ABSTRACT
Hybridization poses serious threats to endangered
species conservation. Current regulations provide
little to no framework to address the issue, leaving
many species in jeopardy. The issue is analyzed from
both scientific and legislative perspectives and the
complications associated with addressing
hybridization in policy are identified.
Elizabeth J. Armentrout
joy10@mail.usf.edu
ADDRESSING VERTEBRATE
HYBRIDIZATION IN CONSERVATION
Thesis Director: Dr. M. Hafen
Thesis Committee: Dr. C. Mizak
POLICY
University of South Florida – Honors College
Photo credit: U.S. Fish & Wildlife Service
Armentrout |1
Contents
Introduction.................................................................................................................................................. 2
Definitions ................................................................................................................................................ 2
Science ........................................................................................................................................................ 3
Ecological Impact ................................................................................................................................... 4
Policy ............................................................................................................................................................ 5
Federal Endangered Species Act ........................................................................................................ 5
Hybridization in Policy ........................................................................................................................... 7
Conclusions & Recommendations........................................................................................................... 8
Index ........................................................................................................................................................... 11
Case Study 1: Florida Panthers and Texas Cougars ..................................................................... 11
Image 1: Puma concolor coryi and Puma concolor cougar ....................................................... 11
Case Study 2: Red Wolf and Coyote ................................................................................................ 12
Image 2: Canis rufus and Canis latrans........................................................................................ 12
Case Study 3: Spotted Owl and Barred Owl .................................................................................... 13
Image 3: Strix occidentalis caurina and Strix varia ..................................................................... 13
References ................................................................................................................................................ 14
Armentrout |2
Introduction
Hybridization rates in vertebrates are increasing globally at unprecedented rates due, in
part, to anthropogenic causes. This threatens the continued survival of many endangered
species both directly and indirectly (Allendorf, et al. 2001). However, in populations with little
genetic variation, hybridization may be the best course of action to increase genetic variation
and prevent the extinction of the species or subspecies. The U.S. Endangered Species Act
makes little, if any, attempt to address the issue of hybridization. In this examination, the
biological and ecological implications will be explored, complicated political implications of
hybridization will be considered, and federal policy concerning endangered species will be
analyzed.
Definitions
In the fluid and ever-expanding world of biological definitions, it is crucial to determine
the definitions of the terms discussed herein. The word “hybrid” refers to the progeny of a cross
between parents of different genotype. The word “species” is understood to mean a group of
interbreeding individuals not normally able to interbreed with other such groups. “Speciation”
refers to the evolution of a new species (Lawrence 2005). The term “introgression” refers to the
gradual diffusion of genes from the gene pool of one species into another species when a viable
hybrid backcrosses genetically within a parent population. The phrase “intraspecific
hybridization” refers to the interbreeding of individuals within the same species, but separate
subspecies and breeds. “Interspecific hybridization” refers to the interbreeding of individuals
from different species (Lawrence 2005). The word “viable” may be understood to mean the
capability of the offspring to reproduce. “Pre-zygotic isolation” refers to any number of
mechanisms or factors that contribute to a failure of reproductive individuals to create viable
zygotes; examples include ecological, temporal, behavioral, mechanical, or gametic isolation.
Armentrout |3
“Post-zygotic isolation” refers to any number of mechanisms or factors that contribute to a
failure of hybrid individuals to achieve reproductive success, including hybrid inviability, sterility,
and hybrid breakdown (Krempels 2010). “Inbreeding depression” is the exposure of the
individuals in a population to the effects of deleterious recessive genes through matings
between close relatives. “Outbreeding depression” occurs when offspring from crosses between
individuals from different populations have lower fitness than progeny from crosses between
individuals from the same population (Grant 1997).
Science
Hybridization is a natural process by which evolution of species may occur. It is the
result of breeding crosses between members of separate species or sub-species. In some
cases, this process leads to speciation, at which point the hybrids may be referred to as a
separate taxa distinct from the two parent species. This may occur in as little as 50 generations
(Wolf, et al. 2001). There are many explanations for why hybridization occurs in nature,
including sexual selection, genetic isolation, and population dynamics. In some closely related
taxa there are recognized "hybrid zones" where hybridization is more likely to occur (Haller &
Leonard 2008). Although there are several different types of hybridization, only two types are
discussed herein: intraspecific and interspecific hybridization. Intraspecific hybridization,
reproduction resulting from two individuals within the same species, occurs much more
frequently than interspecific hybridization for various reasons, including the decreased presence
of pre-zygotic and post-zygotic reproductive barriers (Krempels 2010). In interspecific
hybridization, two species may be able to produce viable offspring if the taxa are closely related
genetically. The less closely related the species, the more likely breeding crosses are to
encounter both pre- and post-zygotic reproductive barriers (Krempels 2010).
Armentrout |4
Ecological Impact
Ecologically, hybridization is a natural mechanism of evolutionary biology where
successful hybrids are produced. While most hybrids are disadvantaged as a result of genetic
incompatibility, the fittest survive, regardless of species boundaries (Hochkirch 2013). They may
have a beneficial combination of traits allowing them to exploit new habitats or to succeed in a
marginal habitat where the two parent species are disadvantaged. This increases genetic
diversity in a population and biodiversity in an ecosystem (DeSalle & Amato 2004). The issue of
hybridization is typically addressed on a case by case basis by conservation biologists. For
scientific purposes, intraspecific hybridization is generally accepted as an appropriate method of
endangered species conservation in cases where the remaining population is deemed too small
(Edmands 2007). Bringing in a member of a separate subspecies to interbreed can increase
genetic diversity within the population generally without reducing fitness or threatening genetic
integrity (see Case Study 1). However, this method of conservation is still widely debated and
generally used in situations where alternative conservation methods are either unsuccessful or
not applicable (Edmands 2007).
However, in other circumstances, hybridization poses a serious threat to conservation
efforts, as seen with red wolves (see Case Study 2) (Stockwell, et al. 2001; Haller & Leonard
2008). Hybridization is a natural process, but issues arise when anthropogenic influences
increase the occurrence of and amplify risks associated with hybridization. The scope and scale
of species hybridization has increased exponentially as a result of invasive and exotic species
introductions, habitat fragmentation, global climate change, and etcetera (Allendorf et al. 2001).
This has resulted in the reduced ability of certain species to rebound from the effects of
hybridization within a population, such as introgression. Successful hybrids may return to a
parent population and breed with individuals of the species, introducing new genetic information
and traits into the population (Haller & Leonard 2008). This leads some scientists to speculate
Armentrout |5
that life is a genetic continuum rather than a series of self-contained species (Hochkirch 2013;
Reich, et al. 1999). In most cases, the parental species will re-establish themselves when the
environmental change is reversed, and hybrids will remain in the minority. If the change in
environmental conditions is permanent or is radical enough that the parental species cannot
survive, the hybrids become the dominant form (Stockwell, et al. 2003). This is referred to as
“hybrid swarm” (Krempels 2010). The largest concern, of course, is extinction, which may occur
in as few as five generations (Wolf, et al. 2001). It is for this reason that some scientists propose
the expansion of conservation genetics to allow for more precise conservation decisions to be
made (DeSalle & Amato 2004).
Policy
Currently, the Endangered Species Act is the foremost regulatory framework for
protecting and managing endangered species in the United States. 45 of the 50 U.S. states
have their own endangered species protection legislation, but five states currently rely solely on
the federal act to administer protection of endangered species (Bauer 2010). However, despite
efforts and proposals in 1996, the act does not currently address species hybridization. This
poses serious risk and causes controversy for endangered species that may hybridize with
other species or subspecies, either naturally or artificially.
Federal Endangered Species Act
The Endangered Species Act (ESA) was originally passed by the 93rd U.S. Congress
and signed into law by President Richard Nixon on December 28, 1973. The purpose of the
legislation is “to provide a program for the conservation of such endangered species and
threatened species” and “the ecosystems upon which endangered species and threatened
species depend,” (United States 2006). Since its inception, it has been amended twice, in 1978
and in 1982. In 1978, the act was amended to require critical habitat designation for all
Armentrout |6
endangered or threatened species. The 1982 amendment allowed for species to be taken for
reasons other than scientific or research purposes and gave higher-priority actions more
precedence on the agenda (Allendorf, et al. 2001).
The Endangered Species Act falls under the jurisdiction of two federal agencies – the
United States Fish and Wildlife Service (USFWS) and the National Oceanic and Atmospheric
Administration (NOAA), which includes the National Marine Fisheries Service (NMFS). The
USFWS controls all freshwater fish and land species while NOAA controls the marine species.
Any species that occur in both habitats, such as sea turtles, are jointly managed by USFWS and
NOAA (United States 2006). These agencies are responsible for listing species on the
Endangered Species List, delisting species when health bounds are reached, and developing
regulation relevant to each species or habitat. Critical habitats must be identified and protected
and any exemptions must be listed. Any changes to the list must be announced to the public
and may be subject to judicial review (United States 2006). There are five criteria for listing a
species:

“The present or threatened destruction, modification, or curtailment of its habitat
or range

Overutilization for commercial , recreational, scientific, or educational purposes

Disease or predation

The inadequacy of existing regulatory mechanisms

Other natural or manmade factors affecting its continued existence,” (United
States 2006).
The ESA has been subjected to considerable criticism from interested parties in various
fields for curtailing business practices and recreational activity, but it has also be criticized by
many environmentalists for not providing adequate measures. As of yet, no scientific studies
have been completed in order to measure the negative impact of the policy, but the general
Armentrout |7
consensus among scientists and policymakers is that the positive effects exceed the negative
impacts of the policy (Suckling 2011).
Hybridization in Policy
Currently, the only U.S. policy that addresses hybridization is the Migratory Bird Treaty
Act of 1918, which applies only to North American birds under the act’s protection and any
hybrids of two protected species (see Case Study 3) (Haig, et al. 2004). The Endangered
Species Act fails to address species hybridization of any form, causing conflict among scientists,
legislators, and activists on both sides of the issue. There is some language presented in the
policy that allows some for extrapolation to some cases of hybridization, but the extent of this
protection is limited. In Section 4 (a) (D), “the inadequacy of existing regulatory mechanisms” is
listed as a criteria for labelling species as “endangered” (United States 2006). Hybrids are
currently not protected by any legislation and agencies have little or no framework for
addressing hybridization except on a case by case basis. However, the ESA also states in
Section 8 (b), “the Secretary may, by regulation of commerce or taking, and to the extent he
deems advisable, treat any species as an endangered species or threatened species even
though it is not listed as pursuant to section 4 of this act if he finds that –
(A) such species so closely resembles in appearance, at the point in question, a species
which has been listed pursuant to such section that enforcement personnel would have
substantial difficulty in attempting to differentiate between the listed and unlisted species;
(B) the effect of this substantial difficulty is an additional threat to an endangered or
threatened species; and
(C) such treatment of an unlisted species will substantially facilitate the enforcement and
further the policy of this Act,” (United States 2006).
This language would allow for the protection of any species that closely enough
resembles a protected species in order to prevent accidental take of endangered species. In
Armentrout |8
many cases, hybridization occurs between species that resemble each other enough to be
mistaken by hunters and enforcement personnel (see Index Images). The hybrids of these
crosses therefore often even more closely resemble the endangered species. This policy should
allow protection to be extended to species sharing similarity of appearance, including hybrid
offspring. However, this section has only been invoked seven times as of 2004, and never in a
hybrid situation (Haig, et al. 2004). An amendment to the Endangered Species Act was
proposed in 1996, scheduled to be completed a year later, that addressed ‘intercrosses’ – as
opposed to ‘hybrids’, a term which carried negative connotations (Allendorf, et al. 2001).
However, the policy was never approved and was eventually withdrawn (Haig, et al. 2004).
Conclusions & Recommendations
From looking at both the scientific and legal issues associated with species
hybridization, it is obvious that the issue of species hybridization is a complex matter that
demands close attention. Ecologically, species hybridization is both a solution and a threat to
endangered species conservation. While hybridization may threaten to overwhelm certain
endangered species via hybrid swarm, it also rescues other species from inbreeding depression
(Edmands 2007). In addition, hybrid swarms may also merit conservation if they are the only
remaining evidence of prior species lost to hybridization and other factors leading to extinction
(DeSalle & Amato 2004). Politically, hybridization is so controversial that universal guidelines
have not yet been set, likely due to the difficulty of writing a hybrid policy that would be flexible
enough to apply to all situation, but that would still provide helpful recommendations (Allendorf,
et al. 2001; Haig, et al. 2004). Hybridization has been largely debated for over 25 years both
scientifically and politically, but very little progress has been made thus far. However, a few
recommendations may be made for future examination in this field. One suggestion is that the
genetics of hybridization should be explored more fully to help scientists and policymakers
Armentrout |9
better understand the mechanisms and implications of hybridization. The interdisciplinary field of
conservation genetics should be expanded and funded to contribute to a more informed and
precise decision making process on conservation issues (DeSalle & Amato 2004). This could
have radiating effects in the scientific field, including the possibility of solving the problem of
defining the term “species”.
As of yet, the term “species” is widely debated, but species boundaries are currently
determined by an inability to interbreed with other populations (Lawrence 2005). This definition
poses many more questions and problems than it answers, especially in cases of species
hybridization. Hybrids, especially interspecific hybrids, are vigorously debated as to their status
as members of a taxa and their protection under endangered species legislation. Currently, the
only legislation that addresses hybridization is the Migratory Bird Treaty Act of 1918, and the
scope of that legislation is severely limited (see Case Study 3). The question of where hybrids fit
in determination of species places hybrids in a controversial position where protection may or
may not extend to them and in certain cases, protection may be more harmful than helpful.
Biologists have often had two different objectives in mind when defining the term “species”; one
is to determine a unit of classification and the other is to determine the end product of an
evolutionary process (Amato, et al. 1991). It has been suggested that “species” should be
considered as a continuum rather than a category (Hochkirch 2013). The expansion of the field
of conservation genetics would shed light on this issue and aid scientists in developing a more
appropriate definition.
It may also be recommended that a regulatory framework be established to more
appropriately handle the subject of hybridization. Some scientists have concluded that there are
no plausible solutions to the problem, but other suggest that a formulation of guidelines may be
useful (Haig, et al. 2004; Allendorf, et al. 2001). Hybridization should be handled on a case by
case basis, but some general rules and trends may be developed to aid in the management of
A r m e n t r o u t | 10
hybridization. The main issue is to control the rates and effects of hybridization driven by
anthropogenic influences which may cause the extinction of certain species through competition
and hybrid swarm. Taxa that have originated from natural hybridization, as some suggest is the
case with red wolves (see Case Study 2), should be protected. However, hybrids that result
from anthropogenic pressures should generally be excluded from protection unless hybridization
was employed as a conservation management method (see Case Study 1), or in other
outstanding circumstances (Allendorf, et al. 2001). A set of guidelines that first determines
whether a hybrid situation was driven by anthropogenic pressures should be employed. Once
this conclusion is drawn, the degree of introgression should be determined and the threat to
extant species should be assessed. Hybridization should be managed on a case by case basis,
but assessing the degree of anthropogenic influence, introgression, and potential threat to
extant taxa should facilitate the development of management plans the determination of legal
protection to be extended.
A r m e n t r o u t | 11
Index
Case Study 1: Florida Panthers and Texas Cougars
Image 1: Puma concolor coryi and Puma concolor cougar
Photo credits: U.S. Fish & Wildlife Service
Florida panthers (Puma concolor coryi) were listed as endangered due to a combination
of factors including habitat fragmentation and unregulated killing. Conservation efforts of the
early 1990’s discovered the serious threat posed by inbreeding depression, caused by the
incredibly small population size of only 60-70 individuals (Land & Lacy, 2000). This erosion of
genetic diversity led to declining health, decreased survivorship, low fitness, and threatened
extinction within a few generations. For this reason, the Florida Fish and Wildlife Conservation
Commission introduced eight female Texas cougars into the population to increase gene flow.
By 2000, 36 hybrid individuals were known, with at least 22 surviving (Land & Lacy, 2000). This
effort caused significant controversy as scientists and politicians debated the status of the
hybrids as members of the subspecies Puma concolor coryi and therefore questioned their
protection under the Endangered Species Act. As a result, all members of the species Puma
concolor were listed as “endangered” to prevent accidental killing of Florida panthers and
hybrids (Haig, et al. 2004). This has proved to be a successful effort overall, but progress is
ongoing (Land & Lacy, 2000).
A r m e n t r o u t | 12
Case Study 2: Red Wolf and Coyote
Image 2: Canis rufus and Canis latrans
Photo credits: B. Crawford/USFWS (red wolf), Jerry Murray (coyote)
The red wolf (Canis rufus) have been federally listed as “endangered” since 1967, but
were declared extinct in the wild in the 1980. The occurrence of hybridization with coyotes
(Canis latrans) was identified and only 17 purebred individuals were captured. 14 of these
individuals were utilized to implement a captive breeding program through the U.S. Fish and
Wildlife Service (Reich, et al. 1999). These captive red wolves were reintroduced to North
Carolina in 1987 and as of 2013, nearly 100 red wolves exist in the wild, with another 200 in the
captive breeding program. However, the presence of coyotes in North Carolina has led to
renewed hybridization, which threatens the genetic integrity of the population (Haller & Leonard
2008). Although, some scientists claim that hybridization of these species is to be expected as
red wolves may be a result of hybridization between coyotes and gray wolves (Canis lupus
lupus) (Reich, et al. 1999). As in Case 1, many scientists and other interested parties question
the status of the hybrids as “endangered”, and the genetic integrity of the species is in question.
Residents of North Carolina also fear the development of a “super-coyote” as a result of
hybridization, which may threaten property and livestock (Shaw 2013). As coyotes have become
a greater nuisance in North Carolina, state legislation has attempted to implement coyote
A r m e n t r o u t | 13
hunting, which poses a great threat to red wolves which closely resemble coyotes and often live
in close proximity to them (Shaw, 2013).
Case Study 3: Spotted Owl and Barred Owl
Image 3: Strix occidentalis caurina and Strix varia
Photo credits: U.S. Fish & Wildlife Service
The northern spotted owl (Strix occidatlis caurina) is federally protected under the
Endangered Species Act, but the recent expansion of the range of barred owls (Strix varia) has
led to occurrence of hybridization between the two species. Possible effects on the fitness of the
owls have not yet been fully explored, but it may pose a serious threat to the spotted owls (Haig,
et al. 2004). Under the Migratory Bird Treaty Act of 1918, barred owls, spotted owls, and hybrids
may be protected, as it considers these North America species as protected and hybrids
between protected species are also considered protected (Haig, et al. 2004). However, their
status under the ESA is debatable and scientists question whether the protection of hybrids may
increase or decrease the threat to the spotted owls. Contrary to Case 1, hybridization in this
case may pose a greater threat to the continued survival of the species, and therefore protection
of the hybrids may not be advisable (Haig, et al. 2004).
A r m e n t r o u t | 14
References
Allendorf, F.W., Leary, R.F., Spruell, P., Wenburg, J.K. (2001, November). The problems with
hybrids: setting conservation guidelines. TRENDS in Ecology & Evolution 16(11) 613622.
Amato, G. D., Nowak, R. M., O’Brien, S. J., Glitzenstein, E. R. (1991, July). Species
hybridization and protection of endangered animals. Science 253 50(17) 250-251.
Bauer, D. C., Irvin, W. R. (2010). Endangered Species Act, law, policy, and perspectives.
American Bar Association, Section of Environment, Energy, and Resources. 2nd ed.
DeSalle, R., Amato, G. (2004, September). The expansion of conservation genetics. Nature
Reviews Genetics 5: 702-712.
Edmands, S. (2007). Between a rock and a hard place: evaluating the relative risks of
inbreeding and outbreeding for conservation and management. Molecular Ecology 16:
463-475.
Grant, S. W. (1997). Genetic effects of straying of non-native fish hatchery fish into natural
populations: proceedings of the workshop. U.S. Dep. Commer., NOAA Tech Memo.
NMFS-NWFSC-30, 130.
Haig, S. M., Mulllins, T. D., Forsman, E. D., Trail, P. W., Wennerberg, L. (2004, October).
Genetic identification of spotted owls, barred owls, and their hybrids: Legal implications
of hybrid identity. Conservation Biology 18(5) 1347-1357.
Haller. F., Leonard, J. A. (2008, October). Hybridization among three native North American
Canis species in a region of natural sympatry. PLoS One 3(10) 1-8.
Hochkirch, A. (2013) Hybridization and the origin of species. Journal of Evolutionary Biology 26:
247-251.
A r m e n t r o u t | 15
Krempels, Dana M., Ph.D. (2010). "Reproductive Isolation." UM Department of Biology.
University of Miami. Web.
Land, E.D., Lacy, R.C. (2000). Introgression level achieved through the Florida panther genetic
restoration. Endangered Species Update 17(5) 100-105.
Lawrence, E. (2005). Henderson's Dictionary of Biology. Vol. 13. Harlow, Essex: Benjamin
Cummings/Pearson.
Reich, D. E., Wayne, R. K., Goldstein, D. B. (1999) Genetic evidence for a recent origin by
hybridization of red wolves. Molecular Ecology 8: 139-144.
Shaw, T. (2013, February) "The super-coyote is here." Windsor Star. Web.
Stockwell, C.A., Hendry, A.P., Kinnison, M.T. (2003, February). Contemporary evolution meets
conservation biology. TRENDS in Ecology and Evolution 18(2) 94-101.
Suckling, Kieran. (2011, December). "Endangered Species Act 99 Percent Effective at Saving
Imperiled Species." Center for Biological Diversity. Web.
United States Government. (2006). Endangered Species Act of 1973, as amended through the
108th Congress. U.S. Department of the Interior.
Wolf, D. E., Takebayashi, N., Rieseberg, L.H. (2001, August). Predicting the risk of extinction
through hybridization. Conservation Biology 15(4) 1039-1053.