Effects of Forest Fragmentation and Restoration on Invasive Species

The invasive Asian longhorned beetle, is a wood-boring insect that kills hardwood trees. Guo’s study tested for potential correlations between feeding habits and forest fragmentation. Photo by R. Anson Eaglin, USDA APHIS.

Managing invasive species is one of the largest challenges that land managers face. They threaten the health of natural ecosystems, prevent the growth of native species, and leave landowners with significant amounts of damage.

“More than 4,300 exotic plant species and 66 foreign pest species that can cause negative effects on forest ecosystems and economies have been found in the United States,” says USDA Forest Service research ecologist Qinfeng Guo. “While not all nonnative species are equally invasive, many are especially adaptive to new environments and can quickly dominate them.”

Humans are largely responsible for increasing the range and distribution of pest species. As people continue to change the landscape by converting forest land for development, agriculture, and other uses, scientists are trying to understand the effect that such fragmentation will have on forest ecosystems, including their susceptibility to invasive pests.

Guo, who specializes in the study of biological invasions, collaborated with SRS scientists Kurt Riitters and Kevin Potter to examine the effects that different types of fragmentation and forest edges can have on forest pest invasions. Their results were published in the journal Forests.

They pulled together existing data on land use and pest richness from across the continental U.S. and categorized different types of forest edge. A forest edge was understood as a piece of forest that borders another land type, as opposed to a forest core which borders only forested land. For example, an edge could be classified as a “forest: agriculture” or “forest: grassland” edge. This way, the researchers could separate the correlation between each edge type and pest numbers.

Forests bordering developed land – areas such as roads, housing, or industry – were correlated with higher numbers of pest invasions. This supported the current understanding that humans are a primary source of pest mobility. In contrast, forests fragmented by agricultural or grassland were not correlated with more invasions.

Listen to a brief audio clip by author Qinfeng Guo describing this publication. • Text Transcript

“Forest pests don’t have much use for agricultural land,” explains Guo. “And grass pests are different than forest pests, so they also pose little threat to forests.”

Invasion rates varied among the pests. Specialist species, those with specific diets, are generally more responsive to fragmentation than are generalists, those with a varied diet. And pathogens – disease causing microorganisms – are less sensitive to fragmentation than are insects, likely because pathogens trend to spread easily.

Guo’s work can help predict which types of pests will be the largest threat after fragmentation happens. It can also motivate land managers to balance the ways in which they cut and transform forests.

Guo also wrote an invited review for the journal Invasive Plant Science and Management with guidelines for ecological restoration practices that are minimally susceptible to invasive species – specifically, invasive plants.

As more native ecosystems are lost across the country, greater restoration efforts will be needed. Guo hopes that they will be done carefully and strategically, especially since restoration efforts often focus on previously disturbed environments that are especially susceptible to invasions.

Kudzu is a perennial vine that has invaded southern ecosystems. Detailed plans for site restoration must be carefully crafted and executed in order to avoid kudzu invasions after disturbances such as fire, timber harvesting, or land clearing for development. Photo by Charles Bryson, courtesy of bugwood.org.

“This review is an attempt to put all of the pieces together: ideal restoration practices by habitat, species, methodology,” adds Guo.

Guo explains that a review such as this is important for tackling the broad array of existing restoration scenarios. Factors such as a project’s cost and size can influence the feasibility of restoration options.

Restoration plans depend on the type of site selected. Barren lands may require more effort to facilitate plant growth; lands with existing vegetation may need extra attention ridding the area of invasives.

Choosing the best species to use for restoration can be overwhelming, and it can determine the success of the restoration effort.

“Though the term ‘exotic species’ usually refers to species from other countries, the U.S. is so big that we have ‘native invasives:’ species that may be native to one area of the country but invasive to another,” says Guo. “Because of this, we need to choose seeds very carefully. Some of the best resources for seed selection are local experts, who know well which species are native to the area, which will do well, and which could pose a lot of danger.”

In some highly disturbed habitats, such as abandoned mines, native species can be hard to establish. Thus in certain cases, landowners may choose to first plant some nonnative species that are known as “nursing plants” to help improve the soil quality and nurture native species into existence.

Another technique is to use native species to outcompete invasive ones. Choosing native species genetically similar to invasives found in the area will allow them to occupy the habitats that invasive species would usually use, thereby making it more difficult for invasives to grow.

Though site and species selection generally receive fair attention in the restoration literature, Guo worries that not enough focus is placed on the growth and distribution of planted species.

Prescribed fire is one strategy for managing invasive species during ecological restoration. Controlled burning has its risks and must be applied carefully and with proper consideration for historic fire regimes. Photo by Chris Evans, courtesy of bugwood.org.

It is important to make sure that not everyone is planting the same species over a relatively large area; this could selectively prioritize a few native species and leave a whole region vulnerable to invasions. Guo emphasizes the need for landowners, whether federal, state, or private, to share information on which species each is planting and to make sure that as many as possible native species are being used.

At every step of restoration, it is important to monitor the species growing above- and below-ground. Early prevention against invasive species can help avoid massive takeovers. Once vegetation is growing, it may be necessary to manipulate the amount of biomass within the restored ecosystem; excess biomass can reduce species diversity when less dominant or rare ones can no longer compete.

“It is recommended to mix biomass reduction strategies to catch different species,” says Guo. “For example, grazing with different animals and at different intensities is a safe strategy. Using historic records from the region can also help inform management decisions, because the ideal situation is always the natural one.”

Read the full text of the forest fragmentation article here.

Read the full text of the ecological restoration article here.

For more information, email Qinfeng Guo at qinfeng.guo@usda.gov.

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