ABS 476 Exam 2 Study Guide - Community Ecology and Trophic Cascades

What are the two mechanisms (or factors) that can cause a trophic cascade?

1. Numerical 2. Behaviorally mediated trophic cascade (BMTC)

Explain the two mechanisms, numerical and BMTC, that can cause a trophic cascade. How are they different?

1. Numerical -reduced population density of ungulates 2. Behaviorally mediated trophic cascade (BMTC) -alter behavior of ungulates so they move around the landscape and do not concentrate feeding in one area for too long • e.g., localized recovery of aspen even with high elk density

Describe the four trophic levels when evaluating community ecology. Can there also be a fifth level? Please explain.

1. Producers = green plants 2. Primary consumers = herbivores 3. Secondary consumers = carnivores 4. Tertiary consumers = higher carnivores 5th? Idk

Landscape of Fear

A visual model to help explain how fear could alter an animal's use of an area as it tries to reduce its vulnerability to predation.

How can wolves indirectly benefit song birds?

Increased willow released which in turn increased richness and abundance for song bird

Trophic Cascade

Indirect species interactions that originate with predators and spread downward through food webs.

Bottom-Up

Lower trophic levels influence upper trophic levels

What is the evidence that ungulates respond to a landscape of fear?

Male Elk Do Not Show Difference in Vigilance in Response to Wolves ; Cow Elk with Calves More Vigilant in Areas With Wolves ; Less Time Spent Foraging for Cow Elk w/ Calves in Areas w/ Wolves

For predictions of top-down and bottom-up processes, draw the relationships for (a) predator density vs prey density and (b) prey density vs predator density.

Predator density vs. prey density (Bottom-Up): increase straight line Prey density vs. predator density (Top-Down): decrease straight line

Top-Down

Upper trophic levels influence lower trophic levels

We discussed several examples of trophic cascades. Understand the main take home messages of these examples.

Willow -> song birds Wolf -> ungulates Ungulates -> aspen and willow growth

For the wolf-elk-vegetation trophic cascades in Yellowstone, what is the evidence that a trophic cascade is occurring? Be able to provide a clear and exhaustive explanation of this evidence considering all mechanisms.

• Before / after photos • Missing age classes of trees without predators • Browsing on vegetation• Growth of vegetation (Increase in Young Aspen > 1 m) • Elk population size• Elk behavior / movements• Predators absent: missing age classes of trees due to over-browsing • Reduced or no tree recruitment in the absence of predators • With wolf reintroduction in Yellowstone NP • Reduced browsing• Increased plant growth / height • For willow, recovery outside of exclosure • Growth of two willow species before and after wolf reintroduction• Increased growth after 1995 • Increased willow growth after wolf reintroduction• Reduced herbivory by ungulates

How do daily activity patterns of wolves shape the LOF in elk?

• Elk avoid using risky areas during the day when wolf activity is greatest (predation risk is highest) • Elk are using risky areas during night when wolves are least active (predation risk the lowest) • Low wolf activity leads to less perceived predation risk• High wolf activity leads to greater perceived predation risk • High and low predation risk changes throughout the day, which influences elk use

Define and contrast the concepts of food chains and food webs.

• Food chain -transfer of energy from plants to herbivores to carnivores in a single chain of interactions • Food web -the linking of multiple food chains among species in a community

How did a non-native trout species (the lake trout) potentially cause a decline in the elk calf:cow ratio in Yellowstone?

• Historically, grizzlies fed on native cutthroat trout in spring• Invasive lake trout introduced and reduced cutthroats by 90% • Bears switched to feeding more on elk calves in spring • Post lake trout invasion: (1) more elk in grizzly diet, (2) higher elk calf mortality, and (3) more calves eaten per bear per year (declining calf:cow ratio)

How do wolves influence berry-producing shrubs? Which other species benefits from the increase in berry production? Please explain.

• Number of berries per plant (buffaloberry, serviceberry, and chokecherry) • More berries inside exclosures (no elk) • Many fewer berries outside exclosures • Browsing impacted number of berries • Wolves affect number of elk • Grizzly bears benefit from increase wolf populations (less elf feeding on berries) • As elk density increases, the amount of fruit in grizzly bear diet decreases

How do wolves influence pronghorn populations in Grand Teton NP? Why would pronghorn fawn survival increase with a healthy wolf population?

• Wolves Reduce Coyotes Which Increases Pronghorn Populations • In the absence of wolves, the realized population growth rate for pronghorn was lower (λ = 0.92) based on fawn survival • At wolf abundant sites, pronghorn population growth rate was higher

Are top-down processes and additive mortality the same thing? Why or why not?

• Yes, can be potentially -if top-down processes reduce population size of prey • Also, consider short term effects (e.g., appears to be additive) vs long-term effects (compensatory; will those animals die from other causes eventually, such as starvation due to over-browsing?) • Not necessarily -if top-down processes shape behavior of prey, but not population size of prey