Woodland caribou scientific review to identify critical habitat: chapter 18

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Appendix 6.6 (continued)

RESULTS

Critical Population Size Assessment

The results of the non-spatial PVA indicated that populations of boreal caribou with poor demographic conditions (e.g., low calf survival and moderate adult female survival) face a high risk of quasi-extinction regardless of population size (Figure 1; LHMM). Populations with medium calf survival (high CV) and medium adult female survival (mean CV) required a minimum of 300 individuals to reduce the risk of quasi-extinction to <10% (Figure 1; MHMM). Under the same mean survival rates but using high CVs for calf and adult female survival, a population size of 600 was required to offset the risk of quasi-extinction. Under conditions of low calf survival (high CV but high adult female survival and mean CV), however, a population of 50 animals had a quasi-extinction risk <10%, suggesting that high adult female survival compensated for low calf survival (Figure 1; LHHM). Under good demographic conditions (e.g., relatively high adult female and calf survival corresponding to 75th percentile of survival rates and CVs), a population size of 50 had a 10% chance of quasi extinction over 100 yrs (Figure 1; 75th Percentile).

Appendix 6.6 - Figure 3. Change in risk of extinction (%) per unit increase in rates of the following parameters: adult female survival (S ad) and female calf survival (S calf), and unit increase of coeffi cients of variation (CV) of adult female survival (S ad CV) and female calf survival (S calf CV), under various population sizes (Ni; 100, 200, 400 females).

Appendix 6.6 - Figure 1. The effect of population size on risk of quasi-extinction for models combining various rates of calf survival, CV of calf survival, adult female survival, and CV of adult female survival. Quasi-extinction is defi ned as the risk of the population decreasing below 10 females over 100 yrs. See Table 3 for a description of the models.

Population Trajectory Models

All populations, except the Corner Brook Lakes population in Newfoundland (Mahoney and Virgl 2003), went extinct within 100 yrs, although the time to extinction varied among studies (Figure 2). The three populations with the poorest demographic conditions (Red Wine Mountains late period, Birch Mountains, and Little Smokey) declined to the quasiextinction threshold of 10 females within 20 years, while the Red Wine Mountains early population declined at a slower rate. The risk of extinction P(e) and quasi-extinction P(qe) for all populations except the Corner Brook Lakes population was >10% (Table 7).

Appendix 6.6 - Table 7: Probabilities of extinction, P(e), and quasi-extinctions, P(qe), over 100 yrs for four boreal caribou study populations

Appendix 6.6 - Figure 2. Predicted changes in the mean total abundance of four populations of boreal caribou over 100 years, based on population size, mean adult female survival, calf  survival, and their variance (CVs) reported in a demographic study for each population. We used mean values from 15 boreal caribou population studies to estimate parturition, proportion of yearlings in population and calf sex ratio.

Sensitivity Analysis

Of the vital rates that we tested, adult female survival and calf survival had the largest effect on probability of extinction (Figure 3). The CV of S ad and the CV of Scalfhad minor effects on probability of extinction, depending on the size of the population that was modelled (Figure 3). Relative to survival rates, population size in the range that we modelled (100 – 400 initial adult and yearling females) had little effect on the risk of extinction (Figure 3).

The cumulative percent change in risk of extinction was much greater with increasing adult female survival than it was with increasing calf survival (Figure 4). A change from mean to low Sadincreased the probability of extinction by 72%, while a change from mean to low Scalf increased the probability of extinction by 42%. In contrast, a change from mean to low CV of Scalfor Saddid not change the probability of extinction more than 5%. Relative to survival rates, population size in the range that we modelled (100 – 400 initial adult and yearling females) had little effect on the risk of extinction (Figure 3).

Although the range in modelled adult female survival was smaller (0.70 – 0.94) than the range in modelled calf survival (0.17 – 0.67), the cumulative change in risk of extinction was much higher for adult female survival (78%; Figure 4) than for calf survival (52%), suggesting the importance of adult female survival in boreal caribou population dynamics.

Recruitment

The probability of extinction decreased with increasing recruitment rates (Figure 5). Under conditions of relatively high adult female survival (0.88), populations of 400 individuals required a recruitment rate of 20 calves/100 cows to reduce the risk of quasi-extinction to <10% (Figure 1). Under lower adult female survival (0.80 – 0.84), populations of 400 individuals required a recruitment rate of 30 – 43 calves/100 cows to reduce the risk of quasi-extinction to <10% (Figure 5).

Appendix 6.6 - Figure 3. Change in risk of extinction (%) per unit increase in rates of the following parameters: adult female survival (Sad) and female calf survival (Scalf), and unit increase of coeffi cients of variation (CV) of adult female survival (Sad CV) and female calf survival (Scalf CV), under various population sizes (Ni; 100, 200, 400 females).

Appendix 6.6 - Figure 4. Cumulative change in risk of extinction due to change in adult femalesurvival and calf survival for boreal caribou. Parameter represents adult female survival (S ad) or female calf survival (Scalf ).

Appendix 6.6 - Figure 5. Effect of boreal caribou recruitment (calves per 100 females) on the probability of quasi-extinction as determined from a non-spatial population viability analysis. Quasi-extinction risk was measured as the mean number of years (across 500 replicates) the population reaches 10 females over 100 years (based on an initial population size of 400 females).