• Honeysuckles produce small, aromatic flowers.Wild Honeysuckle 2 image by DelB from Fotolia.com
  
  The invasive shrubs in Pennsylvania's ecosystems typically escaped from cultivation, meaning that they were once a cultivated species that managed to spread from landscapes through the decades. Once in a wild setting, these plants thrive and begin to cause problems for native species, crowding them out and creating so much shade in some instances that native plants cannot grow. These invasive shrubs exist in many states in addition to Pennsylvania, where they adversely affect the ecology of where they grow.
  
  

Exotic Honeysuckles

• Honeysuckles include species such as Morrow's honeysuckle, Amur honeysuckle and Tartarian honeysuckle. All these exotic types can quickly take over and form thick layers of shrubs that create so much shade that native plants suffer. Honeysuckles suck up moisture from the soil as well as nutrients, says the Pennsylvania Department of Conservation and Natural Resources site. These shrubs will also send out substances into the surrounding ground that effectively keep other plants from sprouting. Most of these shrubs exist along the borders of forests and in open areas such as fields and along roadsides, as they require sunshine to flourish. The Morrow's honeysuckle can grow in moist sites such as near swamps and ponds. The original idea behind introducing these shrubs to the United States focused on the honeysuckle's ability to stop soil erosion and provide food for wildlife.
  
  

Buckthorns

• The common buckthorn and glossy buckthorn reach small tree sizes, about 20 feet tall in the larger specimens. Both buckthorns came to the Northeast in the 1800s, often planted to form hedgerows. Native to parts of Asia and Europe, the shrubs grow in states like Pennsylvania and take over entire areas. Buckthorn gets so dense that nothing grows in its shade. Buckthorn is also a problem for agriculture, existing as a host plant for a fungus that causes a disease known as oat rust. Birds such as cedar waxwings, wood ducks and blackbirds consume the purple-black seeds fruits the shrub produces and then pass them as they fly from one place to another. The University of Wisconsin Extension notes that the fruits act as a laxative on wildlife, making them quickly pass through their systems.
  
  

Japanese Barberry

• The Japanese barberry features many sharp spines on a plant that can be as tall as 8 feet with brown, grooved branches and small leaves. The red berries can mature by the end of summer and many stay on the shrub until winter. Japanese barberry does best in well-drained ground and it can survive in partial shade. This allows this invasive species to grow in open woodlands under other trees. Birds spread the seeds. Japanese barberry came to the United States around 1875 as an ornamental species, states the Wisconsin Department of Natural Resources website. Browsing wildlife, such as the white-tailed deer, will refrain from eating this plant, applying even more pressure on the surrounding edible plants and giving the barberry a distinct advantage. The shrubs alter soil pH and affect the amount of leaf litter that accumulates on the forest floor, altering the habitat.