QUESTIONS AND ANSWERS

Causes and consequences of exotic bird introductions to Hawaii.
Jeff Foster, Univ. of Illinois

Q:  Your data and results indicated that survival varied by altitude as well as by species. What do we know about altitudinal clines other than pattern or degree of disturbance? How do we know that other features associated with altitude (e.g., parasite or predator occurrences, climatic, physiological) are not equally or more important to invasion "success?"

A:  Other features associated with altitude could certainly be at play. We have all heard the mantra, "correlation is not proof of causation." Although dietary overlap and corresponding density declines in the species with the most overlap (in sympatry with the native) is suggestive, it is not definitive. I am currently looking at the distribution of Japanese White-eyes (and other exotics) through bird surveys from other islands in the archipelago to see if this is a consistent pattern when native competitors are not present at high elevations. This absence of a native competitor does not occur on Maui but does occur on the islands of Oahu, Molokai, and Lanai. If we still have evidence of declines at higher elevations, that would be good evidence of a feature associated with elevation being the effect, rather than competition. We still must remember, however, that if elevational effects are occurring, they must be manifested demographically through either lower survival or lower reproductive success. My study pretty conclusively shows no differences in reproductive success among elevations for most species and for species with differences, they do better, not worse, at high elevation. In fact, predators and most parasites are far more common at low and mid elevation than they are at high elevations (data from fellow Hawaii researchers). Physiological effects on survival certainly may be occurring and would be a direct effect of climate on demography.

Q:  How generalized or consistent are your findings in describing invasion or colonization patterns for avifauna? What are the major processes or variables that shape invasion success (particular to birds)?

A:  My findings were quite a surprise. We knew that exotic birds (and other taxa) can do well in native areas but had no idea they could do this well. It appears that once they have succeeded in establishing a beachhead, they have passed the most major hurdle to invasion. In establishing a population initially, pretty good evidence exists that the intensity of the introduction effort is the most consistent predictor of invasion success. As for colonizing new sites after initial introduction, my data are certainly suggestive that adult survival, and not high reproductive success, is a characteristic of the most successful invader. Yet, it also shows that high survival is not a prerequisite for success and that invaders can do well with only moderate survival and moderate reproductive success.