Snow geese are a keystone species in the Arctic tundra ecosystem. Keystone species are those that have an outsized influence on their environment and play a critical role in maintaining the structure and function of their ecosystem. Snow geese affect the tundra in several key ways that make them a classic example of a keystone species.
What is a keystone species?
A keystone species is one that has a disproportionately large effect on its natural environment relative to its abundance or total biomass. Keystone species exert strong control on both the structure and function of their ecosystems. The influence of a keystone species is much larger than would be expected from simply their abundance or total biomass.
Some key characteristics of keystone species:
– Their functional role in their ecosystem is crucial and unique
– Their removal initiates major changes in the ecosystem structure and function
– They provide critical resources to a wide variety of other species
– Their impacts are much larger than would be expected from their abundance
Not all abundant or dominant species are considered keystone. It is specifically the disproportionate influence and crucial functional role of keystone species that differentiates them.
Some classic examples of keystone species include:
– Sea otters – control sea urchin populations, which otherwise would devastate kelp forests
– Beavers – transform landscapes and create wetland habitat for many other species through dam building
– Prairie dogs – their burrows provide habitat for many other species and increase landscape heterogeneity
– Starfish – regulate diversity and abundance of invertebrates in tidepools
The ecological role of snow geese
Snow geese occupy a vital role in the food webs of Arctic and subarctic wetland ecosystems. They exert top-down pressure through intensive grazing and bottom-up fertilization effects. This combination of grazing and fertilization makes snow geese a keystone species with disproportionate impacts on tundra structure and function.
Intensive grazing
Snow geese feed heavily on the leaves, roots, and stems of wetland plants. Their preferred foods include sedges, grasses, and forbs. As they grub and dig for food, snow geese remove substantial amounts of live plant biomass.
At high population densities, snow goose grazing can:
– Dramatically reduce the height and density of vegetation
– Decrease the cover of preferred food plants
– Increase the abundance and dominance of unpalatable, grazing-tolerant plants
– Convert vegetated marshland to barren mudflats dominated by algal mats
This intensive grazing removes the existing dominant plant species and causes a shift to grazing-resistant, often less palatable plants. Their feeding habits essentially act as a “reset” button, converting marshlands to an earlier successional state.
Fertilization effects
In addition to reducing live plant biomass through grazing, snow geese also fertilize tundra ecosystems. They do this primarily through deposition of feces as they move and feed across the landscape.
Snow goose feces are rich in nitrogen and phosphorus. Addition of these nutrients from fecal deposits can:
– Increase nutrient availability in otherwise low-nutrient soils
– Stimulate plant productivity and regrowth after grazing
– Shift species composition toward faster-growing, more nutrient-demanding species
Together the effects of grazing and fertilization make snow geese a keystone species – they exert strong control over tundra structure and function through both top-down and bottom-up processes.
Impacts on plant communities
The intensive grazing and fertilization by geese can dramatically alter marshland plant communities. Some of the major impacts include:
Decreased abundance of dominant graminoids
Graminoids are grass-like plants such as sedges, rushes, and grasses. They often dominate undisturbed marshlands. Preferred graminoid foods of snow geese include species like Carex, Calamagrostis, Dupontia, and Arctophila.
Intense, sustained goose grazing can substantially reduce the cover and density of these nutritious graminoids. For example, goose activity decreased the total cover of graminoids by over 90% in some regions of Hudson Bay.
Increased abundance of prostrate shrubs
With reduced abundance of tall graminoids, the low-growing vegetation of heavily grazed sites becomes dominated by prostrate shrub species. Salix spp., Dryas spp., and Rhododendron spp. are examples of low-growing shrubs that increase under intensive goose grazing.
The short stature, mat-like growth form, and tolerance of abrasion of these shrub species allows them to persist under repeated grazing. They gain a competitive advantage with reduced competition from graminoids.
Invasion of algal mats
With removal of living graminoids and other plants, sustained heavy grazing creates open mudflats. These bare substrates are readily colonized by fast-growing, grazing-resistant algae. Blue-green algal mats dominated by Oscillatoria and Phormidium species develop.
These unpalatable algal mats represent an early successional stage. They reduce habitat quality for other species and alter key ecosystem processes like nutrient cycling and energy flow.
Changes in plant diversity
Altered competitive environments under intense goose grazing can lead to declines in plant diversity over time. Unpalatable or highly tolerant species dominate, while less tolerant or more nutritious plants decline.
For example, heavy grazing converted diverse Carex fen ecosystems to low diversity mats dominated almost solely by a single sedge species, Carex subspathacea. This homogenization of the landscape represents a negative impact of excessive goose grazing.
Impacts on soil conditions
In addition to vegetation, goose activities also transform abiotic conditions – particularly soil nutrients, structure, and hydrology. Key impacts include:
Increased soil nutrients
Inputs of nutrient-rich feces from geese elevate levels of nitrogen, phosphorus, and other nutrients in marsh soils. For example, snow geese increased sediment nitrogen nearly 3-fold in some Hudson Bay wetlands. This fertilization effect drives changes in the plant community.
Altered soil structure and hydrology
Intensive grubbing and digging actions of geese modifies soils. Removal of plant cover and trampling destroys soil crusts. This can lead to increased evaporation and altered water flow over the landscape. Grubbing can even break up and erode surface peat layers.
Over time, heavy grazing results in a more homogenous, degraded soil structure. It also alters local hydrological patterns, such as by increasing ponding in some disturbed areas.
Expanded barren areas
Sustained, heavy goose populations remove vegetation cover and degrade soils over large areas. This can expand zones of barren, exposed sediments with poor capacity to support plants and retain water. These degraded sites have long recovery times.
For example, even 17 years after reduction in goose grazing, areas that were heavily impacted still had 50% lower vegetation cover compared to lightly grazed sites. This illustrates the long-lasting effects.
Trophic cascade effects
As a keystone species, snow geese have impacts that cascade through multiple trophic levels of the tundra ecosystem. Reduced plant biomass and diversity under heavy grazing indirectly affects other species dependent on those plants.
Declines in nest density of other bird species
Birds that nest in marsh vegetation are affected by habitat changes caused by snow geese. For example, the density of nests of American coot and several duck species was over 90% lower in heavily grazed sites in Hudson Bay compared to reference marshes.
Reduced nesting habitat quality from loss of tall graminoid cover contributes to these declines. Lower insect abundance in degraded areas may also reduce nesting densities of insectivorous birds.
Decreased small mammal populations
Small mammals like voles, lemmings, and shrews also decline following intensive goose grazing. For example, heaviest grazed areas of coastal Hudson Bay had only 1/3 the vole trap success compared to lightly grazed reference sites.
Loss of food sources, protective cover, and increased predation risk all likely contribute to the reduced mammal densities under heavy grazing. This illustrates a trophic cascade.
Changes in large mammal habitat use
Large mammals such as caribou alter their habitat use in response to goose-driven changes in vegetation. Caribou preferentially use lightly grazed sites with greater graminoid abundance as forage.
But barren, degraded sites are used less. This shifts caribou activity and alters their impacts on the landscape ecology.
Creation of inland salt marshes
Nutrient inputs from goose feces increase soil salinity downslope of colony sites. This can convert freshwater marshes into saltwater-influenced, brackish marshes. These altered salinity regimes favor salt-tolerant vegetation over fresher water species.
In effect, snow geese create inland salt marsh habitat where it otherwise would not exist. This unique habitat supports different invertebrate and bird communities compared to surrounding freshwater marshes.
Positive and negative effects
The transformative impacts of snow geese have both positive and negative dimensions. Evaluating their overall role as a keystone species depends on perspective.
Positive effects
Some beneficial aspects of snow goose impacts include:
– Fertilization increases plant productivity and habitat quality for graminoid-feeding birds like brant geese
– Grubbing helps recycle nutrients trapped in roots back into the active soil ecosystem
– Grazing sets back succession and creates early successional habitat beneficial to some species
– Inland salt marshes increase habitat diversity at the landscape scale
Negative effects
Detrimental effects of excessive goose grazing include:
– Loss of large expanses of vegetation degrades general habitat quality
– Decreased biodiversity as plant communities homogenize
– Declines in density and reproduction of other tundra-nesting birds
– Displacement of sensitive species by more tolerant or invasive species
– Altered migratory traditions and landscape use patterns of caribou and other mammals
So while geese provide some benefits, extreme grazing degrades habitats, decreases biodiversity, and disrupts ecosystem function. Population control is likely needed to reduce these negative impacts.
Conclusion
The disproportionate impacts of snow geese on tundra vegetation, soils, and other species unequivocally qualify them as a keystone species. Their intensive grazing and fertilization alters fundamental biotic and abiotic properties of the ecosystem. Snow geese essentially act as ecosystem engineers that shape the structure and function of Arctic and subarctic wetlands.
Their unique ecological role comes with both costs and benefits. Excessive grazing pressure under high population densities can degrade habitats and induce trophic cascades. But more moderate grazing maintains early successional habitats useful to some species. Overall, the transformative effects of snow geese on tundra ecosystems make them a classic keystone species.
