Invasive Species Management

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European beachgrass (Ammophila arenaria) was introduced around the globe due to its superior stabilizing properties. It can tolerate more sand burial than our native dunegrass, Elymus mollis, and produces more densely-packed shoots from its rhizomes (underground stems), creating monolithic stands that outcompete other species. This photograph was taken at our newest addition, which has not yet been restored.

What is an Invasive Species? 

Invasive species are non-native species whose introduction into an ecosystem in which the species is not native causes, or is likely to cause, environmental or economic harm or harm to human health. At Humboldt Bay National Wildlife Refuge, all of the currently managed invasive species are plants. There are a variety of ways in which an invasive plant can harm native species, habitats, or ecosystems. An example of a direct effect is when the invader is able to outcompete and ultimately replace native species.  This can occur because the invader is able to grow in the absence of natural predators and competitors that would keep it in check in its native range. Impacts of invasive species can also occur at an ecosystem level. An example of this at the refuge is the plant yellow bush lupine (Lupinus arboreus). This plant fixes atmospheric nitrogen and enriches the normally infertile dune soils, causing both chemical and biological alterations. 
Invasive species are widely considered to be the greatest threat to natural areas after habitat loss. They negatively affect 34-46% of endangered species. There are 2.5 million acres (out of a total of 150 million acres) of Refuge System lands infested with invasive plants. One of the challenges of controlling invasive species infestations is the fact that they are not constrained by political boundaries. A tidally dispersed species like dense-flowered cordgrass (Spartina densiflora) can continually re-invade a restored habitat and must be controlled at the level of an estuary or, to prevent seeds from reaching unaffected estuaries, at the scale of the entire west coast of North America. 
Seeds of new invaders can continually arrive at a refuge, through dispersal by wind, ocean currents, or as hitchhikers on a visitor’s shoes or clothing. Once established, they can spread exponentially. For this reason, a policy of “early detection/rapid response” is the highest management priority, and requires initial mapping of known infestations of invasive species (inventory) and keeping records of treatments and their effectiveness over time (monitoring).  A baseline inventory is accomplished by carefully mapping infestations on the ground using GPS (global positioning systems) and thereafter updating records in a geodatabase using GIS (geographic information systems). An example of this type of program can be found on our Forest Invasives page. 

Invasive Species Control at Humboldt Bay National Wildlife Refuge  

The Lanphere and Ma-le’l Dunes Units are unconstrained, naturally functioning components of the dune/estuarine ecosystem of Humboldt Bay, and preventing/controlling invasions on these units is the primary form of active management. Intensive restoration is first carried out to remove invasive species and restore any altered functions (see Dune Restoration). In dune systems, simply removing some exotics, such as European beachgrass (Ammophila arenaria), restores the critical process of sand movement. In general, true “eradication” is not possible, due to the presence of the species on adjoining lands and the likelihood of continued dispersal. An exception to this is Spartina densiflora, an ecosystem-level invader that occurs in all refuge units that harbor salt marsh. This species is currently being managed through a regional eradication program targeting all occurrences in the Mad River, Humboldt Bay, and Eel River estuaries. In other cases, management focuses on one or a suite of interacting invasive species until a “maintenance” threshold has been achieved—that is, the effort required to remove new or recurring infestations is reduced to a minor level that becomes part of an annual “check and treat” strategy. At the Lanphere and Ma-le’l Dunes, we have achieved this threshold for European beachgrass (Ammophila arenaria), iceplant (Carpobrotus spp.), yellow bush lupine (Lupinus arboreus), although recent new additions to the refuge will require additional management.  We are also close to this maintenance stage for our forest invasives (see below). Although there are several persistent non-native species in our dune swales (including pennyroyal (Menthum pulegium), rabbit grass (Polygpodium monspeliensis), Hyssop’s loosestrife (Lythrum hyssopifolium), bird’s foot trefoil (Lotus corniculatus) and yellow glandweed (Parentucellia viscosa), these species do not displace native plants entirely, nor do they prevent the natural succession of herbaceous swales into beach pine forest. For this reason we do not prioritize their management, but instead focus on the two swale invaders known to outcompete native species: pampas grass (Cortaderia jubata) and bull thistle (Cirsium vulgare). Both are easily dispersed from off-refuge and require continued surveillance and treatment.

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Pictures: The above photographs show the extraordinary result of English ivy (Hedera helix) control in a forested section of Humboldt Bay NWR. 
In many systems, the removal of an established invasion merely creates a window for future invasion. However, our foredune and middune habitats are early successional in nature, and gaps are an important part of the system. When we remove invasives we are returning the system to a semi-stabilized, low fertility condition that favors native plants. Only when invasions are in a late stage and native species are nearly absent, is revegetation required.   
The freshwater and brackish wetland-dominated units in the southern part of the refuge are highly managed habitats, most of which originally supported salt marsh prior to the extensive diking that occurred around Humboldt bay in the late 1800s. As such, these areas are managed for wetland functions and wildlife habitat. Invasive species can also be a threat to these systems by outcompeting native species that occur in riparian and wetland habitats. The extensive agricultural wetlands that are managed to provide short-grass habitat for the Aleutian cackling goose and numerous other species of shorebirds, waterfowl, mammals and amphibians are composed largely of non-native species. In these areas, only aggressive invaders such as bull thistle (Cirsium vulgare) and bristly ox tongue (Helminthotheca echioides) are controlled. 

High Priority Plant Invaders By Habitat Type 

Dune Mat (Semi-stable, nearshore dunes) 
Ammophila arenaria - European beachgrass 
Lupinus arboreus - yellow bush lupine 
•Annual/perennial grass complex 
    ◦Bromus diandrus - ripgut brome 
    ◦Vulpia bromoides - brome fescue 
    ◦Briza maxima - rattlesnake grass 
    ◦Aira praecox & A. caryophyllea – European hairgrass 
    ◦Anthoxanthum odoratum - sweet vernal grass 
    ◦Holcus lanatus - velvet grass 
Dune Swales (Seasonal wetlands in nearshore dunes)  
Cirsium vulgare - bull thistle
Cortaderia jubata - pampas grass 
Dune Forest (Stabilized forest dunes, both upland and wetland/riparian) 
Hedera helix - English ivy 
Cotoneaster franchettii - cotoneaster 
Ilex aquilegium - English holly 
Pittosporum tenuifolium - tawhiwhi    
Salt Marsh (Estuarine intertidal marsh)
Spartina densiflora - dense-flowered cordgrass
Riparian (Forested freshwater wetlands occurring along stream courses) 
Calystegia silvatica spp. disjuncta - false bindweed
Freshwater marsh (Herbaceious freshwater wetlands)  
Lotus pedunculatus - marsh lotus
Agrostris stolonifera - creeping bentgrass
Phalaris arundinacea - reed canary grass