What Drives Caribou Cycles?

Caribou and wild reindeer populations cycle between distinct periods of abundance and scarcity. In North America, abundance appears to peak at 40-200 year intervals. Currently many herds are in decline and managers and co-management groups require a better understanding of the mechanisms that initiate declines. A number of theories have attempted to explain caribou cycles: 1) cycles are driven by continental weather patterns; 2) cycles are driven by a density dependent response such that with increasing density, intra-specific competition for forage also increases and this, interacting with environmental variation, explains the transition from peak abundance to a decline in abundance; or, as we develop in this paper, 3) cycles result from increasing demands for protein as density increases. We hypothesize that as density increases transmission rates of intestinal parasites increase and thus forage requirements, specifically protein requirements, increase at the individual scale to meet immune system needs and that individuals become logistically constrained from obtaining enough protein. In this paper we test, with available data, these three theories making the assumption that large migratory herds are ultimately nutritionally regulated. Based on these analyses we discuss the vulnerability of the herds to global changes and the management implications of our findings.