Both black and pale swallow-wort are aggressive and extremely competitive species that, once established, can quickly choke out native flora found in the area. Grazers and herbivores, such as deer, do not eat the invasive plant due to its toxicity. As a result, there is no native biological control, so the plant is able to spread more easily.
As swallow-wort continues to spread throughout a region, a monoculture forms. A monoculture is the existence of a sole single plant within a given area. This contributes to less biodiversity, which ultimately affects all species associated with the local ecosystem, most notably other native plants, birds, and soil organisms.
Several studies have been published regarding the impact of swallow-wort on monarch butterflies. Because swallow-wort and milkweed are in the same genetic family, monarchs may mistake one for the other. If a monarch lays its eggs on swallow-wort leaves, the larvae will not survive. As with swallow-wort outcompeting other native plants, the same also happens with milkweed. As milkweed populations decline, the monarch butterfly’s ability to procreate is negatively impacted.