Using Beaver as an Ecosystem Service Provider on Forestlands in the Snohomish River Basin

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1 FY 2011 Noncompetitive Tribal Projects for Restoration and Protection of Puget Sound Using Beaver as an Ecosystem Service Provider on Forestlands in the Snohomish River Basin Project Deliverable: Skykomish Basin Beaver Population Assessment Tulalip Tribes October 20, 2014 Summary The Tulalip Tribes implemented a project to investigate the ecological benefits of beaver in the Snohomish River Basin. Beaver can improve the structure of the forest, provide greater flood abatement, increase groundwater recharge, provide habitat for fish and wildlife, and improve water quality. The Tulalip project aimed to assess a subset of these ecological impacts to identify the benefit of placing and maintaining of beaver in the forested landscape. Project goals included identifying the extent of the current beaver population within the Snohomish Basin, monitoring ecological benefits, and evaluating the feasibility of relocation strategies including identifying potential relocation sites. This report is one of the project deliverables. It outlines the development and implementation of a beaver habitat suitability index model to identify occupied and available habitat in the basin. It also provides a estimate of the current beaver population status and amount of available habitat. Tulalip Tribes Page 1

2 Introduction This project was initiated in December of 2012 and the field component of the project commenced in June The first steps in the project were to map potential beaver habitat using a habitat suitability index (HSI), which was developed by NOAA Northwest Fisheries Science Center for this purpose. From the HSI, the project team ground-truthed all high priority areas on the model and discovered additional areas for project site selection and population assessment. Ground truthed sites were compared with modeled values to evaluate the accuracy of the model and make a population prediction. Methods Habitat suitability index model development In 2013, Tulalip collaborated with Michael Pollock and NOAA s Northwest Fisheries Science Center to assist in the development of a habitat suitability index (HSI) model for beaver populations in the Skykomish watershed and surrounding areas. The HSI was developed by combining a number of GIS layers to target stream reaches that would likely provide requisite habitat needs for beaver populations. Those layers included LIDAR digital elevation model (DEM), stream lines, stream power, stream gradient, and valley bottom width (Table 1). Table 1 Input layers for development of the Beaver Habitat Suitability Index Model LIDAR (DEM) Stream lines Stream gradient Stream power Valley bottom width Ten meter resolution LIDAR DEM data was used to provided base elevation data, and as an input for calculating valley bottom width. Stream lines derived from Tulalip Tribes, King and Snohomish County were used as the base layer onto which the HSI lines would be superimposed. Stream gradient was derived from elevation change identified in the DEM along each stream reach in the stream layer. Stream power was calculated using the contributing stream basin, derived from the LIDAR at pour points along the stream. Finally, valley bottom width, the lateral length of the valley that the stream flows through, was calculated using the LIDAR. The HSI did not include a vegetation layer, as is common in other beaver HSI s. The rationale for this was that beavers are generalist foragers and will adapt to food availability in each region. The areas that the HSI identified are generally broad, flat, wet areas, and as such, they generally contain deciduous species requisite for their diet. This differs from other project areas, such as an HSI developed in the arid southwest where vegetation was the primary driving factor of the model (Bird et al. 2011). The final product consisted of a GIS line layer with values ranging from 0 to 3 for each line segment. In general, as values increased towards 3, the suitability of a given line increased. Tulalip Tribes Page 2

3 Figure 1 Example image of the Skykomish beaver HSI model. Red is highly suitable, blue is moderate, yellow is low, and small blue lines are not suitable Ground truthing of HSI model Between July 2013 and July 2014 over 30 kilometers of HSI-mapped stream reach on US Forest Service lands were surveyed by field crews. Crews targeted areas with the highest HSI values first. These areas were systematically evaluated until most category ~3 s were visited. Subsequent ground truthing continued through reaches with lower values. Over 90 percent of highly suitable HSI reaches within 1 km of a road were evaluated. Over 50% of areas modelled as suitable, cat 1-3, within 1 km of a road were evaluated. Ground truthing of sites not included in HSI model The HSI model missed potential habitat in some cases. This became apparent through driving to different modeled reaches. Between sites, field crews identified locations that were not included in the model. These areas made up approximately 25% of the total stream length ground-truthed. The omission of these areas was primarily due to the coarse nature of the underlying data used to construct the model. For example the stream layer does not include all streams, present on the landscape, and in some cases, mapped streams are located in the wrong location. Had a wetland layer been available for this analysis, it would have identified some of these areas and likely many that we missed. Similarly, the LIDAR DEM base layer s large pixel resolution of 10m 2 was far too coarse to identify small stream channels and depressions appropriate for occupation. Site Assessment Survey sites were assessed for beaver current occupation (presence/absence), historical occupation, and potential suitability. This was done by surveying the site for various types of beaver sign and assessing the site using a site rating scorecard (see Appendix). The observations and indicators at each site are identified in Table 2. At most sites, it was fairly straight forward to verify current or historical occupation (Figure 2). It was relatively easy to identify new construction or maintenance on dams or lodge Tulalip Tribes Page 3

4 structures, which was the primary method of determining site occupancy. In a few cases, presence was uncertain, and the sites were investigated during differents season and flow conditions. Some reaches had very small amounts of sign, sometimes only a few stems of beaver chew and no site modification. In these instances, we first verified that this was not the demarcation of the extent of a colony range and then assumed that beavers were merely passing through these sites during emigration. The presence of small amounts of chew were noted, but did not directly affect subsequent site rating. Due to the coarse resolution of input data, some sites with low to moderate ratings were found to not be suitable (Figure 3). Sites not included in the HSI (Figure 4) were assessed in the same nature as all others and noted seperately for future model development. Table 2 Site assessment Observation Sign of active foraging Sign of active building Historic sign of activity Historic sign of building Beaver absent, suitable habitat Beaver absent, unsuitable habitat Indicator Newly cut twigs or trees Maintained dams or lodge (new mud or twigs) Weathered cut twigs or trees Dams or lodge in disrepair Habitat rating form score Habitat rating form score, or obvious indicators (e.g.high gradient) Figure 2 Site with high rating for suitability found to have an intact dam present Tulalip Tribes Page 4

5 Figure 3 Site with moderate rating found to have too high gradient for dam construction Figure 4 Image of site with high suitability not included in HSI model. Tulalip Tribes Page 5

6 Following site assessment, the results of field surveys was compared to the model using the rubric shown in Table 3. By combining modeled results, site condition, and occupancy status, we can make an informed assessment for the accuracy of the model, verified presence/absence, how much was missed by the model, and similar determinations. Table 3 Comparison of HSI model prediction, site condition, and site occupancy Code Site Modeled as Actual Site Condition Beaver Occupancy Status SSO Suitable Suitable Occupied SSU Suitable Suitable Unoccupied SUU Suitable Unsuitable Unoccupied UUU Unsuitable Unsuitable Unoccupied USU Unsuitable Suitable Unoccupied USO Unsuitable Suitable Occupied Frequency of occurrence NSO Not Modeled Suitable Occupied NSU Not Modeled Suitable Unoccupied Results Field Observations Site suitability and occupancy was assessed by field crews on over 28,000 meters of stream reach, much of which was targeted using the HSI. Results of field observations show that approximately 43% of suitable habitat was occupied by beaver (Table 4). Approximately 57% of suitable habitat was vacant. Crews determined that 8,250 meters of targeted stream reach were not suitable. Table 4 Observed conditions at stream survey reaches across Skykomish Watershed Observed condition Stream length (m) % of total survey % of Suitable habitat Suitable, Occupied 8, % 42.9% Suitable, Unoccupied 11, % 57.1% Unsuitable 8, % 28, % 100.0% Model Evaluation The HSI evaluated and predicted habitat suitability on over 2 million meters of mapped stream length in the Skykomish watershed (Table 5). Table 5 Modeled habitat suitability on all stream reaches Suitability Score length (m) 1 - Low 130, Moderate 168, High 292,005 All Suitable (1-3) 590,465 Not Suitable (0) 1,648,038 Total length evaluated 2,238,503 Tulalip Tribes Page 6

7 We field verified a non-randomized sample of HSI scores, focusing firstly on sites modeled as high suitability and working our way on through decreasing suitability. The field effort verified a little over 1% of the total modeled data (Table 6). Of the most informative data, those being cat 1-3, over 3% of all data were ground truthed. Table 6 Proportion of model field-verified to total modeled stream length Size of nonrandom subsample ground truthed Length (m) Total length modeled 2,238,503 Stream length ground truthed 28, % Total length of suitable streams in model 590,465 Suitable stream lengths (cat 1-3) ground truthed 20, % The model was assessed for accuracy in determining habitat suitability (Table 7). Of all stream lines rated, the model correctly predicted suitability approximately 60% of the time. While the model assessed suitability along stream lines contained within the stream layer only, we also report on areas of suitable habitat found outside of modeled areas. Of all areas field verified, over 24% was identified in areas outside of mapped stream lines, and consisted of suitable habitat. Table 7 Accuracy of model predictions determined through stream surveys Model accuracy Stream length (m) Model correct (suitable = suitable, unsuitable = unsuitable) 13, % Model incorrect (suitable suitable, unsuitable unsuitable) 8, % 21, % Suitable habitat missed by model (NA = suitable) 7, % 28, % Population Evaluation Table 8 utilizes field-verified and model-based results to extrapolate total habitat length occupide by and available to beaver within the Skykomish Watershed. A very rough population estimate is provided based on that habitat metric. To arrive at total occupied and available habitat, the proportion of occupied to suitable habitat, and available to suitable habitat was caluculated, respectively. These proportions were multiplied by the total legth of modeled suitable habitat to achive total occupied habitat and total available habitat in the basin. Total habitat was multiplied by a colony density of 1 colony/500m. Density is highly variable in the literature(gurnell 1998, Hood 2012, Naiman et al. 1988, Pollock et al. 2003, Rossel 2005); this density represents a moderate estimate and supports what was identified in field surveys. The number of beavers per colony is also highly variable in the literature (Bradt 1938, Hay 1958, Novak 1977, Svendsen 1980) and also dependant upon site conditions. An moderate family size of 6 was chosen; this would assume a breeding pair, 2-3 sub-adults,and 2-3 kits. The estimated current population is approximately 2000 beavers, and there is approximately capacity for 2000 additional beavers within areas modeled. It was not possible to extrapolate and predict occupied and available habitat in areas outside of modeled habitat. Tulalip Tribes Page 7

8 Table 8 Proportions of occupancy and area based on effort between modeled and non-modeled suitable habitat Occupied Stream length modeled suitable (m) Within sample Proportion of suitable habitat 31.5% Modeled as suitable (m) Basin-wide Total habitat Colonies Population estimate length (m) (1/500m) (6 beavers/colony) 186, ,234 20, ,465 Available 30.3% 178, ,145 Modeled carrying capacity 4,379 Discussion Habitat suitability index model applicability and interpretation The lack of a finer scale LIDAR DEM, more accurate stream hydrology layer, and related layers are currently the limiting factor of the Skykomish Beaver HSI model. In its current state, the model frequently misses small streams, small wetlands (<1 acre, but often greater), and side channel habitat.beyond model detection, finer scale spatial data would allow for field crews to target offchannel habitat for evaluation. Model Accuracy The HSI produced spurious results in some cases, such as inhospitable alpine lakes that had appropriate hydrology and gradient. This highlights the limitations of the model. The model does not take into consideration factors such as projected snow accumulation, which may be too great to accommodate beavers in some areas. The model was purposefully designed to be broad to capture all potential habitat, with the express intent of requiring follow-up ground-based exploration. While the reported model accuracy was approximately 60%, the true accuracy of the suitability assessment would be greater if it were assessed by category. For example, reaches categorized as highly suitable were very often found to be suitable. Reaches categorized as Low suitability seemed to be suitable about 50% of the time. We did not break our assessment down into categories as the sample size would have been too low to make appropriate conclusions. It should be mentioned that while some of the sites rated as unsuitable by the model were deemed suitable by field crew, none of those sites were occupied. Further attention should be given to analysis of the relationship between conditions at occupied sites and conditions at sites deemed as suitable, but unoccupied by reviewers. While the methodology for determining suitability is well developed at this point, there may still be some bias in that determination. Population Evaluation An attempt was made to produce a beaver population estimate and estimate of available habitat in the Skykomish watershed. These estimates are a very rough starting point, designed to initiate a dialogue. While the error rate is high, the actual population and carrying capacity are likely much higher than estimated. There are two reasons for this. First, the model missed many optimal habitats. In areas identified by field crews outide the model, the occupancy of sites nearly mirrored that seen in the model, 32.9% and 31.5%, respectively. There are likely many more areas on the landscape that are appropriate. Moreover, the majority of suitable areas found by crews were identified along road corridors. If this much beaver habitat has been identified within 100 meters of roads, there is likely a higher concentration beyond this distance, where conditions are more naturalized. The second reason Tulalip Tribes Page 8

9 that population levels, habitat, and carrying capacity are likely much higher than stated here, is that beavers not only create habitat for themselves, their dam-building activities create additional habitat for other beavers. As population levels continue to increase, potential beaver habitat will increase in an iterative fasion. The extent to which this will continue is beyond the scope of this study and is a ripe question for future studies. Citations Bird B., O Brien M., Petersen, M Beaver and Climate Change Adaptation in North America A Simple, Cost- Effective Strategy. Bradt, G. W A study of beaver colonies in Michigan. Journal of Mammalogy, 19(2), Gurnell, a. M The hydrogeomorphological effects of beaver dam-building activity. Progress in Physical Geography 22: Hay, K. G Beaver census methods in the Rocky Mountain region. The Journal of Wildlife Management, Hood, W. G Beaver in Tidal Marshes: Dam Effects on Low-Tide Channel Pools and Fish Use of Estuarine Habitat. Wetlands 32: Naiman, R., C. Johnston, and J. Kelley Alteration of North American streams by beaver. BioScience 38: Novak, M Determining the average size and composition of beaver families. The Journal of Wildlife Management, Pollock, Michael M., M Heim, D. W Beav Hydrologic and Geomorphic Effects of Bea v er Fishes Dams and T heir Influence on F ishes. American Fisheries Society SymposiumAmerican Fisheries Society Symposium. Rosell, F., O. Bozser, P. Collen, and H. Parker Ecological impact of beavers Castor fiber and Castor canadensis and their ability to modify ecosystems. Mammal Review 35: Svendsen, G. E Population parameters and colony composition of beaver (Castor canadensis) in southeast Ohio. American Midland Naturalist, Tulalip Tribes Page 9

10 Appendix Site Rating Scorecard Tulalip Tribes Page 10