Chapter 5

HumanEnvironment

Chapter 4BiologicalEnvironment

Chapter 3PhysicalEnvironment

Chapter 2ManagementDirection

Chapter 1Introduction andBackground

Appendices

Chapter 4

Biological Environment

© Margaret Tusko
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Chapter 4. Biological Environment 4-1 

Chapter 4 Biological Environment 

4.1 Overview 

4.1.1 Historic Land Use and Conditions 

The Willamette Valley is an area of extremely fertile and rich soils that when mixed with the 

abundant rainfall and relatively mild temperatures of the region provide an ideal system for healthy 

natural ecosystems (see Appendix P, Map 1). With the presence of people, these natural ecosystems 

changed (see Appendix P, Map 2). Prior to European settlement in the 1850s, the Kalapuya were the 

first people of the valley, and one band, the Atfálat’i, made their home here. At that time, much of 

the valley was covered by native grasses and forbs, with riparian forests and wet prairies along the 

rivers. The Atfálat’i observed the benefits of lightning-strike fires and emulated those benefits by 

manually setting fires to serve their needs. Each year, they burned different parts of the valley to 

control insect pests and to keep woody plants from moving into uplands of grasslands and oak 

savannas. Fire also cleared foraging ground for elk and deer. This method of prescribed burning 

added nutrients into the soil and stimulated plant growth. However, with European settlement, fire 

ceased to exist on a consistent and rotational basis, thereby allowing woody plant species to invade 

the grasslands and resulting in less than 1 percent of this habitat remaining in the valley. 

Historically, the Willamette Valley was home to large ungulates such as deer and elk. However, by 

1898 elk populations had diminished to the point that the Oregon Legislature prohibited elk hunting 

until 1910 (a ban that was later extended to 1933). Bears and wolves were present, and bighorn sheep 

lived nearby in the Columbia River Gorge. 

Sturgeon, steelhead, salmon, and other native fish were numerous in most rivers and streams; 

however, by 1898 fish stocks were declining and hatchery operations were underway (Oregon 

Department of Fish and Wildlife [ODFW] 2011b). Beaver, mink, and other wetland-dependent 

mammals found an abundant food source in the riparian forest of the Tualatin River. Trapping was in 

high demand here in the early 1800s, but declined as the demand for farmland increased. In 1899, a 

moratorium on beaver trapping was imposed, but it was lifted 20 years later. In 1931 beaver season 

was again closed as habitat conditions declined, and fish and wildlife resources continued to take a 

downward turn. 

By the 1850s, the grasslands and prairies had been claimed by settlers for grazing and town sites. 

This left riparian forest for farmers who were beginning to move west. To use the land, the farmers 

cut, burned, and plowed riparian forest for farmlands. Widespread burning of habitats was eliminated 

to protect infrastructure and farmlands. Lack of periodic fire allowed succession of shrubs and trees 

into remaining prairie, woodland, and savanna habitat types, which eventually changed many of them 

to Douglas-fir-dominated forest. 

Other habitats also saw modifications. Wetlands were drained for farming, and rivers and streams 

were channelized as flood control measures. Some areas were frequently flooded and were too wet to 

farm or develop and therefore were left as intact habitats. The flora and fauna of the valley began to 

change and support different species. This change continues as exotic species invade and once 

common native species decline. Christy et al. (2006) reported 1,404 plant species in the Portland area 

in 2006, and noted “today there are about half as many common native taxa as there were in 1925, 

and the number of rare natives has increased eleven-fold. Since 1925, the number of common exotic 
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4-2 Chapter 4. Biological Environment 

taxa has nearly tripled, while the number of rare exotics has increased by 71. This trend is to be 

expected in an urbanizing environment, and as time goes on, more native species will become rare 

while the ranks of common exotic species will continue to increase.” 

Currently the Tualatin River Basin is a highly altered system with almost all the land within the 

watershed affected, at least to some degree, by timber harvest, agriculture, urbanization, dams, 

diversions, and stream channel alteration. However, many organizations and individuals are working 

to restore natural processes and habitats. Many species of native plants, fish, and wildlife still thrive 

in the basin, and opportunities exist to enhance and restore additional habitat for greater resource 

enrichment. 

4.1.2 Biological Integrity, Diversity, and Environmental Health 

The National Wildlife Refuge System Improvement Act of 1997 directs the U.S. Fish and Wildlife 

Service (Service; USFWS) to ensure that the biological integrity, diversity, and environmental health 

of the National Wildlife Refuge System (Refuge System) are maintained for the benefit of present 

and future generations of Americans. To meet this mandate, the Service developed a Biological 

Integrity, Diversity, and Environmental Health (BIDEH) Policy to provide implementation guidance. 

Elements of BIDEH are represented by native fish, wildlife, plants, and their habitats as well as those 

ecological processes that support them. The Refuge System policy on BIDEH provides guidance on 

consideration and protection of the broad spectrum of fish, wildlife, and habitat resources that 

represent BIDEH on refuges and in associated ecosystems. This policy provides refuges with a 

process for evaluating the best management direction to prevent the additional degradation of 

environmental conditions and to restore lost or severely degraded environmental components. It also 

provides guidelines for dealing with external threats to the BIDEH of a refuge and its ecosystems. 

Maintaining and perpetuating the ecological integrity of the Tualatin River National Wildlife Refuge 

(the refuge) fits well within this policy. By providing for the full range of natural processes and 

native floral and faunal diversity, the refuge will be implementing this policy. 

To preserve BIDEH, we examined priority habitat types and determined the species that were the 

best indicators of the health of those plant communities. 

4.2 Selection of Priority Resources of Concern 

4.2.1 Analysis of Priority Resources of Concern 

Wildlife and habitat goals and objectives were designed directly around the habitat requirements of 

species designated as “priority resources of concern” (ROC). ROCs are called conservation targets in 

conservation planning methodologies used by other agencies and nongovernmental organizations 

(NGOs). In developing objectives, the comprehensive conservation plan (CCP) planning team 

followed the process outlined in the Service’s Writing Refuge Management Goals and Objectives: A 

Handbook (Handbook) (Adamcik et al. 2004). 

As defined in the Service’s policy on habitat management plans, ROCs are “all plant and/or animal 

species, species groups, or communities specifically identified in refuge purpose(s), System Mission, 

or international, national, regional, state, or ecosystem conservation plans or acts. For example, 

waterfowl and shorebirds are a resource of concern on a refuge whose purpose is to protect 
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Chapter 4. Biological Environment 4-3 

‘migrating waterfowl and shorebirds.’ Federal or State threatened and endangered species on that 

same refuge are also a resource of concern under terms of the respective endangered species acts 

(620 FW 1.4G).” The Handbook goes on to say that “Habitats or plant communities are resources of 

concern when they are specifically identified in refuge purposes, when they support species or 

species groups identified in refuge purposes, when they support NWRS [Refuge System] resources 

of concern, and/or when they are important in the maintenance or restoration of biological integrity, 

diversity, and environmental health.” Therefore, ROCs for a refuge may be a species or species 

group, or the habitat/plant community that supports a priority species/species group. 

In developing a list of ROCs, the planning team selected not only species mentioned in establishing 

documents for the refuge, but also species that captured the ecological attributes of habitat types 

required by larger suites of species (see Appendix E, Biological Resources Of Concern). The 

ecological attributes of habitats were analyzed with regard to the life history requirements of ROCs 

and are critical to sustaining the long-term viability of the ROC and other benefitting species. 

Ecological attributes of habitats include vegetation structure, species composition, age class, and 

patch size and/or connectivity with other habitat types; hydrologic regime; and disturbance events 

(e.g., flooding, fire). From the ROC list, we developed a list of focal species to represent each habitat 

type on the refuge. These provide measurable indicators that strongly correlate with the ability of a 

plant community to support a given species or group. The descriptions of habitat types on the refuge 

incorporate “desired” conditions that are based on scientific literature review and team members’ 

professional judgment. These desired conditions for specific ecological attributes were used to help 

design habitat objectives, as presented in Chapter 2. 

Limiting factors were also considered in developing objectives. A limiting factor is a threat to, or 

degradation of, the natural processes responsible for creating and maintaining plant and animal 

communities. In developing objectives and strategies, the team gave priority to mitigating limiting 

factors that presented a high risk to ROCs. In many cases, limiting factors occur on a regional or 

landscape scale and are beyond the control of individual refuges. Therefore, objectives and strategies 

may seek to mimic, rather than restore, natural processes. For example, pumps and water control 

structures may be used to control water levels in wetlands in areas where natural hydrology has been 

altered. Also, the structure of plant communities used by ROCs can be created, rather than restoring 

the original native species composition. For example, mowing and/or grazing may be used to 

maintain a desirable vegetation structure in restoring native grassland communities when prescribed 

fire is impractical. With the consideration of BIDEH, the refuge will maintain all appropriate native 

habitat types and species. Table 4-1 lists the focal and benefitting species that were considered using 

the following criteria: 

 The resource must be reflective of the refuge’s establishing purposes and the Refuge System 

mission; 

 The resource must include the main natural habitat types found at the refuge; 

 The resource must be recommended as a conservation priority during preplanning efforts; or 

 The resource must be federally or state listed, be a candidate for listing, or be a species of 

concern. 

Other criteria that were considered in the selection of the ROCs included: 

 Species groups and/or refuge features of special management concern; or 

 Species contributing to the biological diversity, integrity, and environmental health of the 

ecosystem or community. 
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Table 4-1. Focal Species and Other Benefitting Species for Each Priority Habitat Type 

Habitat Type Focal Species Other Benefitting Species 

Bottomland 

riparian forest 

Pacific slope flycatcher Yellow warbler, song sparrow, willow 

flycatcher, and rufous hummingbird 

Swainson’s thrush 

Beaver, northern red-legged frog, 

anadromous fish, yellow-breasted chat, and 

black-capped chickadee 

Brown creeper 

Chestnut-backed chickadee, anadromous 

fish, downy woodpecker, and pileated 

woodpecker 

Western wood pewee 

Wood duck, yellow warbler, warbling vireo, 

western screech owl, spring Chinook, winter 

steelhead, chorus frog, and hooded 

merganser 

Mixed 

coniferous/decidu

ous forest 

Orange-crowned warbler 

Bewick’s wren, spotted towhee, downy 

woodpecker, olive-sided flycatcher, and 

pileated woodpecker 

Oak savanna 

Western bluebird 

Chipping sparrow, coyote, grasshopper 

sparrow, mourning dove, rufous 

hummingbird, and western meadowlark 

White-breasted nuthatch Western gray squirrel, acorn woodpecker, 

black-tailed deer, and lesser goldfinch 

Wet prairie Wilson’s snipe 

American bittern, savanna sparrow, western 

meadowlark, northern harrier, dusky Canada 

goose, migrating and wintering waterfowl, 

and Nelson’s checker-mallow 

Herbaceous 

wetland 

Northern pintail 

Tundra swan, cackling Canada goose, 

wintering waterfowl, river otter, mink, 

northwestern pond turtle, bald eagle, and 

peregrine falcon (foraging) 

Greater yellowlegs 

Least sandpiper, dunlin, killdeer, lesser 

yellowlegs, long-billed dowitcher, spotted 

sandpiper, tree swallow, and great blue 

heron 

Virginia rail 

Wintering waterfowl, American bittern, 

sora, common yellowthroat, common garter 

snake, and chorus frog 

Shrub-scrub 

wetland Willow flycatcher 

American bittern, sora, Virginia rail, redwinged blackbird, common yellowthroat, 

northern red-legged frog, and northwestern 

salamander 

Rivers, streams, 

and backwater 

sloughs 

Coastal cutthroat trout, 

winter steelhead 

Northwestern pond turtle, mink, beaver, 

belted kingfisher, spring Chinook, and 

Pacific lamprey 

Cropland Cackling Canada goose 

Tundra swan, dusky Canada goose, northern 

pintail, American wigeon, northern harrier, 

and western meadowlark 
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4.3 Habitat Types 

4.3.1 Bottomland Riparian Forest 

4.3.1.1 Overview 

There are currently about 388 acres of riparian forest on the refuge supporting a myriad of breeding 

and migrating songbirds, amphibians and reptiles, resident mammals, and anadromous fish. Mature 

bottomland riparian forests are characterized by Oregon ash, bigleaf maple, scattered grand and 

Douglas-fir, and western red-cedar in the overstory, with sword fern, snowberry, and willow in the 

understory. There are many acres of this habitat type on the refuge that have been recently restored 

and would not have all these attributes yet. This forest type typically occurs in floodplains and other 

areas with moist soils. Plants here are adapted to winter and spring flooding and may be inundated or 

experience saturated soil for several months of the year. This habitat type is a priority habitat in the 

Oregon Conservation Strategy (ODFW 2006). It typically occurs along streams and rivers and 

provides movement corridors for wildlife among habitat patches. 

Photo 4-1. Riparian forest during winter flooding. Peter Schmidt/USFWS. 

4.3.1.2 Regional Distribution, Conditions, and Trends of Bottomland Riparian Forest 

Bottomland riparian forest in the greater Willamette Valley has been severely reduced since historic 

times due to draining and timber harvest, conversion to agricultural and urban uses, and stream 

alterations. Riparian forest within the Tualatin River Basin is highly fragmented. Metro, Clean Water 

Services, and others are working to restore riparian forest on many of their land holdings within the 

basin. The refuge has also engaged in restoration of bottomland riparian forest on several sites within 

the Sherwood Units. 
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4-6 Chapter 4. Biological Environment 

4.3.1.3 Key Species Supported 

Bottomland riparian forest supports a host of bird species and guilds including warblers, 

woodpeckers, raptors, flycatchers, and hummingbirds. Amphibians such as northern red-legged 

frogs, salamanders, and rough-skinned newts may spend part of their life cycles in riparian forests. 

Reptiles such as western pond turtles and garter snakes frequently use these habitats to aestivate over 

winter or to escape the hot summer sun. Mammals such as beaver, mink, black-tailed deer, and bats 

use this habitat type for foraging, resting, and finding escape cover. Anadromous fish such as 

steelhead and Pacific lamprey migrate up rivers and creeks shaded by riparian forest. In winter, 

juvenile salmonids may benefit from riparian forests for foraging and escape during flood events. 

4.3.1.4 Refuge Management Activities 

In existing stands of riparian forest, management activities are typically limited to controlling nonnative invasive plant species. Plants such as English ivy, nightshade, and Himalayan blackberry 

crowd out native plants, to which native wildlife are adapted. Non-native trees such as cherry, 

English hawthorn, and English holly are removed or girdled and treated with herbicide following 

integrated pest management (IPM) methods, to create snags. 

In areas of new riparian forest restoration, native trees and shrubs are planted and maintained for 

several years to ensure successful establishment. Trees are typically planted and protected with tubes 

to prevent browsing by deer and other herbivores, and girdling by voles and other rodents. Grasses 

and non-native plants are mowed and/or sprayed with Service-approved herbicide between shrubs 

and trees to reduce competition and shading until the desired plants are of sufficient size to compete 

on their own. After 5 to 10 years, once the trees have reached sufficient size and girth to resist 

damage from herbivores, tree tubes are removed. 

In some areas where natural recruitment of native plants is strong, management activities include 

placing tubes on naturally established trees and mowing non-native plants and grasses until the trees 

are tall enough to compete on their own. 

4.3.2 Mixed Coniferous/Deciduous Forest 

4.3.2.1 Overview 

There are currently about 49 acres designated as mixed coniferous/deciduous forest on the refuge. 

This habitat type is typically above the floodplain on drier, well-drained soils. This forest type is 

dominated by Douglas-fir, with grand fir, western red-cedar, western hemlock, bigleaf maple, red 

alder, Pacific yew, and Pacific dogwood. The understory contains sword fern, snowberry, hazel, 

cascara, vine maple, and various other shrubs and forbs. Snags are an important habitat component in 

a mature forest. Snags provide foraging substrate and nesting cavities for a variety of bird species. 

This upland forest type supports a host of bird species and other wildlife. 
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Photo 4-2. Mixed coniferous/deciduous forest in the Rock Creek Unit. Peter 

Schmidt/USFWS. 

4.3.2.2 Regional Distribution, Conditions, and Trends of Mixed Coniferous/Deciduous 

Forest 

Historically, upland forest was the dominant habitat type in the greater Portland area (Christy et al. 

2006). By the time the General Land Office surveys were undertaken in the 1850s, much of this 

habitat type was already burned or logged. In the area surrounding the refuge, there is no old-growth 

forest remaining, but there are some tracts of mature second growth with intact native understory 

plants and few invasives. The forests surrounding the refuge were logged during the late nineteenth 

and early twentieth centuries to provide lumber for an expanding population and to provide more 

agricultural land. Currently, remaining forest tracts occur in areas too steep to farm or build 

effectively, or on land managed for timber production. Some tracts of remaining forest are found on 

conservation lands such as those recently purchased and protected by Metro and others. This forest 

type has also expanded into areas that were historically oak woodland, oak savanna, or prairie, as a 

result of fire suppression. 

4.3.2.3 Key Species Supported 

A myriad of species spend all or part of their life cycles in mixed forest habitat types. A host of 

resident and migratory birds such as woodpeckers, owls, wood warblers, and corvids use mixed 

forests for foraging, resting, and nesting. Small mammals such as squirrels, shrews, and mice are 

abundant in mixed forest habitat types, and bats often roost in cracks and cavities of larger trees. 

Larger mammals such as black-tailed deer and coyotes are also present in smaller numbers. Reptiles 

and amphibians such as common garter snakes and chorus frogs also use this habitat type. 

4.3.2.4 Refuge Management Activities 

Management activities for this habitat type are similar to activities carried out in riparian forest. In 

established forest, management of invasive species is the primary activity, while maintenance of 
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4-8 Chapter 4. Biological Environment 

planted trees and shrubs will be the focus in restoration areas. In addition, prescribed fire may be a 

management tool employed to manage fuel and provide a diversity of habitat structure. 

4.3.3 Oak Habitat 

4.3.3.1 Overview 

There are currently about 149 acres of oak habitat on the refuge. This land is in various stages of 

restoration. Restored areas have been planted with oaks, ponderosa pine, native grasses, and other 

native species during the past 12 years. The oaks in these areas are in the sapling stage and range 

from 3 to 20 feet in height; they have not reached a mature stage. In some areas the ponderosa pines 

have grown quickly and may be reaching 30 feet in height and almost 12 inches DBH. Only a few 

relic oaks over 100 years old remain on refuge lands. Mature oak savanna is characterized by large 

single or a very small cluster of Garry oaks, generally widely scattered throughout a grassland with 

few to no shrubs and abundant native wildflowers. Oak/pine woodland is characterized by more 

densely growing trees with a scattering of shrubs in the understory. Snags may be present in small 

numbers, and areas of bare ground may also be present. Native grass species may include California 

oatgrass, California brome, meadow barley, Roemer’s fescue, and blue wildrye. Native forb species 

include blue-eyed grass, tiger lily, tarweed, and pearly everlasting. Overall grass height is usually 

less than 3 feet. Sites are typically dry, but may have saturated soils for a short duration during 

winter. This habitat type was historically maintained by frequent, low-intensity fire, which prevented 

most shrubs and encroaching species, especially conifers, and maintained a low density of oaks. 

Currently the refuge manages this habitat type by annual mowing. However, mowing does not 

remove accumulated thatch, which may inhibit growth of some species of wildflowers. Spot spraying 

of herbicides is also employed to control invasive species such as Himalayan blackberry and Canada 

thistle. This habitat type is a priority habitat in the Oregon Conservation Strategy (ODFW 2006). 
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Photo 4-3. Garry oaks. © Ed Bustya.

4.3.3.2 Regional Distribution, Conditions, and Trends of Oak Savanna 

Oak savanna is a rare habitat type in the Willamette Valley. Noss et al. (1995) reported a loss of 99.5 

percent of this habitat type in the Willamette Valley since Euro-American settlement. Metro and 

other organizations are conserving and restoring this habitat type; however, few large areas with 

mature oaks remain in the Tualatin River Valley and distribution is fragmented. Restoration efforts 

may take up to 100 years before the habitat can support a full range of species. 

4.3.3.3 Key Species Supported 

Oak savanna supports a variety of species, especially grassland bird species, oak specialists, 

invertebrates, and listed plant species. Grassland birds such as western meadowlark and savanna 

sparrow nest and forage in grassland communities. Oak specialists such as acorn woodpecker, Lewis’ 

woodpecker, white-breasted nuthatch, and western gray squirrel use oaks for nesting and foraging. 

Invertebrates such as Fender’s blue butterfly rely on native savanna and prairie plants for survival. 

Listed plant species native to oak savanna habitats include Kincaid’s lupine, Willamette daisy, and 

golden paintbrush (USFWS 2010b). Western pond turtles frequently use this habitat type for 

aestivating and overwintering. 

4.3.3.4 Refuge Management Activities 

The refuge has undertaken several oak savanna restoration projects and annually maintains these 

areas by mowing to reduce encroachment of woody species and by applying Service-approved 

herbicides, following IPM practices, to control non-native and invasive species. As land has been 

acquired, restoration activities have included cooperative farming as a means of purging weeds 
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through tilling and application of herbicide. Further application of herbicide is usually necessary 

prior to planting of native seed. Native seed may be drilled in using a no-till drill or may be broadcast 

following discing. Typically, native grasses are planted first and allowed to become established while 

using a broad-leaved-specific herbicide to control invasive species. Once native grass is established, 

native forbs are seeded. Research is still needed to determine best management practices for 

establishing native oak savanna plants in this area because restoration of this habitat type is still in its 

infancy. 

4.3.4 Wet Prairie 

4.3.4.1 Overview 

Wet prairie covers about 27 acres on the refuge. Wet prairie is dominated by native herbaceous plants 

such as sedges, rushes, grasses, and forbs that are generally less than 3 feet tall. There are few to no 

trees or shrubs. Wet prairie is characterized by generally poorly drained soils that may be saturated or 

have standing water from November through April. These habitat types on the refuge are typically 

inundated by either back flooding of the river onto floodplain areas or by ponding of rainwater in 

shallow basins. There are currently two areas of wet prairie restoration on the refuge. One was 

planted in 2001-2002 with native grasses, oaks, and ponderosa pine. Plants in this area are well 

established. The other area was restored in 2005, but is currently being rehabilitated to remove 

invasive reed canarygrass. This habitat type is a priority habitat in the Oregon Conservation Strategy 

(ODFW 2006). 

Photo 4-4. Wet prairie habitat, Atfálat’i Unit. Kim Strassburg/USFWS. 

4.3.4.2 Regional Distribution, Conditions, and Trends of Wet Prairie 

Similar to oak savanna, this habitat type has undergone a dramatic decline since Euro-American 

settlement due to conversion to other uses and lack of fire, which has allowed the encroachment of 

woody species, thus changing the vegetation community. Noss et al. (1995) state that 99.9 percent of 
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all prairie habitats in the Willamette Valley have been lost. Much of the habitat that remains is in 

degraded condition due to the influence of invasive and non-native species and lack of periodic fire 

to properly maintain function. Further, restored sites are confounded by a lack of available native 

seed types to foster adequate diversity. However, more native seed and plant varieties are becoming 

available as more entities embark on restoration of wet prairies. As with other rare habitat types, 

Metro, Natural Resources Conservation Service, and other organizations are conducting restoration 

and enhancement of this habitat type throughout the Tualatin River Valley. As these projects 

proceed, knowledge is gained and shared about methods and tools used to effectively manage wet 

prairie habitats. 

4.3.4.3 Key Species Supported 

Wet prairie supports a number of listed plant species (Bradshaw’s lomatium, Willamette daisy, 

Nelson’s checker-mallow) and species of concern (upland larkspur, peacock larkspur, Sierra 

horkelia, Columbian whitetop aster) (USFWS 2010b). A number of other species use wet prairies 

during all or part of their life cycles. Chorus frogs, northern red-legged frogs, northwestern and longtoed salamanders, and rough-skinned newts breed and lay eggs in shallow, vegetated wet areas in 

spring; wading birds such as great blue herons and great egrets also forage in the shallow pools or 

dry margins of wet prairies. Wilson’s snipe nest in the wet grasses of the prairie. Black-tailed deer 

forage on grasses and forbs, while mink hunt for birds and rodents. In winter, large numbers of 

waterfowl rest and forage on the numerous seeds and plants in the flooded prairie. 

4.3.4.4 Refuge Management Activities 

Refuge management includes restoration, enhancement, and maintenance of wet prairie. As with 

other habitat types, restoration usually begins with efforts to purge non-native and invasive plant 

species using mechanical means (mowing and discing) and/or application of Service-approved 

herbicide, following IPM practices. Plants may be installed as pots or plugs, or seeded using a no-till 

drill or broadcast method. Areas requiring enhancement may have plants installed as mentioned 

above and invasive species removed using mechanical means or spot spraying with herbicide. 

Current maintenance includes annual mowing to prevent encroachment of woody species and/or spot 

spraying with herbicide to control non-native and invasive species. 

4.3.5 Herbaceous and Scrub-shrub Wetlands 

4.3.5.1 Overview 

There are currently about 294 acres of herbaceous wetland and 20 acres of scrub-shrub wetland 

habitat on the refuge. The herbaceous wetlands have all been restored, but the scrub-shrub wetlands 

are in various early stages of restoration. According to Christy et al. (2006), prior to Euro-American 

settlement, there were historically few wetlands in the vicinity of the refuge. Wetlands that existed at 

the time of Euro-American settlement were often drained by ditching, diversions, channelizing 

streams, and installing drain tiles to facilitate farming and development. The floodplain in the 

northern part of the Riverboat Unit had extensive scrub-shrub and herbaceous wetlands that were 

likely drained by the early twentieth century. Likewise, the majority of the Wapato Lake basin 

consisted of various wetland types. Even though they represent a small fraction of the landscape, 

wetlands play a vital role in the ecosystem, providing habitat for a variety of wetland-dependent 

plants and wildlife, and native resident and anadromous fish. 
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Herbaceous wetlands are characterized by areas with heavy, poorly drained clay soils. Wetlands may 

be saturated or flooded for a large part of the year, usually from the first fall rains until early to midsummer. Water depths may range from just a few inches to 3 feet or more, with an average of 4 to 18 

inches. The extent and depth of water is dynamic, changing throughout the season and year to year as 

periods of rain and flooding from creeks and rivers come and go. Plant composition includes 

perennial species such as sedges, rushes, wapato, common cattail, and softstem bulrush, and a large 

variety of annual plants such as smartweed, beggars tick, and wild millet. 

Scrub-shrub wetlands are characterized by a somewhat more stable hydrological regime. Soils may 

range from heavy clay to organic peat soil, and water may be present year-round. Water is typically 

deeper than on herbaceous wetlands and may range from 1 to 10 feet deep at times. Plants include 

woody species dominated by willow, as well as Douglas spirea, swamp rose, red-osier dogwood, and 

emergent species such as cattails and softstem bulrush. Fewer annual plants would be expected in 

this habitat type. This habitat type is a priority habitat in the Oregon Conservation Strategy (ODFW 

2006). 

Photo 4-5. Herbaceous wetland plants provide habitat and food for wintering 

waterfowl. Kim Strassburg/USFWS. 

4.3.5.2 Regional Distribution, Conditions, and Trends of Herbaceous and Scrub-shrub 

Wetlands 

As noted above, considerable effort was historically expended to convert wetlands to human-oriented 

uses. However, the value of wetlands for many beneficial uses, and their intrinsic value, has been 

recognized, and substantial work has been accomplished to restore or create wetlands throughout the 

Tualatin River Basin for the benefit of both human endeavors and wildlife needs. Wetlands currently 

exist on every unit of the refuge. Many of these wetlands are intensively managed to mimic historic 

conditions by using water control structures, diversions, pumps, canals, and levees. Managing and 

maintaining these systems is expensive and time consuming, but provides a level of certainty that 

habitat would be available no matter what the seasonal climate might provide. Other wetlands are not 

managed and depend on seasonal rain to fill and evaporation to naturally draw down in summer. 
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In addition to wetlands on the refuge, Metro, Clean Water Services, local duck hunting clubs, and 

others have worked to restore or create wetlands in the Tualatin River Basin. These wetlands provide 

a host of benefits such as groundwater recharge, water quality benefits, dissipation of flood waters, 

and habitat for fish and wildlife. 

4.3.5.3 Key Species Supported 

Wetlands support a number of fish and wildlife species throughout their annual cycle. Marsh birds 

such as sora and Virginia rail, and songbirds such as common yellowthroat and marsh wren nest in or 

near wetlands. Mammals such as mink, muskrat, and beaver use wetlands. Amphibians such as 

chorus frogs, northern red-legged frogs, and salamanders use wetlands for breeding. Reptiles such as 

common garter snake and western pond turtle use wetlands during part of their annual life cycle. 

Migratory shorebirds such as least and western sandpipers, greater yellowlegs, and black-bellied 

plovers forage on wetland mudflats and margins. Waterfowl such as northern pintails and cackling 

Canada geese forage and rest on wetlands. Raptors such as bald eagles and peregrine falcons prey on 

waterfowl that use wetlands. 

4.3.5.4 Refuge Management Activities 

Of all habitat types encountered on the refuge, wetlands require the most management and 

maintenance. During fall and winter, water is managed to fill and maintain wetlands. Fish screens are 

maintained to allow water passage and to prevent both native and non-native fish from entering 

wetlands from diversions. Pumps are sometimes operated to begin filling wetlands prior to fall rains. 

Water control structures such as flashboard risers are managed to precisely control water levels in 

some wetlands. Supplemental planting of woody species such as willow is conducted in newly 

restored scrub-shrub wetlands. During spring, water levels in managed wetland basins are drawn 

down to stimulate growth of desirable wetland plant species. Invasive species may be controlled by 

timing of flooding, mechanical means such as mowing or discing, or application of Service-approved 

herbicide. In late summer, some wetland basins are mowed to reduce vegetation height at the wetland 

margin to allow waterfowl access to adjacent uplands. Some wetlands are periodically disced to 

stimulate growth of annual plants dependent on a disturbance regime. In newly restored scrub-shrub 

wetlands, mowing is conducted around woody species to reduce competition with invasives such as 

reed canarygrass. 

4.3.6 Rivers, Streams, and Backwater Sloughs 

4.3.6.1 Overview 

The Tualatin River and several tributary streams run through or are adjacent to refuge lands (see 

Appendix P, Maps 6 and 7). In addition, several areas back up with water during periods of flooding 

or high water. These waterways have been impacted by alterations since the time of Euro-American 

settlement. Stream banks were fortified to prevent flooding; adjacent lands were drained and ditched, 

causing increased runoff and erosion; vegetation along stream banks was removed, reducing the 

amount of shade and resulting in increased water temperatures, reducing woody input, and causing 

additional erosion; timber harvest, farming activities, and urbanization caused increased runoff, 

chemical and trash inputs, and reduced groundwater recharge; and water diversions for industrial, 

agricultural, and municipal uses caused a reduction in water flow. 
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4-14 Chapter 4. Biological Environment 

Tributary streams such as Chicken Creek and South Rock Creek in the Sherwood Units were 

channelized in the early twentieth century to facilitate drainage and farming of adjacent lands. Under 

the management direction, the refuge will restore at least parts of these creeks to their natural 

meandering channels. In addition, construction and/or reconnection of backwater sloughs will be 

explored on some refuge units. This habitat type is a priority habitat in the Oregon Conservation 

Strategy (ODFW 2006). 

4.3.6.2 Regional Distribution, Conditions, and Trends of Rivers, Streams, and 

Backwater Sloughs 

The Tualatin River and its tributaries have total maximum daily loads (TMDL) listed by the Oregon 

Department of Environmental Quality (DEQ) for several water quality parameters including 

temperature, dissolved oxygen, phosphorus, and bacteria (DEQ 2001). Efforts have been underway 

for several years in the Tualatin River Basin to reverse some of the negative effects that have taken 

place over the past 150 years. Many organizations have been promoting stewardship of the watershed 

and are engaged in restoration and enhancement projects that have greatly improved conditions in the 

river and local streams. The Service has partnered with the U.S. Geological Survey (USGS) and 

others to conduct a water quantity and quality study at Wapato Lake that will help inform and guide 

future refuge management activities at that unit. A few small backwater areas on the refuge have 

been enhanced to provide benefits to anadromous fish during flood events and provide benefits to 

native wildlife. 

Photo 4-6. Backwater flooding along the Tualatin River. Peter Schmidt/USFWS. 

4.3.6.3 Key Species Supported 

Rivers, streams, backwater sloughs, and their streamside vegetation support a vast array of fish, 

wildlife, and plant species. Native resident and anadromous, and non-native fish species, may be 

found in local waters. Resident native fish include coastal cutthroat trout, rainbow trout, western 

brook lamprey, and three-spined stickleback. Native anadromous fish include steelhead and Pacific 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 4. Biological Environment 4-15 

lamprey. Non-native fish include a number of sunfish species, bass, catfish, mosquito fish, and carp. 

Coho salmon, which were not historically present in the Tualatin River system because they could 

not get over Willamette Falls during the fall when they migrate, were introduced into the system as 

mitigation for Scoggins Dam. The coho stocking program ended in 1998, but coho are still present 

and reproducing with help from fish ladders at the falls (ODFW 2011c). 

Wildlife also abounds in and along waterways. Mammals such as beaver, mink, and muskrats are 

water-dependent furbearers. Birds of all types are associated with streams and rivers including 

mergansers, wood ducks, belted kingfishers, and ospreys. Many warblers and flycatchers are 

associated with streamside riparian habitats. Reptiles such as western pond turtles and painted turtles 

use the slow-moving lower reaches of the Tualatin River, often basking on logs in spring and 

summer. Amphibians such as rough-skinned newts are abundant in all waters of the refuge. 

Many plant species thrive in and around refuge streams. Submergent vegetation such as pondweeds 

and wigeon grass grow in the slow-moving waters of the river and adjacent sloughs. Trees and shrubs 

commonly associated with streams include willow, Oregon ash, red-osier dogwood, Oregon grape, 

and Douglas spirea. 

4.3.6.4 Refuge Management Activities 

One of the major management activities related to streams is operation of a water diversion structure 

on Chicken Creek on the Atfálat’i Unit. A portion of the water in Chicken Creek is diverted yearround to supply water to managed wetlands. In summer, a small maintenance flow is diverted to 

maintain freshwater flow to a minimal number of wetland acres. In fall and winter, more water is 

diverted to fill and maintain approximately 226 acres of wetlands. There is a fish passage structure 

associated with the diversion structure that was designed prior to listing of upper Willamette salmon 

and steelhead, and it may no longer be adequate based on current standards. There are fish screens on 

the structure to help prevent fish from entering the wetland system and becoming entrapped. 

Upstream and downstream of the diversion, the creek was channelized decades ago and runs straight 

to the Tualatin River. Under the management direction, Chicken Creek will be at least partially 

returned to its historical channel and a new diversion structure will be constructed in a different 

location and designed to current fish passage standards. 

Similar to Chicken Creek, South Rock Creek is channelized through both the Rock Creek and Onion 

Flats Units. Under the management direction, this creek will also be restored. 

Management of water in the Wapato Lake Unit is another important activity. Currently the refuge is 

maintaining cooperative farming operations in the lake bed and using the pumps in place to dewater 

the lake during spring. Until restoration begins, it is anticipated that this practice will continue. 

Other management actions along streams include planting native vegetation, and controlling nonnative and invasive species including reed canarygrass, English holly, and Himalayan blackberry. 

4.3.7 Croplands 

4.3.7.1 Overview 

Cropland management is used as a tool when lands are acquired to provide wildlife habitat and 

control invasive species until restoration of the property to native habitats can commence. Croplands 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-16 Chapter 4. Biological Environment 

are currently managed under cooperative farming agreements with local farmers who plant, maintain, 

and harvest select crops. A share of the crop is left for wildlife use. Crops grown include cereal 

grains, such as corn, wheat, barley, and oats. In the case of cereal grains the grower is required to 

leave 30 percent of crops standing as forage for wildlife. Another crop grown is green forage, such as 

pasture grass for hay or clover for seed. In this case, the grower harvests all the crop and then mows 

the area in late summer or early fall to provide green forage primarily for geese. 

The refuge currently has cooperative farming agreements on the Onion Flats and Wapato Lake Units. 

Crops on the Onion Flats Unit are naturally flooded as fall rains begin and standing water 

accumulates or as the Tualatin River backs up into this area. Similarly, Wapato Lake starts to fill 

following fall and early winter rainy periods. Typically waterfowl will wait until rains begin before 

they begin foraging in grain fields, but they will begin to use green forage as soon as they arrive in 

fall. During periods of drought, when fields may not become flooded, waterfowl will begin to forage 

in dry grain fields after other sources of food in the vicinity have been depleted. 

4.3.7.2 Regional Distribution, Conditions, and Trends of Croplands 

The refuge is surrounded by a mixture of urban and rural land uses. There are many farms in the area 

supporting a variety of crop types including small grains and green forage. The primary difference in 

private management is that most small grains are tilled under after harvest in early fall to prepare for 

the next crop and green forage areas may also be tilled under if on a rotational schedule. However, 

many fields retain forage values for wildlife whether tilled or not. Many farmers rotate crops from 

year to year, and some crops may not be attractive to wildlife. Landowners may also convert field 

crops to orchards, berry crops, or nursery stock, which have different values for wildlife. 

4.3.7.3 Key Species Supported 

Croplands primarily support foraging for waterfowl such as dabbling ducks and grazers such as 

Canada geese and American wigeons. Other birds commonly observed in this habitat type include 

great blue herons, shorebirds, and grassland songbirds. Many small rodents such as Oregon voles 

reproduce here, providing a prey base for coyotes and raptors such as northern harriers, red-tailed 

hawks, and American kestrels. 

4.3.7.4 Refuge Management Activities 

Management activities are typically limited to overseeing cooperative farming activities and, in the 

case of the Wapato Lake Unit, managing water levels necessary for planting and irrigation. 

4.3.8 Ruderal Lands 

4.3.8.1 Overview 

Currently there are approximately 181 acres of ruderal uplands and 68 acres of ruderal wetlands on 

the refuge. Ruderal uplands are lands that are typically above the annual floodplain but may 

experience occasional flooding from extreme events. These lands have been fallow for some time 

and typically are dominated by non-native vegetation. Ruderal wetlands are lands typically within the 

annual floodplain, or they are saturated or inundated during some part of the year. These lands were 

often former pasture lands used for summer grazing. 
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Chapter 4. Biological Environment 4-17 

4.3.8.2 Regional Distribution, Conditions, and Trends of Ruderal Lands 

Ruderal lands in the surrounding area are usually limited to farm field edges or transitions between 

habitat types that remain unmanaged. Occasionally, growers will abandon a field or orchard and 

weeds will invade, creating a ruderal condition. If the field is left unmanaged for some time, native 

species may pioneer into the area and begin to transform the tract to a mix of native and non-native 

species. 

4.3.8.3 Key Species Supported 

Ruderal lands may support a number of species such as great blue herons, sparrows, and grassland 

songbirds. Many small rodents such as Oregon voles reproduce here. Raptors such as red-tailed 

hawks and American kestrels may forage here. 

4.3.8.4 Refuge Management Activities 

Many of these lands have undergone pretreatment activities prior to restoration. Pretreatment 

activities may include removal of non-native trees, mowing, discing, or application of herbicide. It is 

the refuge’s intention to convert ruderal lands to native habitat types. 

4.4 Major Species Groups 

A wide variety of fish and wildlife use refuge habitats during all or part of their life. Appendix E 

contains a listing of ROCs that inhabit the refuge, and Appendix N is a listing of all known plants, 

fish, and wildlife that have been documented on the refuge. Large numbers of waterfowl use refuge 

wetlands during fall and winter, and some remain to breed during spring and summer. Shorebirds and 

marsh birds use the refuge to some extent year-round, but peak numbers typically occur during 

spring. Some species in these guilds breed on refuge lands or nearby, while others are migrants. Both 

resident and migrating songbirds use all refuge habitat types. Many species of neotropical migrant 

songbirds breed here during spring and summer, and many species of raptors use the refuge during 

most seasons of the year. Mammals, amphibians, and reptiles are present on the refuge year-round, 

but some of these species are secretive and may not be readily observed during most of the year. 

Many species of fish, some of which are migrants, inhabit waterways on and adjacent to the refuge. 

4.4.1 Waterfowl 

Providing high-quality habitat for waterfowl has been one of the priorities on the refuge since its 

establishment. Waterfowl are primarily associated with wetlands; however, they also use upland 

grasslands and croplands. Detailed weekly winter waterfowl surveys have been conducted on the 

refuge since 2000, and the area surveyed has increased over time as land has been acquired and/or 

restored. Surveys are conducted from September through at least February each year. Typically, few 

numbers and species of waterfowl breed on the refuge and surrounding areas. A limited number of 

western Canada geese, mallards, gadwalls, cinnamon teal, hooded mergansers, and wood ducks 

annually raise a few broods. Buffleheads, northern shovelers, and blue-winged teal are rare breeders 

here. In early fall, waterfowl numbers remain low as migration begins (Figure 4-1); their numbers 

then increase sharply during November. Numbers remain relatively high to the end of January when 

waterfowl hunting season ends, and then birds tend to disperse to nearby private lands before 

migrating north near the end of March. The refuge lies in the Pacific Flyway, where birds migrate 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-18 Chapter 4. Biological Environment 

primarily from breeding grounds in Alaska, British Columbia, Alberta, the Yukon, and Northwest 

Territories, and to a lesser extent from Saskatchewan. For many of these birds, the refuge is an 

important stopover to wintering areas in California and Mexico, and for others it is their southern 

terminus, where they spend the winter. 

Photo 4-7. Waterfowl. Gary Kramer/USFWS. 

Average and Maximum Number of Waterfowl Counted 

2004-2011

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

25-Aug 25-Sep 26-Oct 26-Nov 27-Dec 27-Jan 27-Feb 29-Mar

Date

Waterfowl Count

Average Ducks

Average Geese

Max Ducks

Max Geese

Figure 4-1. Average and maximum number of waterfowl counted on refuge lands during 

regular weekly surveys from 2004-2011. 

Note: On December 16, 2010, a maximum of 71,601 ducks were counted, but this data point was 

removed to make the chart more readable. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 4. Biological Environment 4-19 

4.4.2 Songbirds 

The refuge supports a host of both resident and migratory songbirds that use all of the habitat types 

represented here. About 100 songbird species have been identified on the refuge and many of those 

species breed here. Point count surveys have been conducted to identify species using the refuge, and 

a bird banding program has been established under the Monitoring Avian Production and Survival 

protocol to assess productivity and survival of locally breeding birds. These surveys will help to 

determine long-term trends in songbird populations on both a local and regional scale. 

4.4.3 Shorebirds 

There are relatively few shorebirds that use the refuge, and species diversity is low. Twenty-one 

species have been recorded, but many of those are one-time or rare sightings. Killdeer, spotted 

sandpipers, and Wilson’s snipe are among the most numerous; all of them breed at the refuge. Least 

and western sandpipers, dunlin, lesser yellowlegs, and long-billed dowitchers are also commonly 

sighted during migration. Regular weekly surveys are conducted from April through September on 

several of the Sherwood Units. Weekly counts average fewer than 50 individuals, but may number 

several hundred on rare occasions. Although some shorebirds do breed here, the majority of 

shorebirds are migrants, passing through during spring and late summer and fall. Typically, mudflats 

in wetland margins are available for shorebird foraging from early spring through summer as 

managed wetlands are drawn down on their annual cycle. 

Photo 4-8. Long-billed dowitcher. © Ed Bustya. 

4.4.4 Marsh Birds and Waders 

Marsh birds use both herbaceous and scrub-shrub wetlands, and to a lesser extent adjacent uplands. 

Surveys for sora, Virginia rail, American bittern, American coot, and pied-billed grebe are conducted 

using recorded calls from April through June each year. Breeding has been verified on the refuge for 

each of these species, although they occur in relatively low numbers. Early spring drawdown of 

wetlands to meet other management objectives may have a negative effect on these species. The first 
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4-20 Chapter 4. Biological Environment 

confirmed breeding of American bitterns on the refuge was observed at a recent scrub-shrub wetland 

restoration site. 

Wading birds such as great blue herons, great egrets, and green herons are surveyed in conjunction 

with shorebird and waterfowl surveys. Great blue herons and great egrets are present nearly yearround on the refuge, while green herons are usually only observed during summer. 

4.4.5 Raptors 

At least 18 species of raptors have been recorded on the refuge including eagles, hawks, falcons, and 

owls. Bald eagles, red-tailed hawks, American kestrels, and great horned owls are year-round 

residents. Other species such as peregrine falcons are usually only present during winter when 

waterfowl populations are high. Some species are regular summertime visitors, such as ospreys that 

forage on fish as wetlands are drawn down. Raptors are specialized in the habitat types that they use 

and may be observed in most habitat types on the refuge. Eagles and peregrine falcons use wetlands 

for hunting waterfowl. Great horned owls hunt rodents in the forest at night while Cooper’s hawks 

hunt in the forest during daylight in search of songbirds. Northern harriers hunt in the uplands, 

croplands, and wetlands during the day, while barn owls hunt open fields during the night and 

evening. The refuge has installed American kestrel nesting boxes and will monitor these from time to 

time to determine nesting success. There are no other surveys that specifically target raptors, 

although raptors are recorded during songbird point count surveys. 

Photo 4-9. American kestrel. Robert Burton/USFWS. 

4.4.6 Mammals 

Many mammal species, large and small, use the various habitat types on the refuge. Large mammals 

such as black-tailed deer and the occasional elk are observed on the refuge. Smaller mammal species 

such as coyote, river otter, bobcat, raccoon, beaver, muskrat, and mink are present on the refuge, but 

are less frequently observed. No regular surveys are conducted for the above-mentioned groups of 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 4. Biological Environment 4-21 

mammals. Smaller, yet more numerous, are squirrels, voles, mice, and shrews. Bats are also known 

to use refuge lands, but no surveys have been conducted to determine species present. From 2003 to 

2005, a small mammal survey was conducted, which identified 14 species on one parcel. By far the 

most numerous species in this study was deer mouse. 

Photo 4-10. Black-tailed deer at Atfálat’i Unit. Peter Schmidt/USFWS. 

4.4.7 Fish 

Numerous fish species have been noted using the waters in and adjacent to refuge lands. The refuge 

has conducted fish trapping in refuge wetlands following river flood events to determine if salmonids 

were entering wetlands from the river and subsequently escaping as water levels receded. No 

salmonid species were detected during any surveys. Several species of both native and non-native 

fish were detected during these surveys. Native species included western brook lamprey, three-spined 

stickleback, and sculpin. Non-native species included carp, mosquito fish, and bluegill. From 1999 to 

2001, ODFW conducted fish surveys in the Tualatin River Basin (Leader 2001) and recorded 16 

species of fish from seven families. The survey noted that only 0.16 percent of the total sample were 

non-native fish. 

Upper Willamette River steelhead were listed as threatened species in August 1999 (Federal Register 

1999a) and Chinook salmon were listed as threatened species in March 1999 (Federal Register 

1999b), and critical habitat was designated in 2005 (Federal Register 2005) in the upper Tualatin 

River Basin (roughly defined as the Tualatin River and its tributaries upstream of Highway 219 near 

Hillsboro), excluding North Rock Creek. Leader (2001) identified both rainbow trout (genetically 

identical to steelhead) and Chinook salmon in the Tualatin River system. Coho salmon, while not 

native to the Tualatin River system, were introduced and are now a self-sustaining population in the 

basin. 

Non-native fish such as carp, bass, catfish, and sunfishes are disruptive to and can outcompete native 

species. Carp uproot and consume submergent vegetation important to native species and cause 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-22 Chapter 4. Biological Environment 

turbidity, thus inhibiting further growth of vegetation. Bass, catfish, and sunfishes directly consume 

native species, such as red-legged frogs, and also indirectly affect native species by consuming food 

such as invertebrates that native species forage upon. 

Native fish species such as lampreys, sculpin, and three-spined sticklebacks have also been recorded 

in refuge waters and are an important part of the aquatic community. The presence of lampreys is a 

good indicator of water quality as they will not persist in poor-quality habitats. Three-spined 

sticklebacks, which were numerous in our sampling, may be beneficial in removing mosquito larvae 

from slow-moving waterways (Sharp 2000). 

4.4.8 Reptiles 

Reptiles observed on the refuge include western pond, painted, and non-native red-eared slider 

turtles, alligator lizards, rubber boa, and various varieties of garter snakes. Non-native snapping 

turtles have been documented in the Tualatin River but have not been observed by refuge staff or 

volunteers. Turtles are commonly observed basking on logs in the river or on herbaceous wetlands. 

Garter snakes are observed in most open habitat types throughout spring and summer. No surveys for 

reptiles are currently being conducted. 

Photo 4-11. Garter snake. © Ed Bustya

4.4.9 Amphibians 

Many species of amphibians have been documented on the refuge including chorus frogs, red-legged 

frogs, and non-native bullfrogs; long-toed and northwestern salamanders; and rough-skinned newts. 

These species breed in refuge wetlands and use adjacent uplands for part of their life cycles. Surveys 

for amphibian egg masses were conducted in late winter and spring during 2010 and 2011 on several 

refuge wetlands. Results showed red-legged frogs, chorus frogs, and long-toed salamanders were 

breeding in refuge wetlands. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 4. Biological Environment 4-23 

4.5 Threatened, Endangered, and Other Special Status Species 

One goal of the Refuge System is “to conserve, restore where appropriate, and enhance all species of 

fish, wildlife, and plants that are endangered or threatened with becoming endangered.” In the policy 

clarifying the mission of the Refuge System, it is stated, “We protect and manage candidate and 

proposed species to enhance their status and help preclude the need for listing.” In accordance with 

this policy, the CCP planning team considered species with Federal or state status, and other specialstatus species, in the planning process. 

Currently there is one plant species (Nelson’s checker-mallow) that is federally listed as threatened, 

and two fish species (upper Willamette River run Chinook, and upper Willamette River run 

steelhead) that are federally threatened and occur or could occur on the refuge. These three species 

are also state-listed as threatened. Meadow checker-mallow is a state-listed candidate plant species 

that occurs on several areas of the refuge. Birds considered species of concern that have been 

observed on the refuge are olive-sided flycatcher, yellow-breasted chat, acorn woodpecker, Lewis’ 

woodpecker, and band-tailed pigeon. Reptiles and amphibians considered species of concern include 

western pond turtle and northern red-legged frog. Fish species considered species of concern that 

may occur in refuge waters are Pacific lamprey and coastal cutthroat trout. 

Photo 4-12. Nelson’s checker-mallow. Jeff Dillon/USFWS. 

In February 2008, Nelson’s checker-mallow was planted in seven plots on three of the Sherwood 

Units: Atfálat’i, Tualatin River, and Riverboat. Nelson’s checker-mallow was known to occur on one 

parcel of the Atfálat’i Unit prior to this planting effort. In 2010, additional plants were added to the 

plots on the Atfálat’i and Riverboat Units. The plants have experienced good survival since 

installation and will continue to be monitored. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-24 Chapter 4. Biological Environment 

4.6 Invasive and Nuisance Species 

Refuge personnel expend considerable resources in terms of funding and staff and volunteer time 

managing invasive and nuisance species each year to provide quality habitat for native species. 

Management of invasive and non-native species follows the practices set forth and described in the 

IPM Plan in Appendix G. This plan describes priorities for response by the refuge and the various 

methods used to manage these species. The refuge has and will continue to follow the practice of 

“Early Detection and Rapid Response” (EDRR). Preventing the introduction of invasive species is 

the first line of defense in the battle against new invasions. However, it is inevitable that new 

introductions will occur. The next best line of defense is to manage these invasions through EDRR. 

EDRR is the principle of targeting noxious weed infestations when they first arrive in a given area, 

while their populations are small and localized. This effort greatly increases the likelihood that new 

invasions are addressed immediately, before the species can become established and widespread. 

Table 4-2 lists the target non-native and invasive species for the refuge; however, this list is not 

comprehensive. Plant species are by far the most problematic, but non-native fish and wildlife 

species also have important direct and indirect impacts to native habitat types, native fish and 

wildlife, and refuge infrastructure. 

Table 4-2. Common and Scientific Names of Target Non-native Invasive Species on 

Refuge 

Common Name Scientific Name State Noxious Weed* 

Plant Species 

Bermuda grass Cynodon dactylon 

Bull thistle Circium vulgare B 

Canada thistle Circium arvense B 

Cocklebur Xanthium strumarium B 

Cut leaf teasel Dipsacus fullonum B 

English holly Ilex aquifolium 

English ivy Hedera helix B 

Field bindweed Convolvulus arvensis B 

Himalayan blackberry Rubus discolor B 

Japanese knotweed Polygonum cuspidatum B 

Poison hemlock Conium maculatum B 

Purple loosestrife Lythrum salicaria B 

Red hawthorn Crataegus monogyna 

Reed canarygrass Phalaris arundinacea 

Scotch broom Cytisus scoparius B 

Tansy Tanacetum vulgare 

Velvet grass Holcus lanatus 

Velvetleaf Abutilon theophrasti B 

Wildlife Species 

Bullfrog Rana catesbeiana 

Nutria Myocastor coypus 

Feral cat Felis catus 

Red-eared slider (turtle) Trachemys scripta elegans 
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Chapter 4. Biological Environment 4-25 

Table 4-2. Common and Scientific Names of Target Non-native Invasive Species on 

Refuge 

Common Name Scientific Name State Noxious Weed* 

European starling Sturnus vulgaris 

House sparrow Passer domesticus

Fish Species 

Bass Micropterus sp.

Catfish Ameiurus sp.

Common carp Cyprinus carpio 

Mosquito fish Gambusia affinis 

Sunfish Family Centrarchidae 

*State B-listed plants are those defined as “a weed of economic importance which is regionally 

abundant, but which may have limited distribution in some counties” (ODA 2012). 

There are species such as reed canarygrass listed in Table 4-2 that are not on the state’s list of 

noxious weeds but are very problematic for the refuge because of their aggressive nature, difficulty 

of control, and abundant distribution. This list is not presented in priority order, and some plant 

species may be treated more aggressively in one season than another depending on staff availability, 

extent of distribution, or threat presented to native wildlife or habitats. The invasive plant species 

listed above often form a monoculture and compete with native plants. Many species of native 

wildlife have evolved with native plants and either cannot use non-native plants or have reduced 

benefits from them. Many plants such as Himalayan blackberry form very dense thickets that may 

actually hinder wildlife movement. Reed canarygrass invades wetlands, forming dense monocultures 

and preventing beneficial native plants from growing. Both purple loosestrife and Japanese knotweed 

have been found on the refuge and have been vigorously removed before they could become 

established. Invasive tree species such as English holly and red hawthorn crowd out native species 

and spread prolifically by means of thousands of seeds annually per tree. These trees are removed as 

time allows in established habitat types and are vigorously controlled in new restoration areas. 

Invasive wildlife regularly observed on the refuge include nutria and bullfrogs. Nutria, a native of 

South America, were introduced for fur farming and escaped or were released as fur prices dropped. 

They are well established in our area, and eradication efforts would only be successful if adopted on 

a regional basis. Climate change may contribute to their spread if winter climate becomes more 

favorable for breeding and survival. Nutria directly affect wetland and adjacent habitat types by 

foraging on native plants, and they also affect native wildlife such as muskrats by direct competition 

and displacement. They affect refuge infrastructure such as levees and water control structures by 

burrowing into levees, causing leakage through the levees and/or causing cave-ins of dens presenting 

safety hazards to vehicles traveling on levees; they also burrow along the side of culverts, causing a 

“piping” effect whereby water leaking along the outside of the pipe erodes the earthen material and 

causes a washout of the pipe. Lethal means such as shooting and trapping, wherever feasible, are 

used to control nutria. 

Bullfrogs, which are native to the southeastern United States, eat native frogs and many other 

wildlife species such as ducklings. Bullfrogs thrive in wetlands that retain water year-round and can 

be at least partially controlled by completely drawing down wetlands during summer. The refuge 

contains managed wetlands that cannot be completely drawn down due to either incorrectly situated 

water control structures or incorrectly contoured basins that do not allow for complete drainage. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-26 Chapter 4. Biological Environment 

European starlings are cavity nesters and displace and sometimes kill native cavity nesters such as 

Western bluebirds, American kestrels, and white-breasted nuthatches. 

Photo 4-13. Non-native nutria. Peter Schmidt/USFWS. 

The Tualatin River watershed contains many species of non-native fish. These species were either 

brought in for sport fishing, as commercial species, or were accidentally introduced. Non-native fish 

displace and compete directly with native species by preying on native species, and/or disrupting 

native plant growth. Sunfish may eat larval amphibians and native fish, carp eat submergent 

vegetation important to wildlife species and cause turbidity, which further reduces plant growth, and 

bass eat native fish and wildlife species. Completely drawing down wetlands in summer may help 

reduce these species on a site-specific basis, but they likely will not be eradicated in the watershed as 

a whole. As mentioned above in the discussion on bullfrogs, drawing down the wetlands completely 

would help reduce non-native fish on a localized basis but is not a likely scenario. Screening water 

diversions from creeks and the river would help reduce the presence of non-native as well as native 

fish in wetland areas. 

Nuisance species on the refuge may include such native animals as beavers, elk, Canada geese, and 

others, depending on circumstances. Beavers are native and are encouraged in most areas. However, 

they can become problematic by plugging up water control structures on a daily basis or cutting 

down trees in restoration areas. Sometimes it is necessary to control beavers using lethal means. 

Relocation is seldom successful as most suitable beaver habitat is already occupied and problem 

beavers are likely to return to their original capture site. Elk have not been a problem on the refuge to 

date, but with acquisition and restoration of large tracts of land in the Wapato Lake Unit this may be 

an issue in the future. ODFW considers the Willamette Valley an “elk de-emphasis zone” to prevent 

conflicts with agricultural interests. A population of elk residing on refuge lands and foraging on 

neighboring agricultural lands would cause conflict with neighboring landowners. In addition, elk 

crossing Highway 47 or Springhill Road to and from refuge property would create a safety hazard to 

vehicles. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 4. Biological Environment 4-27 

4.7 Wildlife and Habitat Research, Inventory, and Monitoring 

There are several regular inventory and monitoring surveys, and occasional research studies, 

conducted on the refuge. Table 4-3 lists the regular surveys that are conducted almost every year to 

determine habitat conditions, restoration/enhancement response to management activities, and 

wildlife response and population trends. In addition, research is conducted by refuge staff or outside 

organizations such as universities and USGS to help inform management decisions. 

Table 4-3. Regular Surveys Conducted on Refuge Lands and Their Usual Timetables 

Habitat Type or Guild Survey Type Timetable 

American kestrel Nest box Spring/early summer 

Amphibians Egg mass Late winter/spring 

Fish Trapping/identification Winter/flooding 

Marsh birds Recorded call-back April-June 

Nelson’s checker-mallow Inventory June-July 

Shorebirds Instantaneous scan April-September 

Songbirds Point count April-June 

Songbirds Monitoring Avian 

Production and Survivorship 

mist netting 

May-August 

Tree and shrub Fixed plots: survival and 

growth 

Summer/early fall 

Wading birds Instantaneous scan Year-round 

Waterfowl Instantaneous scan September-February 

Western bluebird Nest box Spring/summer 

Wetlands Transects: plant 

species/density 

Summer/early fall 

In addition to surveys listed in Table 4-3, other surveys are conducted periodically by refuge staff 

and others: 

 Mosquito larvae surveys were conducted during summer for several seasons and revealed 

very low numbers of larvae in refuge wetlands. 

 A feasibility study was conducted in 2004 to determine if boxes for cavity nesting waterfowl 

would be beneficial. The study revealed adequate natural cavities existed on the refuge and 

brood water was limited. 

 A survey for freshwater mussels was conducted in 2009 and revealed the presence of native 

western pearlshell mussels in Chicken Creek. 

 USGS conducted a survey for non-native crayfish and found none in waters in and adjacent 

to the refuge. 

 University researchers conducted a study over a three-year period to inventory small 

mammals and determine prevalence of hanta virus. 

 Extensive surveys have been conducted by qualified volunteers to identify, collect, and 

catalog plant species throughout the refuge. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

4-28 Chapter 4. Biological Environment 

 Refuge staff and volunteers have undertaken a project to map invasive plant species 

throughout the refuge using global positioning system (GPS)/geographic information system 

(GIS). 

 In 2011 the refuge began a collaborative study with USGS and other partners to obtain a 

detailed topographic survey and water quantity/quality study at the Wapato Lake Unit to 

guide future restoration efforts.