In teaching classical, Clementsian ideas about ecological succession to undergraduates, we often describe how early successional species modify the habitat in ways that make the establishment of other species possible. Vitousek [15] points out that invasives can do the same thing to the ecosystems they invade.
In Hawaii, for example, firetree (Morella faya, formerly Myrica faya) is native to northern Macaronesia (the Azores, Madeira, and the Canaries), and it was probably introduced to Hawaii in the late 19th century by Portugese immigrants [9], and it is now found on all of the high islands. Firetree is a nitrogen-fixer, meaning that after it invades nitrogen levels in the soil are enhanced. Experiments summarized in [15] show that:
yr of nitrogen, which vastly exceeds nitrogen inputs from others
sources. Rainfall adds at most 5 kg ha
and native
nitrogen-fixers (cyanobacteria and decomposers) add only about 0.5
kg ha .
As a result of the substantial changes in nitrogen availability, areas invaded by firetree are more susceptible to invasion by exotics than other areas. Firetree is an ``ecosystem engineer.''
Badano et al. [1] show that a native cushion plant in the high Andes, Azorella monantha, facilitates the invasion of dandelion (Taraxacum officinale) and chickweed ( Cerastium arvense). The effect is especially pronounced at high altitudes. Survival of transplanted seedlings is enhanced at both the lowest (3200m) and the highest (3600m) elevations, but the effect is especially pronounced at the highest elevation (Figure 1).
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