Northern people and caribou are so inter-related that without caribou, the Arctic would be indeed the barrens. The caribou in their central role in the tundra and taiga ecology and inter-connection with the culture of many aboriginal people have parallels with the role of salmon on Canada's Pacific West Coast.
At its simplest, the caribou role in the ecosystem is the net effect of forage removal, production of greenhouse gas and return of nutrients through fecal pellets. Annually, a caribou removes 900 kg of food (2.5 kg per day), produces 20 kg of methane (55 gm per day) and returns to the ecosystem, nutrients in the form of fecal pellets, 270 kg (30g x 25 times a day). At the herd scale,  170 000-350 000 caribou remove 140-320 million kg forage, produce 3-7 million kg of methane and return 38-77 million kg fecal pellets spread over the annual range (150-300 kg/km2). As caribou travel and rest on frozen waterways, the nutrient return from fecal pellets is to aquatic as well as terrestrial ecosystems.


However the role of caribou in the ecosystem is more intricate and complicated than the mere removal of forage, emission of gasses and return of nutrients. The Boreal and Arctic ecozones food webs have relatively few links which does not mean, however, a simple system. The links are complex in the dynamics of their inter-relationships. The northern ecosystems are nutrient limited as so much carbon is inaccessible because only a shallow active layer of the soil thaws each year. Caribou through their forage intake and output (fecal pellets) have complex and cascading effects strongly patterned over time and space. As well caribou support a diverse group of other species including external parasites such as blood-feeding mosquitoes. Mosquitoes in turn through the filter-feeding of their larvae are a key element in nutrient cycling in aquatic systems.  Further up the food webs, caribou support large-bodied and medium-sized predators and scavengers. Earlier debates about top-down (predator) or bottom-up (forage) regulation for caribou are now replaced by an appreciation of how nutrition and predation interact.


Relationships between plants and caribou include the plants' responses to caribou's highly selective foraging. Caribou are highly selective for individual plant species and forage for buds and unfolding leaves to maximize nutritional value. The gregarious and migratory behaviour of migratory tundra caribou forces their role in ecosystem structure and functioning to be strongly scale dependent. As caribou convert plant tissue into their body mass and fecal pellets, their local foraging movements and seasonal migrations mean caribou are re-distributing nutrients within and across ecozones. In the taiga ecozones, the effects of caribou herbivory lag a season as caribou are foraging during winter when most plant growth and nutrient cycling is quiescent due to sub zero temperatures. Over the timescale of decades, caribou winter ranges expand and contract and the herds cycle from high to low abundance. Abundance can vary three-fold with cascading effects on plants and nutrient cycling as the plant communities shift from one state to another. Succession of plant communities as a response to density of foraging includes, for example, lichen- dominated shifting to moss and moss to grass states.


Nitrogen is a limiting factor for plant growth. Caribou summer grazing can increase the rate of soil nitrogen cycling through influencing the amount of plant litter which changes the soil microclimate for decomposition and mineralization processes, and by adding soluble nitrogen from faecal pellets and urine. The changes vary with season and time and intensity of grazing. For example in the boreal forests moose browsing on willows causes them to respond by compensatory tissue growth above ground but reduce their fine root production. Foraging usually increases leaf litter which leads to higher nitrogen concentrations and higher rates of decomposition. The effects of browsing also include effects on plant demography such as changes in sex ratio although the effects are complicated. 


The contribution that caribou make to the predator links is relatively low at least in the northern Arctic ecozone where Krebs et al (2003) argue for less of a top down system and more one driven by variance in weather. In the southern Arctic ecozone, the Bathurst herd of 350 000 caribou in the mid-1990s was estimated to support some 1500 wolves which likely removed  40 000 caribou and  500-1000 grizzly bears which have caribou as 80% of their diet - a bear annually may eat 9 and 18 adult caribou. 


For people in the Arctic ecozones, caribou are the basis of their cultures and still play in central role in their lives. A measure of the importance is the annual harvest, which in Nunavut (1996 - 2001) averaged 24,522 caribou. In the Northwest Territories, Dene, Inuvialuit, Metis and non-aboriginal people from almost all communities hunt the migratory herds and the minimum annual harvest is 11,000 caribou, valued as at least  $17 million dollars (includes meat replacement and outfitting). Assuming an average carcass weight of 45kg, the NT and NU harvest is about 1.6 million kg caribou. At a beef replacement value of $20/kg, the annual average harvest of caribou just for meat replacement is $35 million. This excludes any commercial harvesting or any value for hides let alone the intangible cultural strength and value of hunting. A recent study commissioned by the Beverly/Qamanirjuaq Caribou Management Board determined the total annual net value of the Beverly and Qamanirjuaq herd harvest is $19.9 million annually.