Foraging Methods
The different foraging activities of the geese are described based on observations made mostly in coastal areas of Hudson and James Bays or reported in the literature. Understanding these differences is essential to a clear perception of impacts at different times of the year and in different habitats that geese have on vegetation and soil processes.
Grubbing of below-ground biomass
This refers to the digging and uprooting of roots and rhizomes of plants, in general, and graminoids (grasses and sedges), in particular (Fig. 2.10). The grubbing which takes place mainly in spring (but not exclusively) is dependent on the upper layers of sediment thawing for at least some hours each day. It is restricted to snow-free sites where the layer of vegetation and plant litter is thin and where seasonal above-ground growth of vegetation has not started. Where these conditions occur the birds are able to break open the turf and forage on below-ground biomass. Greater and lesser snow geese and Canada geese grub extensively immediately after snow melt. Some further grubbing may occur at the end of the season in late July and August, when above-ground tissues are senescing and reserves are being transported to below-ground organs. We have observed only lesser snow geese foraging in this manner in late summer, but in degraded environments with a shortage of food resources it may be practised by other species as well. Grubbing is a major foraging technique of LSGO on migration routes and in winter in the Gulf of Mexico coastal marshes, and of GSGO on the Gulf of St. Lawrence tidal marshes, and in east coast tidal marshes of the United States.
Shoot pulling of sedges
This type of foraging occurs in spring and is restricted to fresh-water mires (wetlands with an organic substratum). Lesser snow geese pull individual shoots of large sedges (>20 cm in height from a tussock) (Fig. 2.11). They eat the white basal portion of the shoots which is rich in soluble nitrogen compounds and carbohydrates and discard the remainder (Gadallah and Jefferies 1995a). Removal of shoots by geese can occur in oligotrophic, mesotrophic and eutrophic sedge communities. Carex aquatilis, Eriophorum angustifolium and Carex X flavicans are examples of preferred forage species. The only grass species in wetlands from which shoots are pulled frequently are Dupontia fisheri and Arctophila fulva. However, in sandy or gravel areas the geese pull up shoots of lyme grass (Elymus arenarius).
Grazing
A large number of species are grazed on different occasions during the snow-free season (Fig. 2.12). The selectivity of different species appears to be linked to plant phenology and nutritional quality (Jefferies et al. 1994, Gadallah and Jefferies 1995a). Some coastal salt-marsh graminoids show enhanced compensatory shoot growth following defoliation and are subject to multiple defoliations throughout the season (e.g. Puccinellia phryganodes) (Cargill and Jefferies 1984, Hik and Jefferies 1990, Hik et al. 1991). In contrast, other species either show only limited compensatory growth or no growth following defoliation (Zellmer et al. 1993). Under conditions of intense grazing, the pseudostem of grasses such as Puccinellia phryganodes may be damaged and the regrowth of swards severely impaired. Ross’ geese may be able to graze swards of P. phryganodes lower than lesser snow geese, because of the shape of their bills. In damaged, heavily grazed swards the former species may be capable of obtaining some forage, unlike lesser snow geese.
In addition to the salt-marsh graminoids described above, the following species are grazed at different times of the season in the Hudson Bay region: Senecio congestus, catkins of Salix species (early spring); Potentilla egedii, Plantago maritima, Stellaria humifusa, Triglochin palustris, Triglochin maritima, Carex aquatilis, Eriophorum angustifolium (all of these species in early to mid-summer); Festuca rubra and Calamagrostis deschampsioides (mid-summer). In fall birds eat Potamageton filiformis, the seed heads of Triglochin species, as well as the shoots of Equisetum and a range of Carex species (Prevett et al. 1979).
Berry-feeding occurs on ericaceous tundra during migration (Reed, pers. comm.). Baffin Island LSGO and Atlantic Canada geese appear to feed heavily on the berry crop on the Ungava Peninsula in both spring and fall. Similarly, LSGO feed on berries on tundra ridges along the Hudson Bay coast (K. Abraham, pers. obs.).
Prime Forage Species
Plant species selected by greater and lesser snow geese as prime sources of forage differ depending on the geographical location of the breeding colony (see below). The prime forage species from Wrangel Island and Bylot Island grow at sites where there is a well developed peaty substratum that may have a high water content and which is mesotrophic or oligotrophic. In contrast, the forage species from the Hudson Bay lowlands grow either on mineral soils or where there is only a thin veneer of organic material. Because of the different tolerances of species to foraging, and the different intensities of foraging, the effects of the geese on plant communities at the various geographical locations are not necessarily similar. Most breeding colonies of snow geese are coastal in distribution in the Arctic and sub-Arctic, and much of the damage to vegetation described below is based on changes that have occurred to vegetation in the coastal zone of the Hudson Bay lowlands. The mid-continent population of lesser snow geese breeds in this region.
Experimental field evidence from feeding trials at La Pérouse Bay with captive goslings, as well as evidence of the correlation between amounts of standing crop and gosling weight, indicate that in early life the salt-marsh graminoids meet the nutritional requirements of goslings more successfully than other types of forage (Gadallah and Jefferies 1995b; Cooch et al. 1993). Elsewhere in the Arctic, other species such as Dupontia fisheri and Eriophorum species are a source of high quality forage (Gauthier et al. 1995) and meet the nutritional demands of goslings. These species grow on an organic substratum rather than in mineral sediments.
Isostatic Uplift and Development of Salt-Marsh Plant Communities in the Hudson Bay and James Bay Region
The Hudson Bay region is undergoing isostatic uplift at approximately 1 cm/yr. The actual rate has been estimated to be between 0.5 and 1.2 m per century, depending on the proximity of sites to epicentres of uplift (Andrews 1973). The present-day coastal zones have emerged within the last 1000 years. The rate of uptake and associated modifications of the soil environment influence plant successional processes. Although grazing by geese of salt-marsh vegetation retards the development of dicotyledonous plants (the apical meristem is destroyed by grazing, whereas in graminoids the meristem is basal and clipped leaves continue to grow, (e.g., a lawn) and maintains the Puccinellia phryganodes - Carex subspathacea grazing lawn, so that the successional “clock” is being reset each year, eventually the effects of isostatic uplift modify the physical environment and result in the replacement of Puccinellia and Carex by Calamagrostis deschampsioides, Festuca rubra (grasses) and dicotyledonous plants. The interaction between the grazer and the vegetation maintains the Puccinellia - Carex community, as long as it can overcome the environmental constraints imposed by isostatic uplift.
Although this type of vegetation is well developed at the seaward end of the marsh and is renewed continually by uplift even in the absence of grazing, its continued presence in the upper salt-marsh is strongly dependent on the foraging activities of the geese. In their absence, rapid vegetational changes occur within 5 years leading to the development of a Calamagrostis - Festuca grassland in which herbaceous plants and willows grow. These swards are not as heavily grazed as swards of Puccinellia and Carex. The nutritional quality and digestion efficiency of the forage are lower than comparable data for Puccinellia - Carex forage (Gadallah and Jefferies 1995a,b).
Coastal salt marsh plant communities
The coastal zone of the Hudson Bay lowlands and vicinity consists of a large number of salt marshes, the most notable of which are the marshes on the north shore of Akimiski Island (Northwest Territories); Cape Henrietta Maria-Sutton River, Shell Brook, and Pen Islands (Ontario); Cape Churchill - La Pérouse Bay and the estuaries of the Knife and Seal Rivers (Manitoba); and the McConnell River - Wolf Creek system (Northwest Territories). However, most river estuaries have small areas of salt marsh adjacent to the mouth of the rivers and there are many small fringe salt marshes landward of barrier beaches. The marshes are dominated by two species, the stoloniferous grass, Puccinellia phryganodes and the rhizomatous sedge, Carex subspathacea, both of which are prime forage species of the lesser snow goose. Large breeding colonies are located at the geographical locations mentioned above where there is the strongest evidence of damage to vegetation. Some of these localities, such as the Cape Henrietta Maria-Sutton River salt marsh system are over 120 km in length.
All of these marsh systems show evidence of grubbing, although the scale of damage varies. Grubbing on Pen Island marshes is restricted, probably because they are covered by ice and snow until late in the spring, whereas at La Pérouse Bay, Cape Henrietta Maria, Akimiski Island and the McConnell River-Wolf Creek system grubbing is extensive (Fig. 2.13). At these localities the size of the grubbed patches increases each year and the graminoid vegetation fails to recover. Measurements of abundance of species based on 805 metres of transects across the salt marshes at La Pérouse Bay indicate that since 1985 approximately 70% of the salt-marsh graminoid swards have been severely damaged or destroyed by geese. On 5 of the 12 transects no vegetation remains (R. Jefferies, unpublished data).
The change in the state of the vegetation initiated by the geese acts as a trigger for a further series of changes that leads to increased destruction. The reduced area of salt marsh vegetation and the high numbers of goslings and adults result in intense foraging during the post-hatch period. The above-ground biomass of the heavily grazed swards may be only 10-15 grams dry weight (g dwt) per square metre (40 g dwt in undamaged grazed swards) (Cargill and Jefferies 1984, Williams et al. 1993). The effect of these foraging processes is to reduce the thickness of the vegetation mat (live and dead material) that insulates the underlying marine sediments from the air. These sediments were laid down when the Hudson Bay lowlands were part of the Tyrrell Sea. Rates of evaporation from the surface sediments increase and inorganic salts from the marine clays produce hypersaline conditions ranging from 32-120 g of dissolved solids per litre (32-120 0/00) (Iacobelli and Jefferies 1991, Srivastava and Jefferies 1995a,b; 1996). Experimental studies have shown that salinities above 32 0/00 (oceanic sea water) reduce the growth of the preferred forage plants, Puccinellia phryganodes and Carex subspathacea. This, together with the intense foraging, maintains open swards and hypersaline conditions and results in a positive feedback producing increased destruction of salt-marsh swards and desertification of the landscape (Srivastava and Jefferies 1996) (Fig. 2.14).
A group of species that may invade these grubbed sites are good ecological indicators of disturbance of swards by geese. Salicornia borealis and Atriplex patula var. hastata colonize bare sediments which are highly saline. These species do not grow in some localities such as the McConnell River-Wolf Creek system, but they are present at others, (e.g., Walker Bay, Central Arctic, B. Bromley, pers. comm.). Neither species is eaten by the geese; about 45% of the dry weight of plants of these species consists of salt. In late summer large areas of marsh appear reddish-purple in colour as S. borealis produces anthrocyanin pigments at this time of year.
Within intertidal marshes a number of measures described above indicate ecosystem dysfunction. They include a low level of above-ground biomass, damage to pseudostems of graminoids and the presence of indicator species such as Salicornia borealis. At sites where the vegetation has been killed, bare mud flats remain. At some sites, such as on the foreshore between the north and south arms of the McConnell River, the remains of below-ground biomass of former Puccinellia swards are visible. Elsewhere the sediments are eroded, exposing underlying glacial till and marine gravels.
Damage to sand dunes and beach ridge plant communities
Beach ridges and dunes are widespread in coastal zones around the Bay. Lyme grass (Elymus arenarius) is an active colonizer of these dunes and ridges. Shoot pulling of developing shoots of this grass is widespread in early spring. The grass is no longer a common species at La Pérouse Bay. Some beach ridges where the grass was formerly abundant are now devoid of the species (B. Ganter, pers. comm.). In early spring these ridges are roosting sites for geese and dung heaps are deposited by the birds. The same scenario has developed where spring migrant Interior Canada geese and moult migrant giant Canada geese congregate near Cape Duncan, Akimiski Island, Northwest Territories (K. Abraham and R. Jefferies, pers. obs.). A flora characteristic of the overall disturbance by geese has colonized these degraded sites. Senecio congestus, Matricaria ambigua and Rumex maritimus invade; the leaves of the first species are eaten by the geese in very early spring. In addition, moss carpets (mainly Bryum species) are common on the tops of frost-heave hummocks, where the higher plant vegetation has been removed. In summer, the carpets dry out and the moss mat is blown away, exposing the underlying sand and gravels.
Damage to coastal meadow grassland and willow communities
In dry sites, immediately inland from the intertidal salt marshes, meadow grassland is widespread along the southern coast of Hudson Bay and in James Bay. The grassland is often colonized by Salix brachycarpa and Salix myrtifolia. The most common grass species include Festuca rubra, Calamagrostis deschampsioides, and to a lesser extent Elymus arenarius. These sites are covered by tidal water on rare occasions. Grubbing of these grasslands, which have a thin veneer of organic matter on the surface of sediments, is common in spring. Both lesser snow geese and Canada geese grub the turf and remove grass roots and rhizomes. Where the grubbing of turfs extends to the base of willow bushes, the bushes die as a result of the hypersalinity and exposure of roots (Iacobelli and Jefferies 1991) (Fig. 2.13c). In some areas where the thin veneer of soil organic matter has been removed Puccinellia phryganodes and Carex subspathacea colonize the exposed saline sediment. Patches of these graminoids inland from the intertidal marshes are often indicative of secondary succession following goose disturbance. Further grubbing of these secondary swards can be expected.
Overall Comment
The descriptions given above indicate that the effects of the geese on vegetation development are not symmetrical. The geese interact with physical processes (i.e., the positive feedback described above)(Fig. 2.14), so that discontinuous and irreversible transitions in the serial stages of plant succession may occur (Hik et al. 1992). Intense grazing of the Puccinellia-Carex intertidal swards results in removal of apical meristems of leaves and shoots of dicotyledonous plants (Plantago maritima, Potentilla egedii and Ranunculus cymbalaria). This precludes further shoot growth for the remainder of the season. In effect, the geese reset the successional clock each year and delay successional processes. The species diversity (number of species per unit area) may be used as an indicator of intense goose foraging, particularly at sites close to the mean high water mark of spring tides. In the absence of grazing the plant assemblage switches to one dominated by Festuca rubra and species diversity increases rapidly (Bazely and Jefferies 1986).
Long-term destruction of sub-Arctic freshwater wetland vegetation by lesser snow geese
The Hudson Bay lowlands is one of the largest wetlands in the world. It provides food resources for several million migrating waterfowl, including the mid-continent population of lesser snow geese, the Eastern Prairie, Tall Grass Prairie, Mississippi Valley and Southern James Bay populations of Canada Geese, half of the Atlantic Brant population, and significant populations of pintails, black ducks, green- winged teal and mallards (Ross 1982, Thomas and Prevett 1982). When snow geese migrate north in spring they follow the coastline of Ontario, Manitoba and Northwest Territories northwards, staging just south of the retreating snowline. They feed primarily in fresh-water sedge meadows adjacent to coastal habitats. One of the most conspicuous types of damage that occurs in spring is shoot pulling. The birds forage intensively on the shoots of fresh-water sedges, particularly Carex aquatilis, but also C. atrofusca, C. x flavicans and C. vaginata. Later, during nesting, breeding birds also remove large numbers of shoots. At some localities shoot pulling has been intensive and the effect is accumulative. In shallow ponds, where dense stands of Carex aquatilis were formerly present, the death of stands has created open ponds of standing water devoid of vegetation in which large amounts of organic matter and peat debris are mixed by surface winds (Fig. 2.15). The system is dysfunctional and the depth and mobility of the debris appear to restrict germination and seedling establishment of wetland species. The muds are anoxic and some ponds have remained in this state for a decade or more (Kotanen and Jefferies 1997). In saturated mossy areas, where sedge assemblages formerly occurred, the disappearance of sedges produces extensive moss carpets in which only scattered shoots remain (Fig. 2.16). Again the thick moss carpets, composed of Drepanocladus uncinatus and Aulacomnium species, appears to restrict seedling establishment of sedges (Kotanen and Jefferies 1997). Close to frost heave mounds, Salix reticulata and S. arctophila, together with Potentilla palustris and Petasites sagittatus, have grown across the moss carpet creating a new plant assemblage. The four latter species and the mosses are not eaten by the geese (Jefferies 1988a,b). This type of assemblage and moss carpets can be recognized at a number of goose colonies where sedges have been removed by geese (Kerbes et al. 1990; Kotanen and Jefferies 1997, K. Abraham, pers. obs.). Community development appears to have been altered irreversibly by the geese.
The large scale destruction of sedge meadow communities and the exposure of peaty sediments, or peat, can be seen in the McConnell River-Wolf Creek area (Kerbes, Kotanen and Jefferies 1990). In much of the coastal hinterland of string and flark (ridges and intervening wet zones) bogs and fens (oligotrophic / eutrophic mires), the geese have grubbed vegetation, exposing underlying peat over large areas. These dysfunctional systems have been called peat barrens (Kerbes et al. 1990) (Fig. 2.17). Plants of woody species, such as Betula glandulosa and Empetrum nigrum, which are confined to the strings, also die. At some sites erosion of the peat has occurred as a result of water movement (spring run-off) and chemical oxidation of peats as they dry-out in summer. Where this has occurred, the underlying glacial gravels and till are exposed. At present, examples of this kind of habitat damage along the west coast of Hudson Bay occur from the Tha-anne River north to the Maguse River, a distance of almost 200 km. The area is used by both breeding and staging birds (lesser snow geese).
Rates of destruction of vegetation
Most of the changes described in the above sections have occurred at La Pérouse Bay, Manitoba since 1978 when intensive studies of plant-herbivore interactions began there. Based on our knowledge of the changes in the vegetation, it is clear that this is an on-going process and that there are sites at La Pérouse Bay and elsewhere which show comparable changes in vegetation that occurred much earlier than 1978. How much earlier is very difficult to determine. Although the present colony at La Pérouse Bay was established in the late 1950s (Cooke et al. 1995), the area may have been a major staging location prior to those years. What is different today compared to earlier times, is that the scale of destruction associated with the increase in the population size of lesser snow geese has increased in magnitude. Similar changes have occurred elsewhere along the coastline in response to the foraging activities of large numbers of birds (e.g., McConnell River, Cape Henrietta Maria, and Akimiski Island).
The damage to vegetation and soils of these coastal habitats is cumulative, although the same amount of damage does not occur each year. Prevailing weather conditions throughout the Hudson Bay region determine both migration rates of geese and which sites serve as staging areas in a given year. One approach that offers considerable promise for measuring rates of destruction of vegetation is the application of remote-sensing techniques, particularly the use of LANDSAT imagery. This approach has been used to detect vegetational changes since the 1970s at La Pérouse Bay and the north shore of Akimiski Island, and a similar study is underway to examine vegetational changes along the coast between the Knife and Seal Rivers. At La Pérouse Bay LANDSAT imagery based on spectral differences in the red and far-red bands between 1973 and 1993 indicates that the vegetation has been destroyed or severely damaged over an area of approximately 2400 ha (Jano, unpublished data) (Fig. 2.18). Much of the initial damage was on the beach ridges, dunes and in the salt marshes, but in the last 10 years the damage has extended to the fresh-water sedge meadows, as the birds seek alternative sources of forage. As a result, we are beginning to see an early stage in the development of peat barrens there, as described for the McConnell River-Wolf Creek area.
Estimates of damaged areas of coastal marshes
No formal estimate has been made of the total area of destroyed coastal marsh, or of the area of vegetation swards that have been partially damaged by the foraging activities of the geese. Such an analysis requires the following minimum information: (1) LANDSAT remote-sensing imagery of changes in vegetation from 1973 to the present along the entire Hudson Bay coastline; (2) the necessary ground- truthing of the vegetation and the preparation of a classified vegetation map; (3) calculation of damaged areas and ground-truthing of partially damaged areas in order to calculate this area; (4) Measurements of above-ground biomass at all sites.
Given that these data are unavailable at this time, the following, tentative estimates of the scale of destruction have been made. Long-term monitoring of ground transects in the inter-tidal marsh at La Pérouse Bay indicate the changes that have occurred to the vegetation since 1985. Approximately 35% of the vegetation swards have been destroyed and another 30% are so badly damaged, that they no longer provide a source of forage for the birds (because geese are present each summer recovery is impossible). In effect, two-thirds of the land base monitored in inter-tidal marsh is now non-productive. The remaining 35% is “overgrazed”, such that damage to grass shoots is occurring. Independently, the results from the remote-sensing imagery show that approximately 2400 hectares of vegetation have been destroyed, or very badly damaged between 1973 and 1993 at La Pérouse Bay. Some of this area includes supra- and inland salt-marsh vegetation and sedge meadow vegetation. However, as most damage has occurred in intertidal areas (salt marshes and dunes/gravel ridges), an approximate estimate of 2,000 hectares of vegetation have either been destroyed, or so badly damaged that the vegetation is unproductive. This is equivalent (approximately!) to 65% of the former area of inter-tidal vegetation at La Pérouse Bay, as indicated above.
Observations of the state of inter-tidal salt marsh swards of Puccinellia phryganodes and Carex subspathacea along the entire coastline from Attawapiskat, Ontario to the Maguse River, NWT, together with measurements of above-ground standing crop (g m-2) give no reason to doubt that the estimates for La Pérouse Bay are applicable to other sites where snow geese feed during migration and breeding. In short, the “35%-30%-35%, pattern” discussed above can be applied to other sites. Major exceptions include the Pen Island marshes on the Ontario/Manitoba border where there is little damage and at river estuaries (e.g., Nelson, Albany) where large volumes of fresh water discharge, reducing the salinity and producing tall growth forms of both Puccinellia and Carex which are little grazed. Additional exceptions are small patches of salt marsh fringing beach ridges which are heavily grazed by Canada geese during brood rearing but remain productive. Andrew Jano (Ontario MNR, unpublished data) has estimated the area of intertidal marsh from the Kettle River (Ontario/Manitoba border) to Attawapiskat (Ontario, James Bay) in contiguous 5 km segments of coastline. The total area is 35,329 hectares, including 2,000 hectares for the north coast of Akimiski Island. Figures for supratidal marsh of the same area are also available but comparable figures have not been calculated from remote sensing imagery yet for Manitoba and NWT. In Manitoba, the area of grazed Puccinellia-Carex (i.e., intertidal) salt marsh is estimated to be 14,500 hectares and for the NWT as far north as the Maguse River, the estimate is 5,000 hectares (R. Jefferies, unpublished data). Hence, out of a total of 54,829 hectares of intertidal salt marsh for the entire coastline it is estimated that approximately 35,638 hectares are no longer productive and the majority of the remaining areas of salt-marsh swards are heavily utilized. It should be emphasised that these figures for the area outside La Pérouse Bay are very provisional and they do not include supra-tidal marshes.
Similar estimates of damage to sedge meadow-fen vegetation cannot be made at this time as the extent of damage is very poorly known.