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Thousands of elk live in the Yellowstone National Park and their favorite food is Aspen trees. Elk are herbivores that require them to chew on the bark of the Aspen trees and eat the shoots that grow from underground reproduction roots. Elk is the reason for the reduction of Aspen trees within the Yellowstone National Park. This is termed the elk herbivory hypothesis (SimuText, 2022).
Another possible decline in Aspen trees could be fire suppression. Fire suppression has been used by Yellowstone National Park since around the year 1900. William Romme 1988 concluded that fire suppression did not influence Elk and the Aspen tree decline. William Rome did conclude that without fire suppression the wildfires would decrease the Aspen tree population even more disastrously by killing existing trees. This is termed the fire suppression hypothesis. This means we will focus on the elk herbivory hypothesis (SimuText, 2022).
When simulating for a few years an enclosure without elk (using elk herbivory hypothesis) but only Aspen trees we found that stems per 0.045 meter squared. Then running the enclosure simulation for another few years with the same enclosure plot we found that the stems per square meter had increased to 0.055. Once again, we ran the enclosure simulation for a few more years and noticed the stems had leveled off at 0.050 per square meter. This is an increase of Aspen stems per square meter over a nine-year time frame concluding that elk definitely had a part in the reduction of Aspen trees through their herbivore activities indicated by the top-down control experiment (SimuText, 2022).
The study by James Estes on Urchin, kelp, sea otters, and orca whales off the coast of Eastern Alaska concluded that if urchins were left to feed on the kelp, then the kelp population would decrease. However, if sea otters were added to the food chain, then the kelp would thrive, and urchins would decrease. He then decided to add orca to the food chain of urchins, sea otters, and kelp. This caused the sea otter population to decrease, the urchin population to increase, and the kelp population decreased (National Geographic, 2015).
The next step to ensure accurate results was to make predictions from the elk herbivory hypothesis. The first question asked to develop a prediction was “How do you think the addition of wolves will affect the population size of elk, their favorite prey?” The answer chosen was “the elk population would decrease in size.” Another question was “Given what you observed in the marine ecosystem how do you think the addition of wolves will effect the population size of Aspen, the preferred forage of elk?” The answer chosen was “decrease in elk population.”
In 1995 fourteen gray wolves were introduced into Yellowstone National Park and this decreased the elk population allowing aspen trees to thrive (National Geographic, n.d.). Running a simulation of 100 months of three successions in Lamar Valley of elk, aspen, and gray wolves indicated that by adding the wolves there was an indirect benefit of aspen by allowing the gray wolves to affect the elk population. The elk population was not affected by kills but rather where they ate as in the forest rather than in the aspen tree stem locations. This was because the wolves caused elk to watch and be cautious of their surroundings rather than free roam and eat the aspen trees. Aspen trees provide little cover for elk and that caused the elk to rely on forested cover and herbivore activities within that region to hide from gray wolves. Because of this aspen were able to reproduce without the threat of elk eating their stems (Painter, L., Ripple, W., Beschta, R. (2015).
Researchers from Oregon State University studied if the wolves that were introduced into Yellowstone National Park created a behavioral cascade (Statesman Journal, 2018). The research concluded that elk behavior was impacted by the introduction of wolves into Yellowstone National Park creating a behavioral cascade as noted above through the simulations.
Regarding the prediction answers in paragraph five, my answer of the elk population would decrease in size was accurate. Even though it was a small decrease from wolf introduction the elk population did decrease. The next prediction answer indicated that the aspen population would increase because of the introduction of wolves into Yellowstone National Park through elk population decrease. However, the ideology of cascade behavior was not a reason for this prediction but rather expected a decrease in elk population to an increase in aspen tree population (SimuText, 2022).
References:
Painter, L., Ripple, W., Beschta, R. (2015). University of Wyoming Open Journals.
https://journals.uwyo.edu/index.php/uwnpsrc/article/download/3947/3947 brobinson,+fulltext.pdf
National Geographic. (n.d.). Wolves of Yellowstone. https://education.nationalgeographic.org/resource/wolves-yellowstone
Statesman Journal. (2018, November 12). OSU study: Wolves, cougars return helps Yellowstone National Park streams by reducing elk. https://www.statesmanjournal.com/story/tech/science/environment/2018/11/12/oregon-state-university-study-wolves-cougars-return-yellowstone-national-park-streams-reducing-elk/1975219002/
SimuText. (2022).SimUText Student.
SimUText Student Registration (simutext2.com)
National Geographic. (2015, April 1). As sea stars die, new worries about urchins. https://www.nationalgeographic.com/science/article/150401-urchins-sea-stars-monterey-bay-california-animals