Tuesday, February 3, 2015

Could Coyotes Be the Rancher’s Friends?

By Bob Ferris

Coyote in Yosemite By Yathin S Krishnappa
I am amazed at times by some of the comments I receive in response to my various posts and blogs on ecology and predator-prey dynamics.  In this I am baffled at how little many understand about basic ecological relationships and principles, yet how willing they are to opine.  It is hard for me to fathom why things that are so obvious and so core in everything that we observe and experience can be so entirely invisible to some.

A perfect example is the current conflict over bison, jackrabbits, coyotes and cattle in Utah.  Here ranchers thinking that bison were stealing “their” grasses on public lands were surprised to find that jackrabbits, rabbits and pikas were actually a much bigger problem than the 300-400 bison.  In addition, some were probably even more startled to learn that the rural obsession with killing of coyotes was likely a contributing factor to their woes.

So how could critters so small and insignificant possibly be eating more than something so large and how could the coyote so long considered the bane of ranchers actually benefit ranchers in any manner?  It turns out that some of the answers can be found in studies of energetics and life histories.


Let’s start with the energetics.  Many who have studied biology, ecology or physiology are familiar with the so-called “mouse to elephant” curves.  These graphs demonstrate that smaller animals have metabolisms that burn calories at rates that are generally much higher per unit of body weight than do larger animals.





So how does this work?  Above you will find the formulas for the surface area (A) of a sphere and its volume (V).  If we do the above calculations for familiar objects like golf balls (diameter 1.68”) and softballs (diameter 3.5”) and then divide the surface area coverings by the volume they enclose we find that the ratio for the golf ball is about 3.6 square inches of surface area for every cubic inch of volume, but the softball’s ratio is 1.7 or less than half of that.  If the golf ball and softball were animals and they both were endotherms or what we used to call “warm blooded” that generate their body heat via their metabolisms and lose heat  through their surfaces then the assumption would be that the golf balls would have to eat much more than the softballs to maintain the same body heat.  And that is exactly what we see in jackrabbits which consume somewhere between 20-24% of their body weight per day in grasses versus bison which tend to average about 1.6%.

Jackrabbit by Jim Harper
Now in terms of what services the coyotes might perform for ranchers.  The hares (jackrabbits), rabbits (cottontails) and pikas of this area—all lagomorphs—are essentially propagative engines constantly set on full-throttle.  They start breeding early and then do so often producing a half dozen or so young at a clip.  There is real truth behind the “breeding like a rabbit” characterization and at least two cautionary stories—one from,Australia starting in the mid-1800s  and another from the America during the Dustbowl era  —that tell us what can happen when rabbit or jackrabbit populations are allowed to free-wheel in the absence of controlling forces like predation.  So in essence, coyotes along with bobcats, eagles, owls and some other predators act as the critical brakes on these reproductive rockets as well as those in the rodent realm. (None of the above is to say that jackrabbits and other lagomorphs are bad or without ecological value.)

Now I am not expecting that the above findings or explanations will prove an epiphany to all those grumbling over the bison in Utah or elsewhere or looking to buy their next box of varmint loads, but I am hoping that as the real life examples stack up that one day awareness or at least acceptance might start to dawn.  I will also point out the stinging irony here that the ranchers are quick to criticize the bison and call out their purported impact, but so infrequently own up to the impact of their own cattle on wildlife—particularly deer and elk—using public lands.  My hope is that knowledge on all sides of this equation will lead to fair, fact-based dialogues and informed policy decisions for good of all not just a privileged few.

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