Abstract
Caribou (Rangifer tarandus granti) calf:cow ratios (CCR) computed from composition counts obtained on arctic calving grounds are biased estimators of net calf production (NCP, the product of parturition rate and early calf survival) for sexually-mature females. Sexually-immature 2-year-old females, which are indistinguishable from sexually-mature females without calves, are included in the denominator, thereby biasing the calculated ratio low. This underestimate increases with the proportion of 2-year-old females in the population. We estimated the magnitude of this error with deterministic simulations under three scenarios of calf and yearling annual survival (respectively: low, 60 and 70%; medium, 70 and 80%; high, 80 and 90%) for five levels of unbiased NCP: 20, 40, 60, 80, and 100%. We assumed a survival rate of 90% for both 2-year-old and mature females. For each NCP, we computed numbers of 2-year-old females surviving annually and increased the denominator of CCR accordingly. We then calculated a series of hypothetical “observed” CCRs, which stabilized during the last 6 years of the simulations, and documented the degree to which each 6-year mean CCR differed from the corresponding NCP. For the three calf and yearling survival scenarios, proportional underestimates of NCP by CCR ranged 0.046–0.156, 0.058–0.187, and 0.071–0.216, respectively. Unfortunately, because parturition and survival rates are typically variable (i.e., age distribution is unstable), the magnitude of the error is not predictable without substantial supporting information. We recommend maintaining a sufficient sample of known-age radiocollared females in each herd and implementing a regular relocation schedule during the calving period to obtain unbiased estimates of both parturition rate and NCP.
Highlights
Herd composition counts are commonly used in ungulate management (Bender, 2006), but the ratios obtained are the subject of ongoing debate (Caughley, 1974; McCullough, 1994; Bonenfant et al, 2005; Harris et al, 2008)
We totaled the accrued sexually-mature females with the surviving previous-year sexually-mature females; the sum was used as the basis for calculating numbers of calves present at the speci ed net calf production (NCP) and for projecting numbers of mature females surviving through the following winter
calf:cow ratios (CCRs) underestimates of NCP increased with the progressive addition of 2-year-old females to the simulated population, from 0.046, 0.058, and 0.071 at 20% NCP to 0.156, 0.187, and 0.216 at 100% NCP for low, medium, and high rates of survival, respectively (Table 1, Fig. 1)
Summary
Herd composition counts are commonly used in ungulate management (Bender, 2006), but the ratios obtained are the subject of ongoing debate (Caughley, 1974; McCullough, 1994; Bonenfant et al, 2005; Harris et al, 2008). Late winter composition counts may adequately index overwinter calf survival and herd growth where 1) biased sampling is absent, 2) precise calf:cow ratios are available, 3) adult survival is precisely estimated, 4) the variance in juve-. 2-year-old and older females are combined in a single class called “cows” or, inappropriately, “adult females” for the calculation of CCRs
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