Lecture 13

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# Lecture 13
## Life history variation
### <br/><br/><br/>WILD3810 (Spring 2021)

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## Life history variation

#### Organisms have limited resources to investment between growth, reproduction, and survivorship

- **Trade offs**

#### Evolution selects for different combinations of *life history traits*

> Demographic traits that influence fitness (i.e., `\(\lambda\)`)

- size at birth  
- growth pattern  
- age at maturity  
- fecundity schedule  
- mortality schedule  
- length of life

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#### Selection favors life history combinations that maximize the per capita growth rate

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## Life history variation

#### Which species has higher fecundity?

#### Which species has a higher age at first reproduction?

#### Which species lives longer?

.pull-left[
<img src="https://upload.wikimedia.org/wikipedia/commons/e/ed/BRACHYLAGUS_IDAHOENSIS.jpg" width="100%" style="display: block; margin: auto;" />
]

.pull-right[
<img src="https://upload.wikimedia.org/wikipedia/commons/5/5a/Black-bear_with_her_cub.jpg" width="100%" style="display: block; margin: auto;" />
]

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## Life history variation

#### Which species has higher fecundity?

#### Which species has a higher age at first reproduction?

#### Which species lives longer?

.pull-left[
<img src="https://upload.wikimedia.org/wikipedia/commons/1/10/Green-winged_teal_FWS_18507.jpg" width="100%" style="display: block; margin: auto;" />
]

.pull-right[
<img src="https://upload.wikimedia.org/wikipedia/commons/0/0b/Northern_Pintails_%28Male_%26_Female%29_I_IMG_0911.jpg" width="100%" style="display: block; margin: auto;" />
]

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## Life history variation

#### Which species has higher fecundity?

#### Which species has a higher age at first reproduction?

#### Which species lives longer?

.pull-left[
<img src="figs/cutthroat.png" width="100%" style="display: block; margin: auto;" />
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.pull-right[
<img src="figs/lake_trout.png" width="100%" style="display: block; margin: auto;" />
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# Life history trade offs

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## Life history trade offs

#### Current reproduction vs. future reproduction

.pull-left[
#### Semelparity
<img src="figs/Reproductive_effort_semelparous.jpg" width="100%" style="display: block; margin: auto;" />
]

.pull-right[
#### Iteroparity
<img src="figs/Reproductive_effort_iteroparous.svg.png" width="100%" style="display: block; margin: auto auto auto 0;" />
]

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## Life history trade offs

#### Offspring quantity vs. quality (Lack 1954,1968)

.pull-left[
<img src="figs/clutch_size.png" width="100%" style="display: block; margin: auto;" />
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.pull-right[
<img src="figs/The-Lack-clutch.png" width="100%" style="display: block; margin: auto;" />
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## r-K selection

#### Arises directly from logistic population growth model (MacArthur & Wilson 1967; Pianka 1970)

- `\(\large r_0\)`: density-independent rate of population growth

- `\(\large K\)`: carrying capacity

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#### Evolution of life history strategies leads to:

**r-selected species**

- selection for ability to colonize and reproduce rapidly

- good colonizers, poor competitors

--
**K-selected species**

- selection for ability to contribute to `\(N\)` when the population is near `\(K\)`

- good competitors, poor colonizers

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## Life history trade offs

#### Predictions (based on Pianka 1970)

<table class="table table-striped table-hover table-condensed table-responsive" style="font-size: 16px; margin-left: auto; margin-right: auto;">
 <thead>
  <tr>
   <th style="text-align:center;">  </th>
   <th style="text-align:center;"> r-selection </th>
   <th style="text-align:center;"> K-selection </th>
  </tr>
 </thead>
<tbody>
  <tr>
   <td style="text-align:center;"> Mortality </td>
   <td style="text-align:center;"> Variable &amp; unpredictable </td>
   <td style="text-align:center;"> Constant &amp; predictable </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Population size </td>
   <td style="text-align:center;"> Variable &amp; below K </td>
   <td style="text-align:center;"> Constant &amp; close to K </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Competition </td>
   <td style="text-align:center;"> Variable &amp; weak </td>
   <td style="text-align:center;"> Strong </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Selection favors </td>
   <td style="text-align:center;"> Rapid development, early reproduction, small body size, semelparity </td>
   <td style="text-align:center;"> Slow development, delayed reproduction, large body size, iteroparity </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Length of life </td>
   <td style="text-align:center;"> Short </td>
   <td style="text-align:center;"> Long </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Leads to... </td>
   <td style="text-align:center;"> High productivity </td>
   <td style="text-align:center;"> High efficiency </td>
  </tr>
</tbody>
</table>

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## r-K selection

#### The predictions of r-K selection stimulated vast amounts of research on life history evolution

--
#### But...

- Many species don't fall neatly into these categories (combinations of r-selected traits and K-selected traits)

- Predictions are vague enough that many different results are "consistent" with them

- Carrying-capacity is not a demographic parameter so traits that influence resource use do not directly translate to a specific K

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class: inverse, middle, center

# Fast-slow continuum

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## Fast-slow continuum

#### More recent studies view life history variation as existing on a continuum:

<table class="table table-striped table-hover table-condensed table-responsive" style="font-size: 16px; margin-left: auto; margin-right: auto;">
 <thead>
  <tr>
   <th style="text-align:center;"> Slow species </th>
   <th style="text-align:center;"> Fast species </th>
  </tr>
 </thead>
<tbody>
  <tr>
   <td style="text-align:center;"> Low reproductive effort </td>
   <td style="text-align:center;"> High reproduction </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Delayed maturity </td>
   <td style="text-align:center;"> Early maturity </td>
  </tr>
  <tr>
   <td style="text-align:center;"> High survival </td>
   <td style="text-align:center;"> Low survival </td>
  </tr>
  <tr>
   <td style="text-align:center;"> Long generation time </td>
   <td style="text-align:center;"> Short generation time </td>
  </tr>
</tbody>
</table>

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## Fast-slow continuum in birds

.pull-left[
<img src="https://upload.wikimedia.org/wikipedia/commons/c/c4/American_Redstart_%2836640045423%29.jpg" width="80%" style="display: block; margin: auto;" />

]

.pull-right[
<img src="figs/fast_slow_birds.png" width="100%" style="display: block; margin: auto;" />
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## Fast-slow continuum in mammals

.pull-left[
<img src="https://upload.wikimedia.org/wikipedia/commons/7/76/TiptonKangarooRat.jpg" width="80%" style="display: block; margin: auto;" />

]

.pull-right[

##### Age at 1st reproduction vs. adult body mass

##### Adult lifespan vs. adult body mass

]

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## Fast-slow continuum and elasticities

#### Is the fast-slow continuum related to which vital rates influence `\(\large \lambda\)`?

- Explicit connection between evolved pattern of life history vital rates and impact on population dynamics

- Elasticities are useful to guide conservation & management

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## Fast-slow continuum and elasticities

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## Fast-slow continuum and elasticities

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## Fast-slow continuum and elasticities

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## Fast-slow continuum and management

#### Distinctive demographic elasticity patterns across bird, mammal, and plant life histories

- Elasticities can be reasonably assessed from limited knowledge of an organisms life history (e.g., clutch size, age at maturity, etc.)

- Managers can assess whether to focus on managing survival (e.g., through harvest or wintering habitat) or reproduction (e.g., spring and summer habitat)

- Very important for the conservation of rare species

+ Detailed demographic studies not possible