Lecture 19

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# Lecture 19
## Interspecific interactions: Predation, part II
### <br/><br/><br/>WILD3810 (Spring 2020)

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## Readings

> ### Mills 150-153

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## Do predators control the abundance of their prey?

#### If predators kill prey:

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- does predation lower the overall survival probability of prey?

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- does harvest lower abundance of prey (note - for humans, top predators might be "prey")?

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Images courtesy of AWeith and University of Washington, via wikicommons

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## The effect of predation on prey survival

#### Say `$S_0$` is the survival probability in the absence of predation/harvest

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- if the predation rate is `$P$`, individuals must survive other sources of mortality *and* not be predated

`$$\large S_A = S_0(1 - P)$$`

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## The effect of predation on prey survival

#### **Additive** mortality

> Predation/harvest occurs independently of other sources of mortality

`$$\large S_A = S_0(1 - P)$$`

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## The effect of predation on prey survival

#### If predation/harvest is additive:

- removing predators should increase prey abundance

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- harvest of game species should reduce their abundance

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## The effect of predation on prey survival

#### In many populations, some individuals have very low survival:

- floaters

- old individuals

- sick individual

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- the "doomed surplus" (Errington 1956)

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#### If predators/harvest take individuals that would have died from other causes, overall survival rate may be uneffected by `$\large P$`

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## The effect of predation on prey survival

#### **Compensatory** mortality

> mortality due to predation/harvest is offset by lower mortality from other sources

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## The effect of predation on prey survival

#### **Partial compensatory** mortality

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## The effect of predation on prey survival

#### Partial compensatory mortality

- Partial compensation in hunting of willow ptarmigan (Sandercock et al. 2010)
    + Typical of short-lived gamebirds

.pull-left[
<img src="figs/ptarmigan.png" width="772" style="display: block; margin: auto;" />
]

.pull-right[
<img src="figs/ptarmigan_surv.png" width="50%" style="display: block; margin: auto;" />
]

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## The effect of predation on prey survival

#### Partial compensatory mortality

- Partial compensation of predator mortality in elk calves
    + predation offset by winter severity and density-dependent malnutrition (Singer et al. 1997)

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## The effect of predation on prey survival

#### Population dynamics can be compensated by Density-Dependent changes in other vital rates

- Immigration: mortality of territorial animals can open up space for ‘floaters’ and give them a chance to reproduce and gain fitness.

- Fertility: Reduced density through mortality can free up resources for survivors and enhance their reproductive output and success.

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## The effect of harvest on predator abundance

#### Anthropogenic mortality is largely additive in lower-48 wolves (Murray et al. 2010)
- In absence of human pressure, mortality is fairly low, opportunity for compensation is low

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## The effect of harvest on predator abundance

#### Cougars in Utah (Wolfe et al. 2015)
- In absence of human pressure, mortality is fairly low, opportunity for compensation is low

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## Do predators control the abundance of their prey?

#### Did reintroduction of wolves to Yellowstone NP reduce elk populations?

.pull-left[
<img src="https://upload.wikimedia.org/wikipedia/commons/c/cb/Wolves_and_elk.jpg" width="75%" style="display: block; margin: auto;" />
]

.pull-right[
<img src="figs/yellowstone-wolves-species.jpg" width="600" style="display: block; margin: auto;" />
]

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## Do predators control the abundance of their prey?

#### Did reintroduction of wolves to Yellowstone NP reduce elk populations?

- During the time that elk populations started to decline, general and late season anterless elk hunts were at historically high levels

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## Do predators control the abundance of their prey?

#### Did reintroduction of wolves to Yellowstone NP reduce elk populations?

- Age distribution differs between wolf kills and hunter kills
    + Wolves mainly take fawns and older elk
    + Hunters take females at the height of their reproductive value
    
<img src="figs/elk_surv.png" width="65%" style="display: block; margin: auto;" />

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## Do predators control the abundance of their prey?

#### Who gets killed?

- Young and old may be more vulnerable
    + loss of young may be compensated for by additional reproductive effort
    + old may have low reproducive value

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- Effects on population growth depend on elasticities
    + loss of young may have big impact in species with high elasticity of fecundity/recruitment
    + loss of adults may have big impact in species with high elasticity of adult survival
    
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## Do predators control the abundance of their prey?

#### It's complicated; need to know:

- Predation rate
    + Numerical response
    + Functional reponse

- Additive vs. compensatory

- Who gets killed?

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#### Central questions to many modern issues in natural resource management
  - Predator control
  - Hunting regulations
  - Commercial harvest limits