The tundra is a cold, treeless biome found in the Arctic and Antarctic regions. Despite the harsh conditions, many organisms have adapted to survive in the tundra through symbiotic or mutualistic relationships. Mutualism is an ecological interaction between two organisms where both benefit. The scarcity of resources in the tundra makes these cooperative relationships essential. Below are some examples of mutualism found in tundra ecosystems.
Plants and Mycorrhizal Fungi
One of the most important mutualistic relationships in the tundra is between plants and mycorrhizal fungi. Mycorrhizae are fungi that colonize the roots of plants. The mycorrhizal fungi help the plants absorb water and nutrients like phosphorus and nitrogen from the soil. In return, the plant provides the fungi with sugars produced during photosynthesis. Up to 90% of plants in the tundra participate in this relationship. Key mycorrhizal partners include arctic willow, dwarf birch, lichens, and heath plants like cassiope and arctic heather. Without the nutrients provided by the fungi, these plants would not survive.
Some important mycorrhizal fungi in the Arctic tundra include:
- Amanita muscaria – fly agaric
- Cenococcum geophilum
- Hebeloma crustuliniforme – poison pie
- Laccaria laccata – deceiver
- Lactarius glyciosmus
- Russula nana – arctic russula
These cold-adapted fungi allow both lower and higher plants to flourish in the extreme conditions of the tundra.
Lichens
Lichens are another example of mutualism, consisting of algae and/or cyanobacteria living symbiotically with fungi. The algal cells provide food through photosynthesis, while the fungal filaments provide structure and protection. Lichens are abundant across tundra environments, covering rocks and soil. Common tundra lichen genera include Alectoria, Bryoria, Cetraria, Cladonia, Ochrolechia, Peltigera, Stereocaulon, and Thamnolia. Lichens improve soil stability in the thin tundra biome. Their fungi also help extract minerals from the soil, benefitting tundra plants.
Plants and Nitrogen-Fixing Bacteria
Nitrogen is essential for plant growth but is scarce in tundra soils. Some plants have developed a mutualism with nitrogen-fixing bacteria, which convert atmospheric nitrogen into ammonia usable by plants. The plants provide carbon compounds for the bacteria through photosynthesis. Examples include species of clover, lupines, and alder shrubs hosting Rhizobia, Frankia, and cyanobacteria in their root nodules. These nitrogen fixers allow certain vascular plants to colonize the nitrogen-poor tundra.
Key Nitrogen Fixers of the Tundra
| Plant | Nitrogen-fixing Symbiont |
|---|---|
| Arctic sweetvetch | Rhizobia bacteria |
| Lupines | Rhizobia bacteria |
| Alders | Frankia bacteria |
| Sedges | Cyanobacteria |
Plants and Pollinators
Pollinators like insects, birds, and small mammals provide an essential service to tundra plants by transferring pollen between flowers, enabling fertilization. In return, the plants provide nectar and pollen as food sources. Important pollinators of the tundra include mosquitoes, flies, butterflies, moths, beetles, spiders, and collembola. Birds like the Lapland longspur and hoary redpoll also contribute. Even small arctic mammals like voles and lemmings help distribute pollen. Common insect-pollinated plants include dwarf fireweed, arctic willow, saxifrages, dryas, and Arctic poppies. This mutualism is vital for plant reproduction in the harsh tundra ecosystem.
Lichens and Nitrogen-Fixing Cyanobacteria
Some lichen species in the tundra form a mutualism with cyanobacteria, which are photosynthetic bacteria that convert atmospheric nitrogen into ammonia usable by organisms. The lichen provides a suitable environment for the cyanobacteria, while the cyanobacteria provide essential nitrogen compounds for both itself and the lichen fungi. Genera of lichens engaged in this relationship include Peltigera, Nephroma, and Stereocaulon. The nitrogen fixed by cyanobacteria facilitates lichen growth in the nutrient-poor tundra.
Plants and Herbivores
At first glance, the relationship between tundra plants and herbivores like caribou, muskoxen, lemmings, and hares seems completely one-sided. But plant-herbivore interactions also have some mutualistic aspects. As the herbivores feed, they scatter plant seeds in their droppings, dispersing the plants across the landscape. And grazing stimulates plant growth in some species, causing branching and new shoots to form. So while herbivory removes plant biomass, it also aids seed dissemination and improves productivity in the tundra ecosystem.
Predators and Scavengers
Predators like Arctic foxes and snowy owls provide food scraps for opportunistic scavengers like jaegers, ravens, and bears in the tundra. The predators kill prey larger than they can consume completely. The leftovers provide meals for many smaller organisms. And the scavengers clean the environment by disposing of carcasses and waste. So predators get to feed, while scavengers receive free meals, benefitting both groups. This type of mutualism based on leftovers is especially important for cycling nutrients in low-productivity ecosystems like the tundra.
Key Tundra Predators and Scavengers
| Predators | Scavengers |
|---|---|
| Arctic foxes | Jaegers |
| Snowy owls | Ravens |
| Polar bears | Bears |
Lemmings and Lichens
Tundra lemmings have a seasonal mutualistic relationship with lichens. In winter, lemmings primarily eat lichens, digging them out from under the snow. The lichen gains by being dispersed to new ground in the lemming’s feces. And the lemming gets nourishment from the lichen to survive the winter. In warmer seasons, the lemming switches to eating other foods like roots, mosses, and grasses, allowing the lichen time to recover for the next winter. This seasonal give-and-take provides food and propagation benefits to both species.
Willow Ptarmigan and Willows
Willow ptarmigan and arctic willow show a mutualism based on predator protection and food. Ptarmigan use willow shrubs for cover from predators like foxes and hawks. The thick branches protect them and their chicks. In return, willow ptarmigan eat willow buds and twigs, distributing the plant through feces. Willows gain dispersal, while ptarmigan gain protective cover and food. This helps the birds survive the harsh conditions.
Terrestrial and Marine Food Webs
Nutrient cycling between marine and terrestrial food webs also shows mutualistic qualities. Marine mammals like seals transport ocean-derived nutrients to land when they come ashore. Their excrement fertilizes the soil. In turn, vegetation growth on land allows organic matter to runoff into the ocean, providing nourishment for nearshore marine ecosystems. This cycling benefits both environments. Terrestrial and marine ecosystems are more productive due to this cross-ecosystem nutrient exchange.
Conclusion
The harsh conditions of the tundra make mutualistic relationships essential for survival. Through symbioses based on nutrient exchange, reproduction, propagation, protection, and food sharing, species are interdependent. Lichens, fungi, bacteria, plants, animals, and even entire food webs rely on these cooperative partnerships to thrive in a tough environment. Mutualisms allow organisms to access scarce resources that would be impossible to obtain alone, demonstrating the remarkable inter-connectedness of life in the Arctic tundra.
