The story earlier this week by KLAS News said the advocates have applied over 2,000 doses in the past two years and that the herd size “right now is pretty stable,” suggesting that they’ve achieved achieve zero population growth.
Do those numbers make sense? How many horses would they have to eliminate each year to bring the birth rate in line with death rate?
Before they got involved in 2019, approximately 3,000 wild horses roamed on 300,000 acres.
The stocking rate was ten wild horses per thousand acres, well above the target rate of one wild horse per thousand acres that the BLM says is sustainable.
Trailcam photos posted on these pages over the last few years show the horses were in good condition (go to the Index and scroll down to Virginia Range).
The bureaucrats, eager to erase this outlier, said the area should have no more than 600 wild horses and, ideally, just 300, in line with the rancher-friendly management plans of the HMAs.
The advocates, agreeing with the narrative and happy to advance the ranching agenda, offered to help. Better to get rid of them with PZP than helicopters or bait.
The growth rate of a herd depends on the birth rate and death rate:
Growth rate = Birth rate – Death rate
To keep the herd size constant, the advocates shoot some of the mares with PZP darts, so the birth rate is roughly equal to the death rate.
To reduce the herd size, the advocates dart as many mares as possible, driving the birth rate to zero.
This is what the Salt River Wild Horse Darting Group has done, and based on trailcam evidence this year, appears to be what the Virginia Range darting team is doing: Only one foal was photographed this year, not seen in the months hence.
Both groups receive support from the Campaign Against America’s Wild Horses.
If a herd grows at a rate of 15% per year and the death rate is 5% per year, the birth rate must be 20% per year. In a herd of 3,000, 600 foals would be born and 150 horses would die.
If a herd grows at a rate of 20% per year and the death rate is 5% per year, the birth rate must be 25% per year. In a herd of 3,000, 750 foals would be born and 150 horses would die.
To keep the herd size stable, the advocates would have to prevent 450 births at a 15% growth rate and 600 births at a 20% growth rate.
This is equivalent to a roundup every year.
Over a two year period, 900 doses would be needed if the herd was growing at a rate of 15% per year and 1,200 doses would be required if the herd was growing at a rate of 20% per year.
The treatment is not 100% effective so the actual number of doses will be higher, but not over 2,000 as stated in the story.
How do you explain the discrepancy? The advocates weren’t hired to maintain the herd size, they were charged with drastic reduction.
Western Horse Watchers believes the herd was on its way to filling its niche and the growth rate was closer to 15% per year, maybe less, and not 20% per year, the rate used by land managers to predict herd sizes.
So the advocates should have been able to achieve ZPG with around 1,000 doses.
Given that they’ve used at least twice that many, wild horse numbers are probably going down, but they won’t admit it.
Worse, the story said that a mare can no longer reproduce after five to seven years of treatments (five are sufficient) so their goal may actually be sterilization, but they’re not going to acknowledge that either.
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