The Beaver’s Menu
Beaver Foraging Preference in the Maritime
Pacific Northwest
Christopher C. Tran
, acetran@uw.edu
UW School of Environmental and Forest Sciences
ESRM: Wildlife Conservation
Example of beaver pond, where surveys were conducted
Introduction
What woody plants do beavers eat in the maritime Pacific Northwest?
Results
Selectivity Index by species
Electivity Index by species
Skykomish watershed, in Snohomish, WA. The North American Beaver (Castor canadensis) is an ecosystem engineer that can modify the geomorphology, hydrology, and interactions between organisms in the environment. It is important to understand beaver foraging preference, as it determines their available range, how they affect vegetation communities, and improve our understanding of beaver management. Previous studies have shown that beavers tend to prefer aspen and willow, but these species are not prevalent in the maritime Pacific Northwest (PNW) and may not be appropriate for modeling habitat suitability or restoration planning for beaver in this region. In this study I investigated beaver foraging preference in the PNW by surveying vegetation communities with beaver activity in the in the Skykomish
watershed, in Snohomish, WA.
Species
Salmonberry Red Osier Dogwood Pacific Willow Sitka willow Vine Maple
English Ivy Red huckleberry Spiraea
Red Elderberry Pacific crabapple
Cascara
Other Species
4
4
3.618099
3.618099
3.392229
3.260119
2.380628
1.691503
Methods
0.706637
0.464184
‐2.08107
‐4
‐2
0
2
Selectivity 4
6
Selectivity Index by stem diameter
4
Selectivity 3
2
Stem size (cm) # Observed # Cut
0.5 ‐1
1‐2
2‐4
>4
352
131
74
18
35
29
12
8
1
0
‐1
0.5‐1
Abundance
46.6%
9.6%
13.7%
2.2%
3.3%
2.5%
1.8%
2.5%
2.6%
0.7%
0.3%
14.2%
R2 = 0.06893
There is a little correlation between distance and the amount of beaver chew from the water. I found a weak negative trend and a R2 value of 0.06893. Discussion
6
5
# Cut
26
19
8
9
3
6
3
4
6
7
4
0
•Strongest selectivity was for Cascara and Pacific crabapple, but these plants had low observations and all their observations had beaver chew.
• The most abundant observations were salmonberry, pacific willow, and red osier dogwood.
• I found avoidance for salmonberry and species with no observed cuts (Swordfern, Stink currant, Himalayan blackberry, and Trailing blackberry).
Positive values indicates selectivity, negative values indicates avoidance
I conducted vegetation surveys in riparian areas with active beaver foraging activity adjacent to 3 beaver ponds. Species composition and abundance were identified in quadrats along transects as shown in the figure above. I counted woody vegetation species by stem size and recorded recent beaver chew on stems.
# Observed
265
78
55
19
19
14
14
10
10
7
4
74
1‐2.
2‐4.
> 4
‐2
‐3
•High selection for Cascara, but uncertainty whether beavers are eating or clearing.
•Low selection for salmonberry, but anecdotal observations suggest otherwise
•Beavers have a preference for larger diameter stems. Provides both food and dam material.
• Proportion of stems chewed and distance from water have little correlation.
Current literature suggest that beavers prefer salmonberry, vine maple, and red alder in the PNW. My results differed from these studies and indicate that beavers have a preference for many species. Willow a favorite for beavers in other regions in the U.S was not highly selected here, possibly due to low numbers. Further sampling may be necessary to increase observations of rare species and will offer a fuller depiction of beaver foraging preference in this region.
‐4
Size Class (cm)
Examples of beaver foraging and tree cutting activity recorded in this study
I performed statistical analysis for beaver preference of plants by using an electivity index formula. Selectivity by plant species and by their stem size class was determined. I also investigated proportion of stems with chew and distance from water. Comparing beaver selectivity for stem diameter size, there is evidence for preference for large diameter stems (>4 cm). However observations of stems > 4cm are low. Large stems are not only used for foraging, but for dam building materials for beavers and could be why they prefer to select larger stems. Understanding beaver foraging will help improve management of beavers and the effect they have in their ecosystems. Identifying beaver preference in the maritime PNW is important because beavers are being used and explored as a management technique to restore wetland and riparian ecosystems.
Acknowledgements
Thank you to Ben Dittbrenner for your guidance and support. To Susan Bolton for the equipment for the project and to Jason Schilling for the company and fieldwork help.