Invasive Species

Natural habitats within Pennsylvania are threatened by the invasion of exotic (non-native) species.  These invasive species are plants, animals, and other organisms that do not naturally occur in the area and are likely to cause harm to the natural environment, the economy, or to human health.  Because they have been removed from the control of their natural predators and competition, they often spread rampantly, and once established, it is extremely difficult to control their spread.  Invasive species are recognized as one of the leading threats to biodiversity and impose enormous economic costs to agriculture, forestry, fisheries, and other enterprises, as well as to human health.

The introduction of non-native species into Pennsylvania began with the initial European settlement in the 17th century and continues to this day.  Plants and animals have been deliberately introduced for a variety of reasons including food sources, erosion control, landscaping, and game for hunting and fishing.  Other species have been accidentally introduced as ‘stowaways’ through global trade and transportation.  These introductions have had drastic effects on Pennsylvania’s biodiversity over time.  For example, more over 37% of the plant species now found in the commonwealth did not occur here during the first period of European settlement.

Invasive Plants

Invasive plants are exotic plants that are not native to Pennsylvania and are harmful to our native plants, animals, and ecosystems. Currently, over 285 invasive plant species are impacting Pennsylvania.  Qualities that make these plants invasive include their ability to reproduce rapidly, spread quickly over the landscape, and the fact that they have few, if any, natural controls (such as herbivores and diseases) to keep them in check.  

Some of Pennsylvania’s most problematic invasive plant species are:

Japanese stiltgrass (Microstegium vimineum) — Stiltgrass threatens native understory vegetation in full sun to deep shade. It spreads opportunistically following disturbance to form dense patches, displacing native vegetation as the patch expands.

Kudzu (Pueraria montana var. lobata) — Kudzu outcompetes other plants by smothering them under a solid blanket of leaves, by girdling woody stems and tree trunks, and by breaking branches or uprooting entire trees and shrubs through the sheer force of its weight.

Giant hogweed (Heracleum mantegazzianum) — Originally planted in ornamental gardens, hogweed appears on Pennsylvania’s noxious weed list because its sap causes severe skin sensitivity to UV radiation that leads to blistering and severe burns. One hogweed can produce more than 20,000 seeds, allowing this species to spread very rapidly.

Tree-of-heaven (Ailanthus altissima) — Tree-of-heaven was introduced to Philadelphia from China in the early 1800s, it is present in disturbed places throughout the county.  This fast growing tree is a prolific seed producer and can also proliferate through vegetative means, outcompeting native vegetation.

Mile-a-minute (Persicaria perfoliata) — A vine that invades open and disturbed areas and scrambles over native vegetation in open and disturbed areas, limiting their photosynthesis. This species is listed as a noxious weed in Pennsylvania and federally.

Purple loosestrife (Lythrum salicaria) — A herbaceous wetland invasive that is present at scattered sites throughout the county. Once established in a wetland this species is difficult to eradicate and will displace native species.

Garlic mustard (Alliaria petiolata) — An increasingly common invasive biennial herb spreading through natural areas throughout the region. Recent scientific evidence has shown that this species can disrupt mycorrhizal relationships that trees depend on for their growth.

Japanese honeysuckle (Lonicera japonica) and Oriental bittersweet (Celastrus orbiculatus) — These species of vines cover and outcompete native vegetation as well as girdle trees by twining up them.

Bush honeysuckles (Lonicera tatarica, L. morrowii, and L. maackii) — Found in a variety of environments from wetlands to uplands. Competes with native plants for moisture, nutrients, and pollinators. Fruits do not provide high energy food for migrating birds.

Autumn olive (Elaeagnus umbellata) — A drought-tolerant species that thrives in many soil conditions. Threatens native ecosystems through competition and alteration of natural succession patterns and nutrient cycling.

Japanese barberry (Berberis thunbergii) — Commonly planted ornamental that escapes and forms dense stands in a variety of habitats, including forests and wetlands, displacing native vegetation.

Winged burning bush (Euonymus alatus) — A shrub that can form dense thickets that displace native woody and herbaceous plants.

Multiflora rose (Rosa multiflora) — Widely planted shrub that invades a variety of habitats excluding most native shrubs and herbs.  May be detrimental to the nests of native birds.

Privet (Ligustrum spp.) — These shrubs can form dense thickets in floodplains, forests, wetlands, and fields that can outcompete native vegetation.

Reed canary grass (Phalaris arundinacea) — This grass, though native to Pennsylvania, forms dense, monospecific stands in open wetlands, wet meadows, and riparian areas.  It effectively excludes all other plant species, causing greatly decreased biological diversity in wetland communities.

Common reed (Phragmites australis) — There is a rare and noninvasive native strain of this grass in Pennsylvania, but the introduced strain is very invasive, forming large, dense stands that exclude all other plants.


Invasive Animals

In addition to invasive plants, Pennsylvania now harbors many non-native invasive species of animals including mammal, bird, fish, reptile, and invertebrate species.  Some of these invasive animals such as the Norway rat (Rattus norvegicus) are all too common in our homes and constructed areas.  These invasive animal species directly threaten populations of native animals through competition, predation, or modification of habitat through the alteration of cover and diversity.

Some of Pennsylvania’s most problematic invasive animal species are:

Common carp (Cyprinis carpio) — Introduced as a food fish, this carp is now found anywhere with warm, slow-moving water.  As it feeds along the bottom, it mobilizes a large amount of sediment.

Emerald ash borer (Agrilus planipennis) — This species has been devastating for populations of all species of ash trees (Fraxinus spp.) across the eastern US. Ashes are an important component of many swamp forests and upland forests, and the pumpkin ash (Fraxinus profunda) is endangered in Pennsylvania. Emerald ash borer larva kill the tree’s sapwood.

European house sparrow (Passer domesticus) — A hardy, adaptable species, the house sparrow can cause crop damage, but it has also been documented killing native adult and juvenile birds or smashing their eggs.  The house sparrow is partially responsible for a decline of birds that nest in tree cavities such as the eastern bluebird (Sialia sialis) in the United States.

Feral swine (Sus scrofa) — Feral swine are highly mobile and negatively impact livestock, property, and natural areas.  Also called wild hogs, they are potentially one of the most influential upcoming invasive animal species in Pennsylvania’s forests.  Their numbers are currently relatively low in Pennsylvania, but the potential exists for the numbers of feral swine and their negative ecological effects to explode and significantly impact forest lands and agricultural activity over the next decade.

Gypsy moth (Lymantria dispar) — Though mainly targeting oak species, gypsy moth caterpillars will eat almost any vegetation when pressed and tree defoliation by this specieshas caused extensive defoliation of Pennsylvania’s forests.  This non-native moth was intentionally introduced to the U.S. from Europe in 1869 as part of a commercial silk production venture.

Hemlock woolly adelgid (Adelges tsugae) — This small aphid-like insect feeds on the leaves of eastern hemlock trees (Tsuga canadensis), an important shade tree for trout streams.  Infestations of the woolly adelgid appear as whitish fluffy clumps of feeding adults and eggs along the underside of the branch tips of the hemlock. Adelgid infestations have caused up to 90% mortality in eastern hemlocks throughout the state.

House cat (Felis catus) — House cats, both domestic and feral, can each kill several small animals every day, causing the death of many amphibians, reptiles, birds, and mammals each year.

Multicolored Asian ladybird beetle (Harmonia axyridis) — Likely introduced in an attempt to control non-native aphids, this beetle now preys on native insects, and invades houses each winter.

Norway rat (Rattus norvegicus) — The Norway rat is typically a pest in human made structures, but is also found around rivers and other water systems.  A known carrier for many diseases, this rat is a threat anywhere it occurs.

Rusty crayfish (Orconectes rusticus) — Native to the midwestern US, rusty crayfish can reproduce in large numbers and reduce lake and stream vegetation, depriving native fish and their prey of cover and food.  Their size and aggressive nature keeps many fish species from feeding on them.  Rusty crayfish may also reduce native crayfish, freshwater mussels, and reptile and amphibian populations by outcompeting them for food and habitat or by directly preying on young individuals.

Zebra mussel (Dreissena polymorpha) — Introduced to the Great Lakes in the 1980s, this non-native mussel negatively affects industry and recreation, as well as native species of fish and mussels, and should be controlled wherever it occurs.


Invasive Fungi

Just like plants and animals, fungi can be introduced outside their native range by human actions, and sometimes they become invasive and cause serious harm to native ecosystems. Pennsylvania has been affected by two of the most destructive fungal infections in our country’s history.

Chestnut blight (Cryphonectria parasitica) — This fungus was introduced to North America accidentally around 1900.  By 1926, the disease had devastated chestnuts stands from Maine to Alabama, where chestnut once comprised one-fourth to one-half of forests. Once prized for its durable wood and as food for humans and wildlife, today chestnuts only exist as stump-sprouts from killed trees.

Dutch elm disease (Ophiostoma spp.) — Introduced in the 1930s and 40s, these fungi infect elm trees (Ulmus spp.), which were once the nation’s most popular urban street tree. Thanks to the disease, elms have now largely disappeared from both urban and forested landscapes, where they originally ranged across all states east of the Rockies.  Dutch elm disease has killed an estimated 100 million trees.

Find out more about Invasive Species in Pennsylvania:

Governor's Invasive Species Council of Pennsylvania 

PA DCNR Invasive Species Resources

The USDA’s Invasive and Noxious Weed List for Pennsylvania

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