A study reported in sciencedaily.com suggests they both are. They show the differences in these studies results is a matter of scale. This Washington University study suggests that examining smaller areas demonstrates more loss of diversity, e.g. fewer native plant species growing that would ordinarily be found there.
"At small scales, invaded plots had many fewer species than un-invaded plots, but they picked up species more rapidly, and at broad scales the invasives' effect on diversity virtually disappeared," Kirsten Powell, one of the authors says. As the study area increases, more native species are found, it just takes longer and more extensive areas to find all the normal species.
Bush honeysuckle which covers the roadsides in northeast Missouri is a good example. We are referring to several Asian invasive species such as Lonicera maackii and L. fragrantissima. An innocent smaller native Diervilla found in the Northeastern US goes by the same common name. L. maackii grows rampantly, leafing out early in the season and thus blocking the sunlight from the native species which open their buds later. It also remains green into November, further blocking out sunlight. It also may produce allelopathic chemicals, toxins that reduce growth of other plants in the vicinity.
By the shear volume of blossoms as they take over an area, they outcompete natives for pollinators, further reducing the native plants' reproduction. The seeds are spread by birds which eat the prolific fruit. Unfortunately the berries are not as nutritious as native species. They are high on sugar content but low on fat and protein required by birds storing up energy for their winter survival or migration. Essentially they are consuming the equivalent of giant soft drink bottles, even in New York.
Bush honeysuckle hasn't become a problem along Bull Creek...yet. My lovely editor would say "Death to all invasive plants," an impossible feat on our tree farm, let alone the whole planet. On the other hand, it proves her thesis that there is benefit in local control, especially when our goal is maintaining diversity in the valley. In a final note of irony, as bush honeysuckle spreads through the US, it has become endangered in Japan. Go figure.
Bush honeysuckle control measures are outlined on the mdc.mo.gov/ site