Impact of Invasive Japanese Barberry Cascades Through Local Food Webs
By Ed Ricciuti
Invasive plants have a bad rap sheet, with a record of trashing native species, bedeviling land managers, and even threatening human health. And that may just be the tip of the iceberg, according to new research that suggests invasives could be even badder than that. A study of the notoriously invasive Japanese barberry suggests that the alien plants may subtly upset foundational ecological relationships at the bottom of food webs, with profound and far reaching consequences.
The study, published in late August in the journal Environmental Entomology, reveals that infestations of Japanese barberry can thin out species and numbers of arthropods in a habitat, both herbivores and predators such as ants and spiders, scrambling basic feeding relationships and generating disturbances that can ripple throughout the strands of the web. Even vertebrate wildlife and, importantly, humans can eventually feel the negative impact of the disruption. In effect, the introduction of an invasive can reduce the complexity of a food web.
“Invasive plants can be successful because they don’t have as many herbivores eating them compared to native plants,” says Robert E. Clark, Ph.D., of Washington State University, lead author on the study. “It makes sense that they would have simplified food webs because there is now not as much prey available for predators.”
Clark and his co-author, Chad L. Seewagen, Ph.D., of Great Hollow Nature Preserve & Ecological Research Center in New Fairfield, Connecticut, note that “arthropod species richness was significantly lower in the leaf-litter around Japanese barberry and on Japanese barberry plants themselves.”
Introduced as an ornamental in 1975, Japanese barberry has spread like topsy through forest and field over much of the eastern and Midwestern United States, clogging the landscape with dense, prickly thickets. It is virtually immune from control by most native herbivores, such as whitetail deer.
The researchers compared differences in arthropod communities inhabiting patches of temperate forest invaded by Japanese barberry with those in landscapes relatively untouched. They worked in the woods of the Pawling Nature Reserve, operated by The Nature Conservancy in Dutchess County, New York. Observations covered both above-ground communities with the plants and those below, in the leaf litter and soil.
Arthropod communities in habitats invaded by Japanese barberry turned out to be much less complex than those in natural forest, with few species and lower abundance. These habitats experienced an ecological process called “trophic downgrading,” a restructuring of the transfer of energy through food webs. When plants such as Japanese barberry take over, they can reshape communities of plants and animals in and on the soil by modifying its structure, function, and nutrient cycling. Soil acidity and decomposition rates in forest floor may change. Ecological relationships may also go awry above ground, in the branches of the plant itself.
Predator-prey relationships among lower animals such as arthropods are considered at bottom of food webs while those among vertebrate animals—wolves and whitetail deer, for example—stand at the top. If food webs are out of balance, the disruption can cascade throughout the network, either from the top down or bottom up. An example of a top-down cascade is when depletion of predator numbers leads to an increase in deer, which causes a decrease in the plants on which deer feed. In the case arthropods impacted by the Japanese barberry, the cascade flows from the bottom up. Higher up, migratory songbirds that rely on insects as food would be the most likely vertebrate predators to feel the negative impact of the chain reaction.
Once an invasive such as Japanese barberry displaces indigenous plants, the insects that depend on the native plants for food are in a bind. Loss of herbivore populations occurs shortly after loss of their host plants, and any predator that feeds on those herbivores will soon diminish as well. Invertebrates that feed, reproduce, and develop on foliage, the foundation of the food-web hierarchy, fare poorly on most invasive plants, especially woody varieties like Japanese barberry. Predatory invertebrates that consume the herbivores suffer the consequences.
The consequences can even impact human health. Research in 2009 by scientists at the Connecticut Agricultural Experiment Station indicates that Japanese barberry thickets may promote transmission of Lyme disease by creating a favorable environment for blacklegged ticks (Ixodes scapularis) that vector it and their white-footed mouse hosts. Subsequent research recommends periodic clearing of barberry when possible, to reduce tick populations. According to the new study, the increase in ticks when Japanese barberry enters the picture may actually stem from a decrease in the numbers of spiders and ants that prey on them. If so, say the authors, this suggests that simplified food webs on invasive plants can have consequences for human health by disrupting food-web interactions that suppress vectors for diseases.
The new research “makes absolute sense,” says the Experiment Station’s Scott C. Williams, Ph.D., a long-time researcher on the relationship between Japanese barberry and ticks. Says Williams, “Japanese barberry infestations are vast monocultures that stifle native plant regeneration, reduce biodiversity overall, and amplify blacklegged tick abundances. The existence of Japanese barberry on the landscape is detrimental to the health of the public and native ecosystems alike.”
Seewagen says further research should look at the next-level impacts on insect predators. “Beyond the obvious implications for invertebrate diversity and conservation, it will be important to investigate how such changes in arthropod community composition might affect the diet quality and composition of insectivores, like many birds, that likely prefer or depend upon certain groups of arthropods for meeting their and their offspring’s nutritional requirements,” he says.
All in all, Clark and Seewagen write, “Much more work on the cascading effects of non-native invasive plants throughout ecosystem food webs is needed before the full breadth of their ecological impact can be appreciated and managed.”
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.