Invasive Species at Humboldt Bay NWR

Invasive species are non-native species (plants or animals) that adversely affect the habitats they invade economically, environmentally or ecologically. At Humboldt Bay National Wildlife Refuge (HBNWR), 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 HBNWR is as 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 2.3 million acres of Refuge System lands infested with invasive plants and this number is increasing. 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 as yet 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; preventing an infestation of a known invader is the least expensive and most efficient defense. Refuge biologists are continuously monitoring for the presence of new, highly invasive species, whether through systematic and repeated searches or just by keeping a vigilant eye out when in the field.

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. 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. Instead, management initiatives focus 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 Dunes, our longest-managed unit, we have achieved this threshold for most of our invasive plants. Restoration at the Ma-le’l Unit is nearing completion. In many systems, the removal of an established invasion merely creates a window for future invasion. However, our nearshore dune 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. For a summary of our major invasive plants and the current status of invasion go to the links below (NOTE: this section of our website is continually updated and more links will be coming soon).

HIGH PRIORITY PLANT INVADERS BY PLANT COMMUNITY AND HABITAT DUNE UNITS

Dune Mat (Semi-stable, nearshore dunes) Ammophila arenaria - European beachgrass Carpobrotus chilensis x C. edulis - iceplant Lupinus arboreus - yellow bush lupine Senecio elegans - purple ragwort 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) Polypogon monspeliensis & P. maritimus - rabbit’s-foot grass Mentha pulegium - pennyroyal Parentucellia viscosa - yellow glandweed Lythrum hyssopifolium - Hyssop’s loosestrife Lotus corniculatus - bird’s-foot trefoil Trifolium repens - white clover Cirsium vulgare - bull thistle Dune Forest (Stabilized backdunes, both upland and wetland/riparian) Hedera helix - English ivy Cotoneaster franchettii - cotoneaster Ilex aquilegium - English holly Pittosporum tenuifolium - tawhiwhi Cirsium vulgare - bull thistle Salt Marsh (estuarine intertidal marsh) Spartina densiflora - dense-flowered cordgrass Parapholis strigosa - sicklegrass Atriplex prostrata - triangle orache		Ammophila arenaria - European beachgrass		Lotus corniculatus - bird’s-foot trefoil
		Carpobrotus chilensis x C. edulis - iceplant		Trifolium repens - white clover
		Lupinus arboreus - yellow bush lupine		Cirsium vulgare - bull thistle
		Senecio elegans - purple ragwort		Hedera helix - English ivy
		Annual/perennial grass complex		Cotoneaster franchettii - cotoneaster
		Polypogon monspeliensis & P. maritimus - rabbit’s-foot grass		Ilex aquilegium - English holly
		Mentha pulegium - pennyroyal		Pittosporum tenuifolium - tawhiwhi
		Parentucellia viscosa - yellow glandweed		Spartina densiflora - dense-flowered cordgrass
		Lythrum hyssopifolium - Hyssop’s loosestrife		Parapholis strigosa - sicklegrass
				Atriplex prostrata - triangle orache