the effects of trophic cascades

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.


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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


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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


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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


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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.


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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).


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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)


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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)


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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,


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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)


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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


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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).


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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)


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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).


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References

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Carrying Capacity (n.d.) Merriam Webster Online. In Merriam-Webster.

Retrieved October 31, 2016 from http://merriam-

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Coughenour, M. B. (2000). The concept of overgrazing and its application to

Yellowstone‘s northern range. The Greater Yellowstone ecosystem.

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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

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https://www.ted.com/talks/george_monbiot_for_more_wonder_rewild_the_

world?language=en

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Terborgh, J. W. (2015). Toward a trophic theory of species diversity. PNAS,

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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,

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trophic-cascades-and-kinship/

the effects of trophic cascades

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