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ECOLOGICALIMPACTOFTROPHICCASCADES
Memorandum
To: John Knight, Comparative Vertebrate Biology; Kris McBride; Fish and Wildlife Program Coordinator From: Aaron Zuwala, Fish and Wildlife Technician Student
Subject: Comparative Vertebrate Biology Technical Report
Date: Thursday November 3, 2016
The technical report enclosed was composed by Aaron Zuwala, a third semester Fish and Wildlife Technician Student, and contains a specialized report pertaining to the concept of trophic cascades. Specifically, the report discusses the reintroduction of Canis lupus (Gray Wolf) to Yellowstone National Park. The report is due on November 4, 2016 to John Knight as per the assignment guidelines.
When Gray Wolves were hunted to extinction within the Yellowstone National Park boundaries in the mid-1920’s, the ecosystem slowly began to destabilize. The populations of animals the wolves predated increased uncontrollably which started to disturb the ecosystem of the park. Cervus canadensis (Elk) and Bison bison (American Bison) over-grazed the majority of the pasture land, devastating the plant life in the park and increasing the risk of large-scale erosion in areas with steep elevation differences and around the lakes and rivers. By the late 1980’s, the ecology of the park was under scrutiny by wildlife biologists across North America, and an investigation into the benefits and risks of reintroducing wolves back into Yellowstone began. The research led to petitions to reintroduce the wolves and to protect them from hunters within the parks limits once they arrived to prevent history from repeating itself. Once the population was replenished in the park, vast changes began to happen. This report will explore those changes and examine the benefits and disadvantages this trophic cascade displayed by investigating the population changes to key species, vegetation diversity, and the changes in the physical geography in the park.
ECOLOGICALIMPACTOFTROPHICCASCADES
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Comparative Vertebrate Biology Technical Report
Ecological Impact of Trophic Cascades: Canis lupus in
Yellowstone National Park
Aaron Zuwala
Comparative Biology- SCIE 32
Attention: John Knight; Kris McBride
Section 64
November 3, 2016
Table of Contents
ECOLOGICALIMPACTOFTROPHICCASCADES
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Title Page………………………………………………………………………………..1
Table of Contents……………………………………………………………………...2
List of Illustrations…………………………………………………………………….3
Abstract………………………………………………………………………………….4
Introduction……………………………………………………………………………..5
Discussion………………………………………………………………………………7
Conclusion…………………………………………………………………………….12
References…………………………………………………………………………….14
Photo References ………………………………………….………..………17
ECOLOGICALIMPACTOFTROPHICCASCADES
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List of illustrations
Figure 1: Yellowstone Food Chain………………………………………………….6
Figure 2: Yellowstone National Park Sign………………………………………...7
Figure 3: Population Trends of Species in Yellowstone……………………….9
Figure 4: Dramatization of the Changes Brought by Wolves………………..12
ECOLOGICALIMPACTOFTROPHICCASCADES
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Abstract
A trophic cascade is a term used to describe the changes an ecosystem displays
when there is a sudden change to the very top or very bottom of the food chain in
a specific ecosystem. Specifically, this report looks at the changes in the
ecosystem of Yellowstone National Park when wolves were extirpated from the
park in the early 1920’s. Visible changes occurred within the park, and these
changes were seen in the diminishing diversity of vegetation species, and the in
the degrading stability of the banks of rivers, streams, and lakes. A change was
also seen in the park’s wildlife diversity as the population of elk was now free
from predation from wolves, their most effective natural predator. Overgrazing
took place in many areas throughout the park causing forests to not be able to
recuperate their over storey as saplings were browsed so frequently they never
reached maturity. When wolves were reintroduced to the park, all of this began
to change and the ecosystem began to restore itself to the strength it was before
the wolves were extirpated. The report examines the changes the park saw in
vegetation, wildlife, and physical geography after the wolves began to influence
the behaviour of the elk once again.
ECOLOGICALIMPACTOFTROPHICCASCADES
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Introduction
A food chain is a visualization of the hierarchy animals hold in their respective
ecosystems, and whichever species of animal sits at the top of their food chain
can generally be called an “apex predator” (Wallach et al. 2014). Apex predators
achieve their status most often by being the largest carnivorous predator,
meaning they rarely are prey to another carnivore (excluding carnivores) and are
responsible for the control of populations of mesopredator’s (medium-sized
predators) (Roemer et al. 2009; Estes et al. 2011; Ripple et al. 2014). This
relationship is vital in ecosystems with little human impact as the loss of an apex
predator would result in an outbreak of mesopredator’s, also known as
mesopredator release (Crooks and Soule, 1999; Prugh et al. 2011). It was once
believed that removing an apex predator from an ecosystem would result in the
mesopredator becoming the new apex predator (Estes et al. 2011) when the
inverse is actually true. Apex predators have proven to be unique by maintaining
their own populations, and the populations of their prey (Wallach et al. 2014).
When the apex predator is removed, the mesopredator’s population density
spikes initially and levels off much higher than before the removal of their largest
predator (Wallach et al. 2014). This has a negative impact on the populations of
animals they prey on and all the organisms under them in the food chain as they
are not as effective at population control of their prey (Ripple et al. 2014). This
process is known as trophic downgrading (Estes et al. 2011).
ECOLOGICALIMPACTOFTROPHICCASCADES
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A trophic cascade occurs when the
introduction or removal of an animal to the
top of the food chain, or the removal of the
organism at the bottom of the food chain,
alters the behaviour and impacts the
abundance of their prey, releasing the next
level of the food chain from predation
(Terborgh, 2015). An issue arises when it is
taken into considerations that most species
of large predators undergo extreme pressure from humans through lethal control
(Ripple et al. 2014). When apex predators are removed, especially by humans
(as opposed to natural causes), it results in their herbivorous prey’s populations
reproducing uncontrollably, which begins to damage their own population (Dolph,
2013). The populations of herbivores overgraze their habitat, degrading the
ecosystem with no pressure from predators to control them (Dolph, 2013). An
apex predator such as Canis lupus (Gray Wolf) becomes essential to an
ecosystems health as they feed upon the most vulnerable prey available (Mech,
2003). When given the opportunity, Canis lupus will hunt the sick, weak, old, or
young, or those affected most by the elements, increasing the overall health of
their prey’s population (Mech, 2003). This is natural selection at its finest as
Canis lupus is selecting the individuals least likely to survive and removing them
from the gene-pool, ensuring that only the strongest of the species are all that
are left to reproduce.
Figure1:AYellowstonefood-chainillustration(Newsome,2012)
ECOLOGICALIMPACTOFTROPHICCASCADES
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Discussion
At 7.25 million hectares,
Yellowstone National Park is one
of the largest temperate-zone
ecosystems on earth and
produces diverse aquatic and
terrestrial life (National Park
Service, 2015). The park
houses 67 different species of mammals, 330 species of birds, 16 species of
fishes, 5 species of amphibians, and 6 species of reptiles, making it an incredibly
diverse ecosystem and the largest concentration of mammals in the lower 48
states of America (National Park Service, 2015). Wolves were extirpated from
the park in the early 1920’s as the western expansion brought settlers into the
area and wolves were frequently targeting the settler’s livestock as easy prey
(National Park Service, 2015). “Overgrazing” was a term used by ecologists to
describe the impact the removal of wolves had on the ecosystem within
Yellowstone (Coughenour, 2000). Specifically, the ecologists stated that
overgrazing happens due to “an excess of herbivory that leads to degradation of
plant and soil resources” (Coughenour, 2000). A carrying capacity is the
maximum population of a species that an area can support without undergoing
deterioration (Merriam-Webster, 2016), and in the case of Yellowstone, the
removal of a major predator such as Canis lupus meant population spikes for
Cervus canadensis (Elk) and Bison bison (American Bison) which raised their
Figure2:YellowstoneNationalPark(Peaco,1992)
ECOLOGICALIMPACTOFTROPHICCASCADES
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respective population numbers well above the carrying capacity of the park
(Coughenour, 2000). This led to the overgrazing of the parks vegetation which in
turn affected the herbivorous and carnivorous populations of the park, the
diversity and strength of the species, and geographic qualities that the
ecosystem was comprised of.
From 1975 to 1993, a process began to reintroduce wolves to Yellowstone by
making a point of the negative impacts that were directly associated with
removing them (National Park Service, 2015). One of the impacts was the
change in ungulate feeding behaviour. An ecosystem such as Yellowstone’s can
be described at tri-trophic (three trophic levels: predator, prey, plants), and in a
tri-trophic ecosystem, predators can have an indirect impact on vegetation by
influencing the population density and behaviours of herbivores (Ripple and
Beschta, 2011). During the 70-year absence of wolves in Yellowstone, the tri-
trophic cascade collapsed and Cervus elaphus (elk) gradually degraded the
wildlife habitats, soils, and plant diversity (Ripple and Beschta, 2011). Tree
species including Salix spp. (willow) and Populus spp. (poplars) were unable to
replenish the over storey of forested areas as saplings were browsed on
extensively with little chance of growing tall enough to avoid this action (Ripple
and Beschta, 2011). In addition to this, species in a riparian buffer suffered
exceedingly as elk could often be found near water, which began to degrade the
stability of river banks, eventually causing erosion and habitat destruction (Ripple
and Beschta, 2011). As vegetation was being overgrazed throughout the park,
ECOLOGICALIMPACTOFTROPHICCASCADES
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the overall plant diversity decreased; species of plants that were more sensitive
were unable to sustain themselves and eventually also became extirpated within
the park (Ripple and Beschta, 2011). This affected populations of omnivorous
mammals as well, however much differently than it did herbivores.
Ursus arctos horribilis (grizzly bear) can have a diet consisting of up to 90%
vegetation, and the other 10% split between mammals as small as rodents, and
as large as a moose (National Geographic, 2016). Hyperphagia is a state most
Figure3:PopulationtrendsofspeciesdirectlyandindirectlyassociatedwiththetrophiccascadeinYellowstoneNationalParkasinfluencedbywolvesbetween1990and2010.ThegraphsshowthatasthepopulationofCanislupusincreasedandstabilized,populationsofCervuselaphusandoverallbrowsingintheparkdecreasedaccordingly.Theheightofaspen,cottonwood,andwillowbegantoincreasewiththeintroductionofwolves,andbisonandbeaverpopulationsroseasthetreesgotbiggerandthepopulationsdecreased.
(RippleandBeschta,2011)
ECOLOGICALIMPACTOFTROPHICCASCADES
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species of bears go through prior to hibernating, and while in hyperphagia they
eat excess amounts of food to build a fat reserve large enough to last the winter
with minimal feeding; during this phase, bears can often gain up to four pounds a
day (National Geographic, 2016). Yellowstone’s northern range has little cover
given by coniferous trees and extremely productive soils, making it an ideal
location for berry producing plants (Ripple et al., 2015). The trophic cascade
involving Canis lupus in Yellowstone National Park changed the way bears
foraged for food, and altered their diet from predominantly being comprised of
berries to forcing them to hunt for food more often (Ripple et al., 2015). This
happens as a result of the increasing population of Cervus elaphus out-competed
Ursus arctos horribilis for berries on shrubs and trees (Ripple et al., 2015). A
similar study found that Canis latrans (coyotes) were vastly outnumbering Vulpes
vulpes (red fox) in areas where Canis lupus were present, however, in areas
where Canis lupus is present, Vulpes vulpes outnumber Canis latrans (Newsome
and Ripple, 2015). This has potential to become significant in areas like
Yellowstone where Canis lupus were historically present, then removed by
humans (bringing in larger numbers of Canis latrans), and then again
reintroduced to the area, as all the populations predated on by Canis lupus,
Canis latrans, and Vulpes vulpes could become unbalanced with each change to
the composition of the area’s top predator (Newsome and Ripple, 2015).
Water is essential to life in an ecosystem and in Yellowstone with the over-
populating species of Cervus elaphus, the behavior of waterways became
ECOLOGICALIMPACTOFTROPHICCASCADES
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increasingly unpredictable (Ripple and Beschta, 2011). With the riparian buffers
around streams, rivers, and lakes becoming thinner and decreasing in quality,
large amounts of erosion began destroying habitats used by fish and species that
nest on the edges of water bodies, and ruining beaver dams and crossing paths
across the water (Monbiot, 2014). Once fast moving rivers or streams began to
meander more and slow themselves down which flooded areas and created
droughts in other (Monbiot, 2014). With noticeable changes to the vegetation,
wildlife, and geography of the park, it has been widely accepted in the scientific
community that Yellowstone National Park displayed one of the most obvious
examples of a full-scale trophic cascade in documented history (Monbiot, 2014).
ECOLOGICALIMPACTOFTROPHICCASCADES
12
Conclusion
Yellowstone National
Park was the center of
several studies on
trophic cascades (a
change at the very top
of bottom of a food
chain that has a
dramatic effect on the
rest of the organisms
in the ecosystem)
(Carpenter, 2010).
When wolves were
forcibly removed from the park in the early 1920’s, the population of elk grew out
of control, and it resulted in changes in the behaviour of a list of other mammals,
specifically grizzly bear, bison, coyote, and beavers (Ripple and Beschta, 2011).
The diversity of the vegetation in the park also decreased as aspen, willow, and
cottonwood trees couldn’t replenish the over storey in forests as they would be
browsed before the tree was able to mature (Newsome and Ripple, 2015). This
also depleted riparian buffers which weakened the banks of rivers and streams
and caused erosion (Newsome and Ripple, 2015). As ecologists started to
observe these negative impacts, a push to reintroduce wolves back into the park
was established and in 1995, wolves were reintroduced (National Park Service,
Figure3:Adramatizationofthechangeinbio-diversityYellowstonesawwiththetrophiccascade (WES,2014)
ECOLOGICALIMPACTOFTROPHICCASCADES
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2015). The wolves were able to change the grazing preferences of the elk and
the vegetation around streams and rivers was able to stabilize, biodiversity
increased, grizzly bears were able to depend on berries as a main source of
nutrients again, and as the river banks were re-fortified with roots of trees and
shrubs, the waterbodies straightened out and meandered less, creating a better-
balanced ecosystem for the park (Monbiot, 2014).
ECOLOGICALIMPACTOFTROPHICCASCADES
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References
Arian D. Wallach, I. I. (2015). What is an apex predator? Oikos Scientific Jounral,
1-9.
Barber-Meyer, S. M. (2015). Trophic cascades from wolves to grizzly bears or
changing abundance of bears and alternate foods?. Journal Of Animal
Ecology, 84(3), 647-651. doi:10.1111/1365-2656.12338
Carrying Capacity (n.d.) Merriam Webster Online. In Merriam-Webster.
Retrieved October 31, 2016 from http://merriam-
webster.com/dictionary/citation
Coughenour, M. B. (2000). The concept of overgrazing and its application to
Yellowstone‘s northern range. The Greater Yellowstone ecosystem.
Redefining America‘s wilderness heritage, 209-230.
Crooks, K. R. and Soulé, M. E. 1999. Mesopredator release and avifaunal
extinctions in a fragmented system. – Nature 400: 563–566.
Dolph, M. (2013, 01 01). What happens when the top predator is removed from
an ecosystem? Seattle, Washington, USA.
Estes, J. A. et al. 2011. Trophic downgrading of planet earth. – Science 333:
301–306.
Kieter, R. B. (1991). The Greater Yellowstone Ecosystem. Redefining America's
Wilderness Heritage. New Haven, London: Yale University Press.
Monbiot G. (2013, July). For more wonder, rewild the world [Video file]. Retrieved
from:
ECOLOGICALIMPACTOFTROPHICCASCADES
15
https://www.ted.com/talks/george_monbiot_for_more_wonder_rewild_the_
world?language=en
Mech, L. D. (2003). Wolves: Behavior, Ecology, and Conservation University of
Chicago Press. University of Chicago Press, 0-15.
Merriam-Webster. (2016, January 01). Carrying Capacity. Retrieved October 24,
2016, from Merriam-Webster: http://www.merriam-
webster.com/dictionary/carrying%20capacity
National Park Service. (2015, October 15). Yellowstone National Park.
Washington, D.C., USA.
Newsome, T. M., & Ripple, W. J. (2015). A continental scale trophic cascade
from wolves through coyotes to foxes. Journal Of Animal Ecology, 84(1),
49-59. doi:10.1111/1365-2656.12258 (William J. Ripple R. L., Trophic
Cascades from Wolves to Grizzly Bears in Yellowstone, 2013)
Prugh, L. R. et al. 2009.Th e rise of the mesopredator. – BioScience 59: 779–
791.
Ripple W. J. et al. 2014. Status and ecological effects of the world’s largest
carnivores. – Science 343: 1241484
Ripple W. J (2011). Trophic Cascades in Yellowstone: The first 15 years after the
wolf reintroduction. Biological Conservation , 1-7.
Ripple W. J (2013). Trophic Cascades from Wolves to Grizzly Bears in
Yellowstone. Journal of Animal Ecology, 1-17.
Ripple W. J (2015). Wolves trigger a trophic cascade to berries as alternative
food for grizzly bears. Journal of Animal Ecology, 652-654.
ECOLOGICALIMPACTOFTROPHICCASCADES
16
Terborgh, J. W. (2015). Toward a trophic theory of species diversity. PNAS,
11415–11422.
Roemer, G. W. et al. 2009. The ecological role of the mesocarni- vore. –
BioScience 59: 165–173.
Wallach, A. D. et al. 2009. More than mere numbers: the impact of lethal control
on the social stability of a top-order predator. – PLoS ONE 4(9): e6861.
ECOLOGICALIMPACTOFTROPHICCASCADES
17
Photo References
Newsome, T. (2014, May 28). Yellowstone’s Wolf Food chain [Conceptual
diagram showing direct (solid lines) and indirect (dashed lines) effects of
wolves in Yellowstone]. Retrieved October 31, 2016, from
https://thomasnewsome.com/2014/05/
Peaco, J. (1992, October 01). Yellowstone National Park sign at the North
Entrance [Digital image]. Retrieved October 31, 2016, from
https://www.nps.gov/features/yell/slidefile/graphics/signs/Page.htm
Ripple, W. (2011, December 01). Population Trends [Population trends of
species directly and indirectly associated with the trophic cascade in
Yellowstone National Park]. Retrieved October 31, 2016, from
http://www.sciencedirect.com/science/article/pii/S0006320711004046
Yellowstone image [The Ecology of Fear]. (2010, March). Retrieved October 31,
2016, from http://wolfawarenessinc.org/wolf-conservation-issueswolf-
trophic-cascades-and-kinship/