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Spatial variability in granivory determines the strength of stochastic community assembly Rachel M. Germain 1 *, Laura Johnson 1 , Andrew MacDougall 1 , Karl Cottenie 1 , and Elizabeth Gillis 2 1 Department of Integrative Biology, University of Guelph, ON; 2 Department of Resource Management, Vancouver Island University, BC

ESA 2011 Austin, TX

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Page 1: ESA 2011 Austin, TX

Spatial variability in granivory determines the strength of stochastic community assemblyRachel M. Germain1*, Laura Johnson1, Andrew MacDougall1, Karl Cottenie1, and Elizabeth Gillis2

1Department of Integrative Biology, University of Guelph, ON; 2Department of Resource Management, Vancouver Island University, BC

Page 2: ESA 2011 Austin, TX

What structures diversity?

• Neutral-based stochastic perspective:– species are functionally equivalent– ecological drift and dispersal

• Niche-based deterministic perspective:– species are functionally distinct– biotic and abiotic interactions

(Adler et al. 2007; Chase 2007, 2010)

Page 3: ESA 2011 Austin, TX

What structures diversity?

• Neutral-based stochastic perspective:– species are functionally equivalent– ecological drift and dispersal

• Niche-based deterministic perspective:– species are functionally distinct– biotic and abiotic interactions

(Adler et al. 2007; Chase 2007, 2010) False dichotomy?

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Evaluating assembly processes withβ-diversity

Stochastic

β-diversity is high

Deterministic

β-diversity is low

Regional scale Local scale Species

Page 5: ESA 2011 Austin, TX

Stochasticity increases with productivity in experimental pond communities

(from Chase 2010 Science)

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Small mammals regulate plant community assembly

• Populations are rapidly increasing

• Selective foragers

• Habitat use is spatially predictable– predator risk– distance from nesting sites

(Kotler 1984; Howe et al. 2006)

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Small mammal impacts on tallgrass prairie assembly

1. Are small mammals agents of deterministic assembly?

2. Are small mammal effects spatially predictable?

3. Do small mammals alter the trajectory of native plant assembly?

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Charitable research reserveCambridge, ON, CA

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Prairie restoration

Ploughed and seeded with 24 native tallgrass prairie species

Seeds randomly applied using a tracker-drawn seeder

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1. Are small mammals agents of deterministic assembly?

2. Are small mammal effects spatially predictable?

3. Do small mammals alter the trajectory of native plant assembly ?

Small mammal impacts on tallgrass prairie assembly

• Prediction: small mammals will selectively remove some species over others

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Cafeteria trial• For each of 8 seed species, we prepared 8 dishes of 10 seeds

• Half of the dishes per species were covered by wire mesh (control)

• Seeds counted after 24 hours in the field

• Analysis:

– nested ANOVA: seeds remaining ~ functional group + seed species

Page 12: ESA 2011 Austin, TX

H. annus

H. dive

ricatus

L. ca

pitata

R. blanda

S. tri

folia

S. cry

ptandrus

B. inerm

is

B. kalm

ii0

2

4

6

8

10

Seed species

Seed

s re

mai

ning

Forb seeds were removed significantly more than shrub and grass seeds

Functional group: F2,23 = 29.97, P < 0.001*Species: F5,23 = 0.35, P = 0.876

Forbs Shrubs Grasses

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Small mammal impacts on tallgrass prairie assembly

1. Are small mammals agents of deterministic assembly?

2. Are small mammal effects spatially predictable?

3. Do small mammals alter the trajectory of native plant assembly?

• Prediction: Seed removal will decrease with distance from the prairie edge

Page 14: ESA 2011 Austin, TX

Small mammal survey• Conducted in mid-April

• Samples of 10 sunflower seeds on bare soil at 100 random locations across our site

• Seeds counted after 14 days in the field

• Analysis:– ANOVA: seed removal ~ distance from edge

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Closedold-field Open prairie

-30 -10 10 30 50 70 90+

Distance from edge (m)

Seed

s re

mov

ed (l

og10

)

Seed removal was strongest in the old-field, and decreased with distance from the edge

F1,84 = 150.9, P < 0.0001*

Page 16: ESA 2011 Austin, TX

Small mammal impacts on tallgrass prairie assembly

1. Are small mammals agents of deterministic assembly?

2. Are small mammal effects spatially predictable?

3. Do small mammals alter the trajectory of native plant assembly ?

• Prediction: α- and β-diversity will increase with distance from the prairie edge where small mammals are less abundant

Page 17: ESA 2011 Austin, TX

Community-wide diversity survey

• We surveyed fifty 5 x 5 m plots at random locations across our study site

• Presence/absence data for all species• Analysis:

– α: species per plot ~ distance from edge– β: Jaccard’s dissimilarity ~ distance from edge

Heliopsis Solidago Desmodium Ratibida

Page 18: ESA 2011 Austin, TX

α- and β-diversity increased with distance from the edge of the prairie

P = 0.012*F1,48 = 12.82, P <0.001*

β-di

vers

ity (J

acca

rd’s

diss

imila

rity)

α-d

iver

sity

(spe

cies

per

plo

t) α β

Page 19: ESA 2011 Austin, TX

Role of granivory during tallgrass prairie assembly

1. Forb seeds were removed significantly more than shrub and grass seeds

2. Small mammal impacts were strongest in the adjacent old-field, and decreased with distance from the prairie edge

3. α- and β-diversity increased along a prairie edge-to-interior gradient

Page 20: ESA 2011 Austin, TX

oldfield

increasingβ-diversity

decreasing seed

predation

Deterministic assembly

Stochastic assembly

Net effect: fine-scale switching in the strength of assembly processes

prairie interior

Page 21: ESA 2011 Austin, TX

oldfield

increasingβ-diversity

decreasing seed

predation

Deterministic assembly

Stochastic assembly

Net effect: fine-scale switching in the strength of assembly processes

prairie interior

Page 22: ESA 2011 Austin, TX

oldfield

increasingβ-diversity

decreasing seed

predation

Deterministic assembly

Stochastic assembly

Net effect: fine-scale switching in the strength of assembly processes

prairie interior

Page 23: ESA 2011 Austin, TX
Page 24: ESA 2011 Austin, TX

Acknowledgements

Carole Ann LeCroix for assistance with plant identification

Mathis Natvik for seeding the field

Peter Kelly--coordinator of the rare charitable research reserve

NSERC and CFI Infrastructure grants