Grazing intensity of herbivores can exert a strong influence on vegetation dynamics within terrestrial systems (Holling 1973; Noy-Meir 1975; May 1977). Herbivores may retard or promote succession depending on the species composition of plant assemblages and patterns of herbivory (Bakker 1985; Pastor et al. 1988; Dublin et al. 1990; Prins & Van der Jeugd 1993; Ungar 1998; Dormann et al. 2000; Zacheis et al. 2000). When a dominant species is the preferred forage, species richness and evenness may increase (McBrien et al. 1983; Furbish & Albano 1994). In contrast, when grazing is intensive and non-selective, competition patterns observed under moderate grazing pressure may disappear (Taylor et al. 1997). In some instances, when consumption rate of forage plants exceeds their growth rate, herbivores may trigger rapid, non-linear shifts of species assemblages towards alternate states, often characterized by either temporary or permanent loss of vegetation and by low species richness (Ludwig & Tongway 1995; Milton & Dean 1995; Whiteford et al. 1995; Magnússon 1997; Van de Koppel et al. 1997).
Vegetational loss and soil degradation have occurred as a result of herbivory in coastal marshes of the Hudson Bay lowlands, one of the breeding grounds of the lesser snow goose, Chen caerulescens caerulescens L., a keystone species in this system (Hik et al. 1992; Jefferies 1997). In recent decades, this mid-continent population has increased in numbers at 7% per year, probably as a result of the high quality, agricultural food subsidy available on the wintering grounds and along migration routes (Abraham et al. 1996; Jefferies et al. 2002). In the early 1980s when the goose population at La Pérouse Bay, Manitoba (58°44′ N, 94°28′ W) was an estimated 5000 pairs of birds (Cooke et al. 1995), changes over 5 years in species composition in intertidal exclosures indicated that grazing slowed vegetational change (Bazely & Jefferies 1986). By 1997 the population had increased to an estimated 44 500 pairs (Abraham, Ross & Rockwell, unpublished aerial survey) and grubbing by adult birds in spring, together with intense grazing by family groups in summer, had led to the destruction of salt-marsh swards and exposure of sediments (Srivastava & Jefferies 1996). In 1993 (the latest available information) over 2500 ha of coastal habitats at La Pérouse Bay showed a decline in vegetation cover compared with that 20 years earlier (Jano et al. 1998). Loss of vegetation cover and increased soil evaporation rates have resulted in hypersaline soils that have limited re-establishment of plants (Iacobelli & Jefferies 1991; Srivastava & Jefferies 1996).
The recovery potential of these degraded systems is largely unknown. Shifts in ecosystem structure caused by herbivores often tend to be asymmetric, such that restoration of the system is seldom as simple as lowering levels of herbivory (Bradshaw 1997). Vegetation transitions may be coupled with discontinuities in abiotic conditions that cannot be easily reversed (Westoby et al. 1989; Bazely & Jefferies 1996; Rietkerk & Van de Koppel 1997). When vegetation change is a non-linear process with apparent discontinuities (Rose & Harmsen 1981), or a series of threshold responses between meta-stable states (Westoby et al. 1989; Rietkerk & Van de Koppel 1997), restoration efforts require knowledge of the various variables that need to be manipulated in order to achieve state transitions (Hobbs 1994; Hobbs & Norton 1996).
We have examined the pattern and rate of natural re-vegetation at La Pérouse Bay in intertidal and supratidal marshes where loss of vegetation had occurred, in order to determine whether recovery was possible in plots from which geese were excluded. In addition, changes in early and later successional assemblages of species were documented over 11 years both in the presence and absence of grazing, in order to evaluate longer term changes in species assemblages. Lastly, experimental transplants were made to test whether later successional species (Festuca rubra and Calamagrostis deschampsioides) could establish directly in degraded sediments, or whether the early successional vegetation template was a necessary precursor. The results have been synthesized with our previous knowledge of the system in a proposed state and transition model of vegetation dynamics.
DESCRIPTION OF SITE
Salt-marshes develop on exposed coastal flats at La Pérouse Bay (Jefferies et al. 1979) in a region where isostatic uplift is occurring at a rate of about 0.8 cm per year (Andrews 1973). Primary colonizers of estuarine brackish soft sediments are Hippuris tetraphylla, Hippuris vulgaris, Ranunculus cymbalaria and Myriophyllum exalbescens. Nomenclature follows Porsild & Cody (1980). On tidal flats, following colonization of bare sediments by diatoms and cyanobacteria, Puccinellia phryganodes is the initial vascular plant to establish, followed by Carex subspathacea and Ranunculus cymbalaria. The early presence of the latter two species is largely dependent on the availability of brackish water from shallow drainage channels. Once Puccinellia-Carex swards are well developed, dicotyledenous plants such as Potentilla egedii, Stellaria humifusa and Plantago maritima var. juncoides establish. As the elevation of the ground increases as a result of isostatic uplift, sediment accumulation and frost-heave, soils become better drained (less saline) and an organic soil layer develops (Jefferies et al. 1979). In the absence of destructive foraging, a relatively species-rich willow-grassland establishes in the rarely flooded supratidal marsh (< 2 times every 3 years) in which Festuca rubra, Calamagrostis deschampsioides, Salix brachycarpa and Salix myrtillifolia are dominant. At more inland sites, where sediments are less well drained, sedge meadows dominated by Carex aquatilis replace the salt-marsh vegetation (Kerbes et al. 1990; Kotanen & Jefferies 1997).
Hik et al. (1992) have argued that the Puccinellia-Carex and Festuca-Calamagrostis plant assemblages are examples of stable vegetation states that shift asymmetrically in response to low to moderate grazing pressure resulting in a standing crop of Puccinellia-Carex swards of c. 40 g m−2 and an increase in ground elevation. In areas of higher elevation in intertidal salt-marshes, when grazing declines or is stopped by erection of exclosures, Puccinellia-Carex swards shift towards the Festuca-Calamagrostis state, but at lower elevation a similar vegetational change fails to occur in the short term (< 5 years) (Bazely & Jefferies 1986). When swards of Festuca-Calamagrostis are moderately grazed, the vegetation does not easily revert to a Puccinellia-Carex sward. However, if foraging pressure is high (standing crop of Puccinellia-Carex swards < 20 g m−2), grubbing and hypersalinity trigger shifts of both Puccinellia-Carex swards and Festuca-Calamagrostis-willow assemblages to a third stable state where the soil is essentially devoid of vegetation (Iacobelli & Jefferies 1991; Srivastava & Jefferies 1996). The transition, which is rapid, creates a mosaic of sediment and vegetation where patches are irregular in distribution and of varying size (1–30 m2).