Invasive species represent an interesting challenge in basic ecology and a growing concern in conservation biology. Invasive species are now regarded as a threat to biodiversity second only to habitat destruction. Why certain species become invasive in their introduced range and why some native communities appear to be more easily invaded than others are central questions in the ecology of invasive species.
My graduate students have addressed these questions in five different systems. Tom Kennedy (M.S. 2004) was interested in oligotrophic stream systems in Florida that have become invaded by the aquatic weed, Hydrilla verticillata. Hydrilla invasion into these systems has been coincident with eutrophication, and Tom's thesis focused on how elevated nitrate levels changed the competitive ability of Hydrilla against species of native, submerged aquatic grasses. He tested his hypotheses in a series of mesocosm experiments conducted at the Blandy Experimental Farm.
Robert Heckman (M.S. 2008) also looked at the effects of nutrient availability on invasive species success. In this case, the nutrient inputs were natural, but the life histories of the plants potentially enabled them to intercept nutrients before they were available to the native plant communities that they had invaded. Garlic mustard (Alliaria petiolata) is an evergreen biennial invader of eastern forests. Because this herb is photosynthetically active during warm periods in the winter, it can potentially take up nutrients being introduced to the soil from the decay of fall leaf litter before these nutrients are available to the native spring ephemeral wildflower community. Similarly the invasive yellow bedstraw (Galium verum, pictured left) is a C3 perennial herb that begins its growing season much earlier than the native C4 grasses that make up the community it is invading. Robert conducted experiments to test whether it was able to intercept nutrients released from spring management burns before the nutrients became available to the native members of the community.
When plants are introduced into new areas, they are often freed from attack by their specialist herbivores and pathogens. The enemy release hypothesis suggests that the absence of these natural enemies allow introduced plant populations to grow unchecked. Recently some investigators have suggested that invasiveness is the result of both enemy release and evolutionary changes that occur in the exotic species within its introduced range. Relaxed selection on defense against natural enemies could favor genotypes that invest more in reproduction and competitive ability. The "Evolved Increased Competitive Ability" hypothesis (EICA) states that this evolved response plays an important part of the invasion process.
Jenica Allen (M.S. 2006) tested the EICA hypothesis with Spartina alterniflora, saltmarsh cordgrass (pictured to the right). This species is native to the eastern United States where it is the dominant species in many coastal marshes from the Northeast, through the Mid-Atlantic and the Southeast. It is fed on by two abundant specialist herbivores, Prokelisia marginata and P. dolens (Heteroptera: Delphacidae). Spartina alterniflora has been introduced to several areas on the Pacific coast of the United States (most notably in Willapa Bay in Washington and San Francisco Bay in California) and is now regarded as an invasive species in these areas. In San Francisco Bay, S. alterniflora has coexisted with its specialist herbivore P. marginata for most of its existence in the Bay. Most populations of Spartina alterniflora in Willapa Bay, on the other hand, have been without this herbivore for a century. This unique situation will allow powerful tests of the EICA hypothesis and its role in plant invasions.
Eric Elton (Ph.D. 2011) was interested in whether air pollution could create opportunities for invasive tree species. Ozone is a major component of smog and can be highly toxic to plants as well as people. Using a common garden containing a mix of common native and invasive trees, he showed that species vary considerably in their tolerance to ozone pollution with a series of fumagation and leaf chamber experiments. Using forestry models, he demonstrated that this variation can cause changes in forest composition if forests are exposed to high levels of ozone.