHNESS, PRODUCTIVITY, AND NITROGEN RETENTION

Species-rich grasslands that become enriched with nitrogen often suffer de- creases in species richness, increases in plant biomass, and invasion by weedy exotic species. Suitable techniques to restore enriched grasslands and reestablish native communities are increasingly needed. Here we report results of a 5-yr experiment in enriched coastal prairie grasslands (Bodega Marine Reserve, Bodega Bay, California, USA), to determine the com- bined effects of mowing and biomass removal on total soil nitrogen, net rates of miner- alization, nitrogen retention, and species richness and biomass. We mowed and removed plant biomass from plots in areas where the N-fixing shrub, bush lupine (Lupinus arboreus), had greatly enriched the soil, and where the community was composed of weedy introduced plants. Our goal was to facilitate the establishment of the native grassland assemblage such as was found at nearby low soil nitrogen sites. Mowing and biomass removal resulted in a dramatic change in the species assemblage, from exotic annual grasses to a mixed exotic/native forb community composed primarily of perennials. Species richness was significantly greater in treated plots than in control plots; weedy exotic grasses diminished in abundance, and both native and exotic forb species increased. In mowed vs. control plots, there was significantly less mean aboveground bio- mass, but significantly greater belowground biomass. This shift in species composition had significant impacts on nitrogen retention. In late fall and winter when plant-available N was highest, much nitrogen leached from the effectively fallow control plots where ger- mination of annual grasses did not peak until midwinter. In contrast, mowed plots retained substantially greater amounts of nitrogen, due to the presence of perennial plants possessing large amounts of belowground biomass early in the season. Despite the cumulative removal of 22 g N/m 2 in biomass over 5 yr, there was no difference between mowed and control plots in total soil N, pool sizes of inorganic N, or net rates of N mineralization. The results indicate that removal of plant biomass by mowing shifted this plant community from an annual grass to a perennial forb assemblage. However, in doing so, N retention by vegetation was increased, making it more difficult to reduce soil N.

POPULATION BIOLOGY OF THE SALT MARSH ANNUAL SALICORNIA EUROPAEA agg.

SUMMARY tion to the adverse effects of hypersalinity in the upper marsh. (3) The demography of the two populations was examined. Permanent quadrats were mapped or photographed to follow the fate of seedlings; the reproductive and growth performances of individuals in relation to plant density were measured. (4) Overall, demographic trends in the two populations were similar. The probability of a seed giving rise to an adult plant was very low; most seeds died without germinating. Mortality of plants appeared to be density-independent, but there was a significant negative density-dependent relationship between number of seeds per plant and the density of Salicornia plants. (5) By midsummer the seed bank in the sediments was exhausted, so each generation of Salicornia at both sites appears to be distinct. (6) A model is given which describes the population dynamics of Salicornia in terms of density-dependent regulation of seed number and density-independent mortality. The reasons for the prevailing densities are discussed. In this study the behaviour of two populations of Salicornia europaea agg., from the upper marsh and from the open sandy mudflats of the lower marsh, respectively, has been examined in relation to environmental heterogeneity at Stiffkey salt marsh on the north Norfolk coast, England. In cultivation, plants from the two populations exhibit different

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.

Environmental change and the cost of philopatry: an example in the lesser snow goose

The consequences of philopatric and dispersal behaviours under changing environmental conditions were examined using data from the colony of Lesser Snow Geese (Anser caerulescens caerulescens) breeding at La Pérouse Bay, Manitoba, Canada. In response to increased population size and decreased food abundance over time, increasing numbers of family groups have been dispersing from the traditional feeding areas. Goslings from dispersed broods were significantly heavier (7.3%), and had longer culmens (3.1%), head lengths (2.6%) and marginally longer tarsi (1.9%) on average than goslings that remained within La Pérouse Bay itself. These differences were consistent in each of 5 years. There was no evidence that the larger size of dispersed goslings was due to either a tendency for larger adults to disperse to alternative sites, or increased mortality of smaller goslings among dispersed broods. The most likely cause for the larger size of goslings from dispersed broods was the significantly greater per capita availability of the preferred salt-marsh forage species at non-traditional brood-rearing areas. The larger goslings in non-traditional feeding areas showed significantly higher firstyear survival, suggesting that the use of deteriorating traditional feeding areas may currently be maladaptive in this population.

The embarrassment of riches: atmospheric deposition of nitrogen and community and ecosystem processes

Anthropogenic sources of nitrogen have exceeded, and will continue to exceed, annual inputs of nitrogen produced by natural processes. Nitrogen enrichment may in plant tissue chemistry and microbial decomposition processes, as well as affecting rates of herbivory, all of which may be expected to result in changes in plant species assemblages Individual concepts, such as nitrogen saturation and critical load, used to describe the effects of enrichment on soil, community, ecosystm processes and species assemblages, cannot accomodate easily the range of interactions and different environmental processes. A number of approaches need to be used in tandem. Major gaps in knowledge are rates of transfer of anthropogenic nitrogen within and between different ecosystem and how these rates affect population dynamic of individual species and trophic relationships. Without this information, predictions of biological effects of enrichment are difficult to make.

BUSH LUPINE MORTALITY, ALTERED RESOURCE AVAILABILITY, AND ALTERNATIVE VEGETATION STATES

Nitrogen-fixing plants, by altering the availability of soil N, potentially facilitate plant invasion. Here we describe how herbivore-driven mortality of a native N-fixing shrub, bush lupine (Lupinus arboreus), increases soil N and light availability, which promotes invasion by introduced grasses to the detriment of a native plant community. Soils under live and dead lupine stands contained large amounts of total N, averaging 3.14 mg N/g dry mass of soil (398 g/m2) and 3.45 mg N/g dry mass of soil (438 g/m2), respectively, over four years. In contrast, similar lupine-free soil was low in N and averaged only 1.66 mg N/g dry mass of soil (211 g/m2) over three years. The addition of N fertilizer to lupine-free soil produced an 81% increase in aboveground plant biomass compared to plots unamended with N. Mean rates of net N mineralization were higher under live lupine and where mass die-off of lupine had occurred compared to soils free of bush lupine. At all sites, only 2.5–4.2% of the total soil N pool was mineralized annually. Soil enriched by lupine is not available to colonists while lupines are alive. The dense canopy of lupine shades soil under shrubs, reducing average photon-flux density in late spring from 1725 μmol·m−2·s−1 (full sunlight) to 13 μmol·m−2·s−1 (underneath shrubs). Stand die-off due to insect herbivory exposed this bare, enriched soil. In January, when annual plants are establishing, average photon-flux density under dead lupines killed by insect herbivores was 370 μmol·m−2·s−1, compared to the photon-flux density under live lupines of the same age, which averaged 83 μmol·m−2·s−1. The availability of bare, N-rich patches of soil enabled nonnative annuals (primarily Lolium multiflorum and Bromus diandrus) to colonize sites, grow rapidly, and dominate the plant assemblage until lupines reestablished after several years. The N content of these grasses was significantly greater than the N content of the mostly native plants that occupied adjacent coastal prairie devoid of bush lupine. Between 57 and 70% of the net amount of N mineralized annually was taken up by introduced grasses and subsequently returned to the soil upon the death of these annuals. Even in the absence of further N inputs, we estimate that it would take at least 25 yr to reduce the soil N pool by 50%, indicating that the reestablishment of the native prairie flora is likely to be long term.

Foraging geese, vegetation loss and soil degradation in an Arctic salt marsh

Abstract The North American mid‐continent population of Lesser snow geese (Chen caerulescens caerulescens L.) has increased by ca. 7% per year, largely as a result of geese feeding on agricultural crops in winter and on migration. We describe the long‐term effects of increasing numbers of geese at an arctic breeding ground (La Pérouse Bay, Manitoba) on intertidal salt‐marsh vegetation. Between 1985 and 1999 goose grubbing caused considerable loss of graminoid vegetation along transects in intertidal marshes. Loss of vegetation led to bare sediment with a plant cover of less than 2%. Changes in vegetation could not be described by simple linear, geometric or exponential functions; most losses occurred between 1988 and 1990 and losses were staggered in time between individual transects, some of which had all vegetation removed. Between 1979 and 1999 the standing crop in July in remaining intact heavily‐grazed swards of Puccinellia phryganodes and Carex subspathacea fell from 40–60 g m−2 to 20–30 g m−2. Intense grazing on remaining patches of sward has restricted growth of these clonal forage plants and hypersalinity of bare sediments has precluded re‐establishment of vegetation. Between 1989 and 1993 numbers of faecal droppings in grazed plots reached maximum values of 15–22 droppings m−2 wk −1. Since then peak values have remained at less than 13 droppings m −2 wk −1. The loss of vegetation and changes in soil conditions have resulted in the establishment of an alternative stable state (hypersaline bare sediment). Nomenclature: Porsild & Cody (1980).

GROWTH RESPONSES OF COASTAL HALOPHYTES TO INORGANIC NITROGEN

Intraand interspecific variation in growth responses to inorganic nitrogen (NH+ or NO-) has been studied in halophytic species grown in sand culture. Populations of Aster tripolium, Plantago maritima, Salicornia europaea agg. and Triglochin maritimna from the upper levels of a salt marsh at Stiffkey, Norfolk, have slower growth rates than corresponding populations from a low marsh at the same locality. Populations of Aster tripolium and Plantago maritima from the upper marsh away from drainage channels grow slower than populations close to channels. Species characteristic of the low marsh such as Halimione portulacoides, Aster tripolium and Suaeda maritima have fast growth rates compared with the rates of species abundant in the upper marsh such as Armeria maritima and Limonium vulgare. Although there is seasonal variation in the concentrations of soluble inorganic nitrogen in the sediments of the two marshes, the values obtained are similar for both marshes, but the upper marsh, unlike the lower marsh, becomes hypersaline in summer. It is suggested that the slower growth response to nitrogen of plants from the upper marsh away from drainage channels is the result of selection for plants with relatively low growth rates, which are able to survive the period of stress during the summer months when the soil is hypersaline.