HomeMy WebLinkAbout09-09-24 Public Comment - R. Crabtree - science behind indiscriminate killing (trapping, snaring, other)From:Robert Crabtree
To:Bozeman Public Comment
Subject:[EXTERNAL]science behind indiscriminate killing (trapping, snaring, other)
Date:Monday, September 9, 2024 9:53:56 AM
Attachments:Crabtree SOL coyote populations.pdf
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Hello Commissioners,
I have been dealing with these kinds of issues for 40 years (coyotes, foxes, skunks, etc.). Ihave received hundreds of calls from probably 100 communities across the US and Canada
that deal with the issues of killing furbearers. That attached SOL is mostly about the scienceand ecology behind indiscriminate killing of coyotes but much of what it contains also applies
to other species as well. When we look at impacts to the environment, you have to look at the"three E" yardsticks in order to justify the proposed impacts, in this case trapping, snaring, or
otherwise killing of wildlife. They are Ecology, Economics, and Ethics. Bottom line: you justcan't justify killing these species on public lands, period. Where's the science (the truth)
behind what they claim?
The ecology says it can't not be justified given the ecological impacts to the non-humancommunity as there are many ecosystem services and assets provided by these species.
The economics has to be balanced. What is income to the trapper? What is the cost to
Nature? What is the value of those species to the non-trapping community?
The ethics couldn't be more clear. And public opinion polls even in Montana indicate anincreased concern for the ethical treatment of animals. It's more than a majority.
Best regards,
Bob
406.570.9545 (cell)
Robert L Crabtree, PhD
Chief Scientist and c0-Founder
Yellowstone Ecological Research Center
www.yellowstoneresearch.org
406.556.1414
May 17, 2023
Dear Interested Person, Party or Organization,
Since the 1990s numerous biologists and NGOs have asked me what effect killing adult coyotes
has on the remaining population. In response, you will find (below) my third update to a
scientific opinion letter (SOL) I first wrote for Brooks Fahy of Predator Defense to address this
question. At the end, you will find a summary showing the 20 effects of coyote-killing in four
categories—demographic compensation, behavioral response, changes in culture/society, and
ecosystem impacts.
While responses vary, overall the science shows indiscriminate killing to be ineffective and
counterproductive. Claims are frequently made that only offending individuals are targeted, and
then killed or removed, but there is no verifiable evidence that this is possible, let alone effective.
What agencies call “control” is actually indiscriminate killing of adult coyotes, which leads to a
higher percentage of females breeding, higher pup production, rapid immigration or replacement
of breeding adults and a myriad of other responses detailed in this letter.
It cannot be over-emphasized how strongly coyote populations compensate for population
reductions after indiscriminate killing. In addition to the population responses, the results of
coyote-killing include multiple detrimental effects on individual coyotes, packs, species
populations, and the entire predator-prey ecosystem due to trophic-level interactions.
OVERVIEW
Because the question at hand is complex, I will present it in a simple scientific and defensible
“impact/response” framework where the human impact is the removal or killing of adult
coyotes—9 months old and capable of breeding—often reported as annual mortality. The
reduction in population size is an impact that elicits a population response over time, hence a
density-dependent response that is possibly compensatory—a rebound back towards the original
population size. Such a compensatory response can vary. It could be weak (the population stays
close to the level it was reduced to), strong, complete, or even hyper-compensatory where the
population rebounds to a higher level than it was before the impact. The timing of the
compensatory response can be immediate or slower over several months to several years.
The compensatory response to density reduction due to removal or mortality of adults can be
grouped into demographic , more at the population level (e.g., average litter size), and
behavioral , often at the individual level within a pack and can act independent of population
demography—for example, a breeding female may move her litter to a safer location after an
adult pack member is killed. Thus, this SOL deals primarily with the demographic response at
the population and pack (social group) level focusing on two demographic events that are
additions to the population: (1) the reproduction or better described as reproductive recruitment
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(the number of pups born that survive to the fall population when they are reproductively
mature) and (2) immigration at the pack level. There are many behavioral and individual
responses that are not included nor addressed in this update. Similarly, there are other alternative
factors that affect the demographic response by adults to human-caused mortality (also called
exploitation) that are mostly outside the scope of this SOL. For example, the influence of
extrinsic factors such as climate and habitat change on wild prey can directly affect intrinsic
compensatory responses.
BACKGROUND
The Goals of “Control”
There is a repeated claim that killing coyotes (often referred to as control and/or human
exploitation) decreases predation on domestic livestock and game animals such as mule deer or
antelope. But before this is addressed, it is important to understand the type of control practices
that attempt to reduce predation. Nearly all, if not all reductions, killing, or culling programs,
often referred to as control practices, are simply indiscriminate killing of adult coyotes. Yet it is
often stated that the offending individual (i.e., culprit) is targeted and then killed or removed.
In fact, there is a near complete lack of evidence from observations or studies that support this
claim and document removal of the offending individual (if any)—let alone the forensic
documentation of predation — and show removal of the offending individual to even be effective,
especially in social species like coyotes and wolves. Thus, regardless of the compensatory
response by coyotes, the inability to verify the removal of the offending individual undermines
their claim at the onset. This is why this SOL deals with this type of impact — indiscriminate
killing — and does not address the question of whether removal of the offending individual is
effective. This SOL also does not address the effectiveness of indiscriminate killing in terms of
ethical standards nor the impact it has on the ecosystem services provided by coyotes, an apex
predator in North America implicated in powerful trophic cascade interactions as first brought
forth in a famous paper that looked at the absence of the apex predator, the coyote (Crooks, K.R.
and Soule, M.E. 1999. Mesopredator Release and Avifaunal Extinctions in a Fragmented
System. Nature 400: 563-566).
Both governmental research (mostly funded and conducted by USDA Wildlife Services) and
government-funded research clearly indicate that offending individuals are most often breeding
adults, often called the “alpha pair,” that provision their pups. There is a strong incentive to kill
large packages of prey to feed fast-growing pups that can starve or succumb to other proximate
factors (e.g., disease) if they become malnourished. This nutrient incentive (and caloric stress)
starts when females are pregnant and are maximized during the summer after weaning and before
pups forage on their own in late summer. The early late-May through July period is also, not
surprisingly, the period when pup mortality from disease (proximate cause) and starvation
(ultimate cause) occurs. Recruitment of pups into the fall breeding population is primarily how
coyotes attain genetic fitness.
Yellowstone Ecological Research Center, 4135 Valley Commons Drive, Suite D, Bozeman MT 59718
Because reproduction is almost exclusively performed by the breeding pair (that is nutritionally
stressed), the primary goal of ‘control’ is to reduce the number of breeding adults and
consequently reduce predation on livestock and wild ungulates. In highly social, territorial
species like coyotes and wolves, the effective population size is measured by the number of
breeding pairs or the number of territories. Thus, to determine the effectiveness of control
(human killing) practices, one must know the number of territorial breeding pairs on the
landscape both before and after the impact of control which attempts to target one or both
breeders within a pair.
There is nothing more conserved, and sought after, by adult coyotes to increase their genetic
fitness than to become a territorial breeding adult. Subsequently, this is a fundamental reason
that indiscriminate killing is ineffective. With annual mortality rates at, or exceeding, 70%
(which is rarely attained and at great costs) the number of breeding pairs remains stable (with the
pack size decreased to 2 or 3 individuals). At even higher rates of mortality, it appears that one
male can breed with several breeding females as was suggested by (Berg and Chesness 1978),
and the number of litter produced is still constant. When food is abundant, a second (beta)
female can sometimes successfully breed (produce a litter) within a pack. This was estimated to
occur 14% of the time in unexploited populations with pack sizes ranging from 3.4 to 6.2
individuals. The mechanism driving the occurrence of a “double litter” is unknown but is a
highly probable response to a surplus of food either within a territory or across a landscape if
there are territorial vacancies.
Common Sense and Evolution
It cannot be over-emphasized how strongly coyote populations compensate for population
reductions. And I will attempt to explain why that is within a general evolutionary perspective
before examining more immediate demographic responses. First off, compensatory or
density-dependent responses to exploitation (human-caused mortality) are common in mammals
and present in all territorial populations at or near carrying capacity—more clearly described as
habitat saturation for non-overlapping territorial species (see Fowler, 1981. Density Dependence
as Related to Life History Strategy, Ecology Vol 62: 602-610). Thus, it should come as no
surprise that coyote populations have strong density-dependent compensatory responses at or
near habitat saturation (few, if any, occupiable territorial vacancies) especially given a locally
available surplus of behaviorally suppressed females capable of breeding. In fact, a large
majority of adult females are pregnant every year, but usually only dominant females (alphas)
successfully reproduce (producing one or more surviving pups).
Second, both evolutionary biology and the results of research (e.g., a 20-year study in
Yellowstone National Park before and after gray wolf reintroduction) suggests that the basis of
their demographic and behavioral resiliency is embedded in their long evolutionary history.
That is, coyotes evolved, and learned to coexist, in the presence of larger competitors capable of
killing them, namely, and more recently, gray wolves—a dominant competitor and natural enemy
that overlapped the historic range of coyotes in North America for tens of thousands of years.
Prior to widespread human persecution of predators starting in the mid-nineteenth century,
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wolves have provided a constant selection factor inflicting mortality, competition, and numerous
other sub-lethal effects as revealed by numerous wolf-coyote investigations. Collectively,
intense selective pressures by larger competitive species (and now humans) result in a species
that has existed in a relatively constant state of colonization with many specialized adaptations.
These demographic and behavioral adaptations are numerous and diverse and allow coyote
populations to easily overcome the relatively less effective human mortality and control practices
compared to the impact of wolves.
Again, the reduction of depredation on domestic livestock (and predation on game animals) due
to indiscriminate killing may or may not occur. And while it is incumbent on the perpetrator to
justify their environmental impacts (e.g., NEPA), few, if any valid studies, demonstrate the
effectiveness of human control practices (see Treves et al. 2016. Predator Control Should Not be
a Shot in the Dark. Frontiers in Ecology and the Environment 14: 380-388). Human control
(killing) is short-term and intermittent, compared to the sustained presence of wolves, from every
day to many thousands of years (24/7/365/1000).
Mechanics of demographic compensation
Thus, there are two main demographic responses in coyote populations to the impact of killing:
(1) an increase in the number of pups surviving to sexual maturity ( reproductive recruitment ),
and (2) increased immigration that can be immediate or delayed. Reproductive recruitment starts
when ova are shed and implanted in territorial breeding females and ends in the fall when pups
become reproductively mature adults when about 9 months old. Replacement of breeders is
known to readily occur by two mechanisms: (1) immigration of an individual from outside the
pack, or (2) internal transition where a non-breeding adult within the pack becomes a breeder
(referred to as ascension of a beta to an alpha). Also, it is clear and obvious that killing adults
does in fact reduce population density and is required in order to drive the compensatory
responses described below. This reduction is often observed as an immediate reduction in pack
size. However, it can be difficult to measure if the replacement is immediate from outside the
pack.
Overall, an important question is not so much if these mechanisms occur but how strong is the
compensatory response (magnitude not mechanism) and does it occur in a way that the annual
window of opportunity to produce a litter is missed or not. Thus, the removal of offending
individuals may temporarily alleviate predation especially if an annual reproduction (a litter) is
lost. If immigration replaces the number of breeding adults so that an annual litter is not lost
then the effectiveness of control is probably short-term if at all because of compensatory
responses to density reduction. The immigration response time may be immediate compared to
the once-per-year chance to reproductively compensate for density-reducing events with an
increase in the number of pups produced. Thus, there are measurable short-term and long-term
demographic responses that result in the ineffectiveness of control and management practices,
and the response to control can result in an increase in predation rates on domestic livestock and
game populations. Whether the breeding adults are removed or not, it is likely that at least some
compensating responses described below will take place anyway. Again, it’s a matter of
magnitude not the mere presence of a mechanism.
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Scientific opinion on whether compensatory mechanisms negates the effectiveness of control:
The following demographic compensatory responses I describe are based on published research
(reviewed up to 1999 in Crabtree and Sheldon, 1999), results of analysis of coyote study
populations subjected to various levels of reduction or exploitation, the work I have conducted
with coyote populations in three study areas over the past 38 years in Washington (4 years,
unexploited population), California (2 years, exploited), and Wyoming (20 year field study of a
pre-wolf unexploited population then exploited by wolves after reintroduction in 1995-96). I
also attribute my knowledge to my experience as a fur trapper in my college years and a special
dog named Chima who was able to communicate to me what her fine-tuned nose was ‘seeing’.
There is little, if any, scientific basis to justify control (reduction by killing) programs that
indiscriminately target adult coyotes, let alone breeding adults. As with any federal action, the
burden of proof should be upon them to demonstrate both the biological and economical
effectiveness and justification of their proposed activities (see NEPA). Even the federal agency
called Wildlife Services often points out the lack of academic research demonstrating
effectiveness. And their research arm has indicated their ineffectiveness. In fact, the
mechanisms described below suggest that control (whether selective or widespread) increases
the immigration, reproduction, and survival of remaining adult coyotes. In several papers, it has
been reported that a sustained reduction of coyote numbers can only be accomplished if over
70% or more of the adults are removed (exploited) on a sustained basis. Review of field research
and modeling (including my own) indicates that even with intensive control efforts, this level is
rarely, if ever, achieved. And if it is achieved, it is likely to be much more expensive than
simply providing monetary compensation for losses. A thorough review and synthesis of
coyote ecology and demography can be found in a book chapter (see Crabtree and Sheldon
1999). Actual reduction in the population density (and the number of coyotes) does occur and is
primarily a function of smaller pack size for a year or more. Betas, yearlings, and 6- to 9-month
old ‘pups’ killed often constitute the majority of coyotes killed. Reproducing adults or alphas
that are older and experienced are killed in a lower proportion than their occurrence in the
population and compose less than half of an unexploited or lightly exploited population.
The primary objective of an adult coyote seeking to increase its genetic fitness is to find a
territorial opening, pair-bond, defend it, exploit the food resources within, and successfully
reproduce thereby increasing their genetic fitness. The competition for a territorial vacancy—a
breeding adult position—is thus, intense. Overall, this is why control practices rarely reduce the
effective population size and subsequently fail to decrease predation on domestic livestock and
wild game. Thus, the primary mechanism or lack of a mechanism for compensatory response is
as follows:
(1) Immigration. This is the most effective and immediate means by which the effective
population size rebounds or compensates for indiscriminate killing. There is little evidence that
non-territorial loners and beta pack members are replaced. Rather, if breeding adults are killed,
they are either replaced immediately or within the next 9 months, especially during the winter
when courtship and territories are set up or reinforced with active defense, scent-marking and
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vocalizations. Approximately 90% of breeding adults are replaced within the following year.
Immediate immigration often occurs within the pack—called internal transition—if a breeding
adult is killed in a pack of 3 or more individuals. A surviving beta pack member ascends to the
alpha or breeding position. Immigration or replacement of breeding adults is also achieved by
non-territorial individuals from outside the pack, especially when both alphas are killed and the
pack disintegrates. This is the expected response for a territorial species with surplus
(non-breeding) adults and dates back to classic studies of territorial birds following removal of
breeding adults involved in courtship and nesting.
(2) Reproduction and Reproductive Recruitment. This is a complex series of interactions and
I have broken “reproduction” into life history segments as well as a section on the mechanistic
basis for why more “pups are produced” when humans indiscriminately kill adult coyotes.
Litter size at birth. Coyotes are monestrous which means the breeding, mated pair attempt to
produce a litter once per year. Reported litter size at birth is highly variable and may be due in
part to the technique used to determine it. Litter size is usually always reported as either the
number of placental scars or the observed counts at den sites (when pups are a month old or
older). Both methods may be biased because they are either well before or after parturition.
Regardless, litter size at birth does not appear to be affected by density-reducing control (killing
adults) as previously reported by Crabtree and Sheldon (1999) which is contrary to
USDA-funded research. Knowlton (1972) reported an inverse relationship between an unreliable
abundance index (number of coyotes per trap line effort) and litter size varying between 4.3 to
6.9 based on placental scars. However, litter sizes reported from den counts varied only from 5.0
to 5.7 pups.
There was no statistical analysis conducted, however, using scars, he inferred litter sizes on 7
counties based on a qualitative assessment of light, moderate, and intensively controlled levels.
This study appears to be the basis of the commonly held notion (called a factoid) that litter size at
birth increases when populations are exploited. Recently, I compared counts of
litter-size-at-birth to concurrent estimates of annual mortality of adults in 14 studies. It resulted
in a slope of ‘zero’ further indicating the fallacy of this assumed mechanistic response. Although
high levels of protein intake around the time of ovulation can cause an increase in the number of
ova shed and implantation sites, there is no empirical evidence that litter-size-at-birth increases
with exploitation (human-caused coyote mortality). Rather, an increase in litter survival (a
decrease in the normal high mortality rates of coyote pups during the summer) is the primary
compensatory response related to reproduction.
Summer Litter Survival is the key to understanding compensatory reproduction. Human
exploitation (killing) reduces the population primarily through a reduction in pack size which
results in more pups surviving during the normal high pup mortality (starvation) during summer.
In unexploited populations, it is typical for about 2 pups—out of an average litter size of 6—to
survive to the fall. A high survival rate of pups from birth to fall when recruitment occurs, can be
measured by the number of young-of-the-year in the fall/winter population sample. A high
proportion of young-of-the-year is commonly observed among exploited populations where pack
size is low (a breeding pair or breeding pair with one adult pup from a previous litter called a
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helper). In our research first-time breeding pairs produce litters that survive at the highest rates
observed. Presumably, first-time breeding pairs have higher fat reserves than packs or pairs that
have experienced the costs of reproduction in the previous year.
A simple mechanistic basis for compensatory reproductive fall recruitment is as follows.
Density-reduction from human exploitation allows the pups that normally die due to starvation
during the summer months in populations, to survive. I believe that the best predictor of
compensatory response to the impact of mortality on coyote populations is: the ratio of the
number of territorial adults to the availability of wild prey biomass. Overall, increases in
wild prey populations in the population region, in particular within the confines of the defended
territory, can cause an increase in this key factor to understand the mechanism underlying the
coyote’s often powerful compensatory response. It is further explained below.
This exploitation causing higher pup survival is fundamentally a function of the general
mammalian reproductive strategy that delays the majority of reproductive energetic investment
beyond the gestation period, the post-partum and neonate state (e.g., young pups). The caloric
demand of offspring reaches an apex in May, June, and July when coyote pups grow very fast.
Thus, the normal litter of six pups has a good chance of (a) surviving the typically high summer
mortality period and, (b) being recruited into the pack the following winter as adults thereby
returning the previously exploited population to normal densities. By contrast, in the two
unexploited populations I investigated, the average litter size at birth was 5 or 6, but due to high
summer mortality, only an average of 1.5 to 2.5 pups survive. In populations subjected to
moderate to high levels of exploitation (25% to 70% removal annually), there appears to be an
ample number of breeding pairs to occupy all available territory openings with high pup survival
rates (more than half of those born survive to adulthood). Compared to an unexploited
population, these levels of exploitation can result in a doubling or tripling of the number of
hungry pups that need to be fed.
It is strongly suggested that the remaining breeding adults (typical pack size of 2 or 3) now have
access to increased food availability within their defended territories and can better provision
fast-growing that might otherwise starve. Summer starvation is widely observed in populations
with low adult mortality resulting in 2 or 3 pups surviving to the fall when recruited into the
breeding population. Thus, surplus food likely improves the nutritional condition of breeding
adults, which translates into higher pup birth weights and higher summer pup survival.
Remember, alpha male coyotes and associate or beta adults, often female, also help
feed pups from May to July. It is no surprise that first time breeders in coyotes and wolves
typically have high survival rates in their first litter
Human control resulting in a reduction in the density of the population results in a smaller social
group size (i.e., pack) which increases the food per coyote ratio within the defended territory.
The food or prey surplus is then biologically available to potentially increase the production of
more pups either through an increase in litter size at birth or a decrease in the normal high
the mortality rate of pups surviving to the fall.
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Topics from previous versions of the SOL
(a) Prey targeting.
Reductions in coyotes capable of breeding (at 10 months of age) result in
smaller pack sizes which leaves fewer adults to feed pups. Although remaining adults may well
have access to more prey within their defended territories, this may further add incentive for the
remaining adults to kill larger prey such as domestic sheep as well as putting pressure on the
adults to select for the more vulnerable prey and venture close to areas of human activity that
they otherwise avoid. Because predators like coyotes also learn what is appropriate food when
they are pups and are reluctant to try ‘new’ food sources unless under stress (such as having to
feed a large litter of pups), reduction programs, in effect, may be forcing coyotes to try new
behaviors (eating domestic livestock) which they would otherwise avoid. Research has clearly
shown that higher numbers of adult pack members provide more den-guarding time and more
food brought to pups. Without the pressure to "maximize" efficiency in hunting for food for
pups, adults provisioning pups may be able to subsist on larger numbers of smaller prey (e.g.,
rabbits and small rodents) rather than going for livestock or other, larger prey like antelope and
mule deer fawns. Although coyotes are exposed to a significant risk of injury when hunting and
killing larger prey, larger litter sizes might ‘tip the balance’ in favor of selecting larger prey and
livestock.
Large packages of prey, (such as sheep, as opposed to the more natural and common prey species
of voles, mice, or rabbits) make for more efficient sources of nutrition because hunting adults
have to invest less energy per unit of food obtained. Research funded by Wildlife Services
clearly indicates that the primary motivation to kill domestic sheep is to provide food for
fast-growing pups.
(b) Females breeding .
Non-selective, indiscriminate killing of adults) cause an increase in the percentage of females
breeding. Coyote populations are distinctly structured in non-overlapping but contiguous
territorial packs. About 95% of the time, only one female (the dominant or alpha) in a pack
breeds. Other females, physiologically capable of breeding, are "behaviorally sterile".
Exploitation rates of 70% or higher are needed to decrease the number of females breeding in a
given area. Either a subordinate female pack member, or an outside, lone female can be quickly
recruited to become an alpha or breeding female. My research has shown that light to moderate
levels of reduction can cause a slight increase in the number of territories, and hence the number
of females breeding.
(c) War zone.
Reduction or removal of coyotes causes the coyote population structure to be maintained in a
colonizing state. For example, the average age of a breeding adult in an unexploited population is
4 years old. By age 6, reproduction begins to decline whereby older, alpha pairs maintain
territories but fail to reproduce. This may eliminate the need to kill sheep or fawns in the early
summer in order to feed pups. Exploiting or consistently reducing coyote populations keeps the
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age structure skewed to the younger more productive adults (average age of an alpha is 1 or 2
years). Therefore, the natural limitations seen in older-aged, unexploited populations are absent
and the territorial, younger populations produce more pups.
(d) Delay Dispersal.
Reductions in adult density of coyotes also cause young adults (otherwise prone to dispersing) to
stay and secure breeding positions in the exploited area where there are vacant territories to
occupy. This phenomenon is well-documented by research conducted by Wildlife Services and
other researchers. Research also indicates that this is the age class most frequently involved in
conflicts.
(e) Alternate prey.
An aspect of coyote predation on livestock that is often overlooked is the availability, or dearth
of alternate prey. Wildlife Services’ research has demonstrated that coyotes will avoid novel
prey, such as domestic livestock. In addition, it is risky for coyotes to predate upon domestic
livestock because of human control actions associated with this behavior. Related research
indicates that predators switch to alternative prey when a preferred prey item is absent or in low
numbers. Voles and other rodents like jackrabbits are a preferred major staple of coyotes in the
West. These prey species require cover and ample supplies of forage (grass and forbs). On many
western rangelands grasses, forbs, and protective cover have been greatly reduced by domestic
livestock grazing, leaving predators with fewer preferred prey to utilize. Present or historic
grazing impacts should be assessed as a likely means of predicting overall predation rates on
other prey species, especially prey like domestic sheep, which are already vulnerable to predators
due to their lack of anti-predator behaviors.
(f) Accelerated selection pressures and learned behaviors.
A relatively unexplored, but promising avenue of research is the long-term genetic and
behavioral changes in coyote populations subjected to decades of exploitation. It seems obvious
that the type of selection pressures and selection rates have been greatly changed for coyote
populations, after a century of exploitation at 20% to 70% per year. More nocturnal, more wary,
more productive, more resilient individuals have probably been intensively selected for. This in
turn may cause coyote populations to resist control practices that previously were effective. In
addition, the possibility of social facilitation and learning may be altered or reduced. Coyotes,
like many mammals, learn to habitually use certain prey or habitats from other individuals in the
population, especially from older adults in their social group (if they have one). Coyotes, already
a highly social and adaptable species, are held in a younger colonizing state when they are
exploited and learned or traditional behaviors may be lost. Individuals are therefore more
susceptible to learning novel prey sources or trying out novel habitat types and are frequently
associated with conflicts such as livestock predation.
(g) Assessment of Alternatives.
There are many questions to be answered such as, "How will coyote populations respond once
predator reduction or control programs are terminated?" or "Are there other management
alternatives, both lethal and non-lethal, that may be effective in reducing predation on domestic
Yellowstone Ecological Research Center, 4135 Valley Commons Drive, Suite D, Bozeman MT 59718
livestock”? "How do economics figure into management options"? This letter and scientific
opinion only addresses the narrow, but important topic of the impacts of human-caused reduction
or ‘control’ on coyote demographic parameters. We see little, if any, evidence to justify control
practices on an ecological basis. This letter also addresses a long-held belief that human control
of coyote populations are ‘necessary’, similar to ‘mowing a lawn’ to keep it from growing out of
control. This belief has no scientific basis whatsoever. Even research conducted by Wildlife
Services reports a variety of factors that keeps the lawn from growing. Their research repeatedly
concludes that the primary means of population limitation is territoriality itself, which imposes
an upper limit on density (or lawn height). Paradoxically the prevalent use of lethal control by
Wildlife Services opens up a ‘Pandora's box’ of behavioral and demographic responses that
negate any long-term effectiveness of control. The predominant responses of coyote populations
to lethal control efforts are to: (1) increase the number of pups produced (recruitment), (2)
increase immigration into the conflict area, and (3) increase behaviors that further exacerbate the
conflict. Collectively, this results in higher predation rates on domestic livestock and wild
ungulates.
(h) The three E yardsticks.
Coyotes are still products of their evolutionary past. Biological, economical, and ecological
evaluation of control practices should be a requirement undertaken before any public or private
effort to reduce losses due to coyotes or any other predator. In conclusion, it is my opinion based
on decades of field research that the common practice of reducing adult coyote populations on
western rangelands are most likely ineffective and likely causes an increase in the number of
lambs, fawns, and calves killed by coyotes.
A Summary of the Effects of Exploitation on
Predator Populations
The 20 responses listed below are divided into four general categories: (1) demographic
compensation, (2) behavioral response, (3) changes in culture/society, and (4) ecosystem
impacts. How many of these occur—and their individual magnitudes—will vary by species, the
severity and type of control action taken, habitat, season, prey availability, and presence of
competing carnivores in the target area. Interactions between the 20 responses listed below can
be unpredictable; however, scientific findings and biological common sense both indicate that
they ‘amplify’ in a manner that renders indiscriminate killing ineffective and results in a
multitude of detrimental effects on individuals, species populations, and the entire predator-prey
ecosystem.
Demographic Compensation: (This is a particularly strong response for coyote populations
because the primary reason they kill ungulate neonates, both domestic and wild, is to feed
fast-growing pups.)
● Breeding adults produce more pups when there is direct reduction in territorial pack size.
There is a weak to negligible effect on litter size at birth; however, the compensatory
Yellowstone Ecological Research Center, 4135 Valley Commons Drive, Suite D, Bozeman MT 59718
response of litter survival is remarkable. For example, prior to wolf restoration, adult
coyote mortality averaged only 9%, pack size was 6, and litter survival was 28%. After
wolf restoration, adult coyote mortality increased to 30% to 50%, pack size fell to 3, and
coyote pup survival abruptly rose to 78%—a nearly three-fold increase. Analysis from
20+ field studies indicated a similar response to human exploitation.
● Immigration of breeding adults into the exploited area to fill vacant territories and find
available mates. This response can be immediate. I have documented successful coyote
litters in territories where the pregnant female was killed one month earlier (ascension by
a pregnant beta female—Wildlife Service’s own research documents this
phenomenon—nearly all non-alpha females are pregnant on an annual basis).
● A higher percentage of females breed and produce pups. Two litters per territory can also
occur with abundant/available prey.
● The average age of reproductive females is lowered, eliminating older, less productive
alpha females. First-time breeders (young alphas) have higher pup survival than older
breeding pairs.
● Increased natal philopatry—yearlings and young betas tend to forego dispersal and
continue to reside in the exploited area.
● Regardless of the level of exploitation, the number of breeding pairs in a target area is
consistent from year to year unless 70% or more of the coyote population is removed
annually. This level of control is extremely difficult and costly to achieve let alone
document.
Behavioral Responses:
● Lower pack size results in selection of larger prey items (e.g., ungulate neonates) over
more numerous small prey items (e.g., rodents). This is particularly detrimental to
livestock when alternate prey abundance is low which is often due to overgrazing
practices.
● Adjust vocal communications—less vocal around humans.
● Activity cycles—more nocturnal and less diurnal.
● Denning behavior (guarding and location)—less susceptible to enemies.
● Avoidance of novel stimuli including control techniques. Perceived avoidance of
sustained control activities.
Changes in the Culture/Society:
● Increases in information sharing within and between new territorial pack members; this
leads to increased exposure to novel prey (livestock).
● Because there is a strong shift to fewer subordinates—betas are immediately recruited to
alpha breeding status—livestock-killing alpha adults are predominant in the population
structure.
● Killing the alpha male results in immediate replacement or the remaining pack breaks
apart and disperses to form breeding pairs elsewhere.
● Indiscriminate control methods have accelerated and amplified selection pressures to
perpetuate a ‘dispersal genotype’ adapted to rapidly colonize and successfully reproduce.
Remember that during the predator eradication era (approximately 1860’s to 1960’s),
Yellowstone Ecological Research Center, 4135 Valley Commons Drive, Suite D, Bozeman MT 59718
large carnivore populations declined substantially (with regional extirpation) while
coyotes tripled their abundance and distribution across North America.
● Their cultural evolution likely interacts with their biological evolution to further
accelerate and amplify selection pressures.
Ecological Impacts:
● Mesopredator release: Decrease in apex predator populations reduces the competition
and/or intraspecific killing rates with other predators or mesopredators (e.g., foxes,
raccoons, skunks, feral cats, etc.). This causes an increase in their abundance (i.e.,
release), which in turn, can have detrimental effects on other species (e.g.,
ground-nesters, songbirds, amphibians, and rodents) and other unintended ‘ripple’ effects
or trophic cascades.
● Loss of ecosystem services: alleviation of control pressures on prey populations (e.g.,
rodents, large herbivores) can lead to vegetation changes.
● Loss of ecosystem services: Disruption and increase of disease spread.
● Loss of subsidies to scavengers (e.g., wolves, coyotes provide food for many other species).
Robert L. Crabtree, PhD
Founder & Chief Scientist
Yellowstone Ecological Research Center
crabtree@yellowstoneresearch.org
(406)570-9545
Yellowstone Ecological Research Center, 4135 Valley Commons Drive, Suite D, Bozeman MT 59718