Two recent stories surfaced about invasive plants and how far reaching their impacts on ecosystems can be.
The first story comes from a recent journal article by Watling et al. In this article, the researchers were looking at bush honeysuckle and how invasions lead to a decrease in amphibian diversity (both richness and eveness). Basically what happened was the slightly cooler temperatures under the honeysuckle lead to the native green frog flourishing, which it then was able to outcompete the other native amphibians, leading to domianance by one species and lower diversity overall. You can find their research in Biological Conservation.
The second story came from NPR's 'Living on Earth' series and it was about invasive plants in Montana's grasslands. The stiff thatch of the invaders, such as spotted knapweed and leafy spurge, has provided structure for a couple of native spiders to build more and larger webs than they would be able to in native prairie. This has led to more insect prey being caught, allowing the spiders to reproduce more and thus build more webs and feed on more insects. They sum up their point very well by saying "The native spiders are thriving because of the new exotic plants. They can eat more insects, and these insects can then no longer keep the growth of certain plants in check, and on and on. You alter one piece of the ecosystem, and the whole web changes." You can read the transcript or listen to the audio of the story on the Living on Earth website .
Both of these stories really do illustrate a point that is often overlooked when considering invasive plants. Their presence in a new environment can have impacts that are unforseen and not restricted to direct competitors. With a cursory glance, one might even think these invaders are having a positive impact on their surroundings (both situations lead to an increase in the populations of native species), but a closer look reveals their ability to throw things out of balance and faciliitate a larger-scale collapse in diversity. I think the authors of the honeysuckle article put it best when they labelled honeysuckle as having the ability to be an "Invasive Ecosystem Engineer", which they describe in the below excerpt:
"Invasive species can have far-reaching impacts on ecosystems. Although some invasive species interact with native taxa primarily through one or few biotic or abiotic pathways (e.g., competition, allelopathy), habitat-forming invasive species may act as ecosystem engineers with the potential to affect many organisms through multiple different pathways. Although the impacts of invasive species are often framed in terms of trophic interactions between organisms (e.g., species that interact as competitors or as predators and prey), an emerging perspective emphasizes the ability of invasive plants to change habitat structure or quality, i.e., to act as ecosystem engineers. Invasive ecosystem engineers may have widespread effects on native species that do not directly consume or compete with the invader. Identifying these non-trophic effects is important because they may be pervasive, yet cryptic consequences of invasion, especially given the extensive realized and potential distribution of many species in invaded landscapes."