Eastern Gray Squirrel Ecology and Interactions with Western Gray Squirrels

Eastern Gray Squirrel Ecology and Interactions with Western Gray Squirrels

Aaron Johnston School of Forest Resources ∙ ∙University of Washington

Aaron Johnston School of Forest Resources ∙ ∙University of Washington

Photo: Rod GilbertPhoto: Mary Linders

Eastern Gray Squirrel

Photo: Mary Linders

Verts and Carraway (1998)

Eastern Gray Squirrel Introductions

Western Gray Squirrel

Photo: Rod Gilbert

Verts and Carraway 1998

Current Distribution of Western Gray Squirrels

Linders and Stinson (2007)

KLICKITAT COUNTY

PUGET TROUGH

NORTH CASCADES

Objective: Investigate competition

• Are they competing? – Test for competitive effects

• How are they competing?– Describe and compare

components of the niche

Niche

Diet

Shelter

Habitat Behavior

Use of time

Use of space

Niche

ENVIRONMENTAL GRADIENT

AB

UN

DA

NC

E

Resource Partitioning

ENVIRONMENTAL GRADIENT

AB

UN

DA

NC

E

Competitive Exclusion Principle

ENVIRONMENTAL GRADIENT

AB

UN

DA

NC

E

Photo: David J. Slater

InterferenceExploitation

Competition

Patch 1Patch 2

Patch 3

Ideal Free Distribution

Patch 1Patch 2

Patch 3

Equilibrium

Density Dependent Habitat Selection

DENSITY

HA

BIT

AT

QU

ALI

TY

Photo: Danielle Reese

Methods

Utilization Distributions

Measuring Spatial Overlap

0.5 Km

EASTERN WESTERN

0

10

20

30

40A

RE

A (

Ha

)

FEMALE GRAY SQUIRREL HOMERANGES

t15 = 5.36, P = 0.0001

EASTERN WESTERN E/W W/E

-5

0

5

10

% O

VE

RL

AP

FEMALE-FEMALE SPATIAL OVERLAP

F3,30 = 2.89, P = 0.052

Utilization Distributions

0.5 Km

1 Km

Eastern Western

DIS

TAN

CE T

O W

ETLA

ND

(m)

Distance to Wetland

P < 0.001, df = 33

Shrub Cover

WesternEastern

% S

hrub

Cov

er

P = 0.007, df = 33

Shrub Height

Shru

b H

eigh

t (m

)

Eastern Western

P < 0.001, df = 33

Western Gray Squirrel Habitat

Eastern Gray Squirrel Habitat

ENVIRONMENTAL GRADIENT

AB

UN

DA

NC

EEastern Western

H0: There is no difference between eastern and western gray squirrels in their selection of habitat.

GIS Layers

Habitat Plots

Nest Plots

Habitat Analysis

Methods: Habitat measurements

Habitat Plot

OVERSTORY

COMPOSITION STRUCTUREDIVERSITY

UNDERSTORY

F7,152 = 8.6, P < 0.001

ebh enh enl ebl wnl wbh wbl wnh2.64 2.58 1.97 1.77 1.25 1.13 1.04 0.98

Total Understory

ebh ebl enh enl wbh wbl wnh wnl

01

23

4

Methods: Removal Experiment

• Randomized block design– Site = Experimental unit– Removal treatment

assigned randomly to a set of sites in 2009 and another set in 2010

1 Km

Eastern gray squirrel Western gray squirrel

Wetland

Trap

BEFORE AFTER

-0.1

0.4

0.9

1.4

% O

VE

RL

AP

OVERLAP BEFORE AND AFTER EASTERN GRAY SQUIRREL REMOVAL

t16 = 0.133, P = 0.896

Hmmm…

SummarySummary

• Territorial behavior and possible interspecies avoidance

• Differential habitat selection

• More to come on resource use and removal effects

• Territorial behavior and possible interspecies avoidance

• Differential habitat selection

• More to come on resource use and removal effects

Photo: Brett PetersPhoto: Brett Peters

Photo: ODFWPhoto: ODFW

Photo: Danielle Reese

AcknowledgementsWDFW – Gene Orth, Matt Vander Haegen, Caitlin McIntyre, AnnMarie Prince, Mary Linders

Fort Lewis – Todd Zuchowski, Dave Clouse

Nature Conservancy – Cheryl Fimbel, Sanders Freed

Volunteers – Danielle Reese, Val Filer, Sara Gregory, Amy Stratton, Scotty Hardwick, Cindi Perez, Tadd Wheeler, Rachel Lipsky, Jesse McCarty

Committee – Steve West, Matt Vander Haegen, John Marzluff, Josh Lawler, Jim Kenagy

Photo: Danielle Reese