North American barren-ground caribou cycles are driven by logistic
trophic dynamics
Abstract
We used the Qamanirjuaq, Bathurst, and George River barren-ground
caribou sine cycles to project numbers (Nt), calculate subpopulation
annual growth rates (λt) and calculate logistic carrying capacity (Kt).
Maximum annual growth rate was 1.196 and maximum annual rate of decline
was 0.836 for the harvested Qamanirjuaq subpopulation sine cycle.
However, the maximum annual subpopulation growth rates for both the
harvested Bathurst and George River subpopulation sine cycles were
greater than the biologically possible maximum intrinsic rate of
increase during the eruption phase. Subpopulation numbers for
Qamanirjuaq, Bathurst and George River barren-ground caribou
subpopulations all closely tracked carrying capacity for one complete
cycle with lag times between Nt and Kt ranging from < 1-year
to approximately 5-years. The short lag times observed indicates that
Qamanirjuaq, Bathurst and George River barren-ground caribou
subpopulations closely track their range condition. Range condition
drives barren-ground caribou subpopulation cycles, but range condition
also cycles; presumably because annual barren-ground caribou grazing
rates are proportional to barren-ground caribou numbers and eventually
exceed range annual growth rates. Immigration from adjacent
subpopulations plays a role in the initiation and acceleration of the
eruption period in some subpopulations, but not all of them. Numerical
synchrony and asynchrony with adjacent subpopulation cycles can affect
the timing of the eruption phase through mediation of immigration. Once
subpopulation range has recovered, the rapid recovery of subpopulation
numbers suggest that subpopulations are not restricted by other factors.
The regularity and symmetry of both the increase and decline phases of
these cycles suggests that the barren-ground caribou cycle is both
stable and resilient. Continuation of barren-ground caribou cycles at
historical levels is likely if habitat conservation measures are adopted
so that annual migration patterns are not disrupted, summer and winter
range remain undisturbed and common-sense harvest management policies
are adopted when caribou are at low numbers.