Dissertation Abstract

MAN AND THE VARIABLE VULNERABILITY OF ISLAND

LIFE: A STUDY OF RECENT VEGETATION CHANGE IN THE BAHAMAS

(Reprinted from Dissertation Abstracts International)

Anthony Roger BYRNE, Ph.D.

The University of Wisconsin, 1972


Supervisor: Professor William M. Denevan


The subject of this dissertation is an analysis of the extent to which man has modified the woodland of Cat Island, a small island in the Bahamas. Cat Island was chosen for the study primarily because it provides an interesting contrast to remote islands such as Hawaii and the Galapagos. From Darwin's time onwards the vulnerability of island life has been widely recognized. Little is known, however, as to what extent islands vary in their vulnerability.

The methods employed in the research were basically of two kinds: analysis of historical evidence for vegetation change, and analysis of the contemporary vegetation.

Historical evidence was uncovered by archival and library research in London, England. It shows that Cat Island has been subjected to the same processes of change that have affected nearly all tropical islands: clearing and burning, grazing and browsing, and selective cutting. Unfortunately the historical evidence is not detailed enough to allow for anything more than a very qualitative reconstruction of change.

The analysis of the contemporary vegetation was carried out with three main objectives in mind. How long did it take the woodland to recover after clearing and burning? How significant were the consequences of other selective pressures, such as grazing, browsing, and the cutting of valuable species? And how important were alien species in the woodland?

In order to try and answer these questions three hundred sampling sites were established. In each, an estimate of the cover was made in 25 by 1 meter square quadrants. In total, 124 woody species were encountered. Each sampling site was unambiguously located on aerial photographs (1: 12,500) and classified according to age of vegetation, height of vegetation, substrate, soil, drainage, and distance from the nearest settlement. These data were then subjected to computer analyses.

The results were surprising in several respects. First, a great many native species are capable of quickly colonizing abandoned fields. Second, although selective pressures have brought about changes in the importance of many species, they have probably not led to any extinctions. And third, although a great many alien species have been introduced, only six were encountered in the sampling, and they accounted for less than 6 percent of the total cover. The conclusion is that the Cat Island woodland is inherently well adapted to withstand the types of disturbances introduced by man.

Several reasons are suggested for this. First, natural disturbances in the form of hurricanes, lightning fires, seasonal-flooding, sea level change, and coastal erosion have been part of the Bahamian environment for a geologically long period of time. Second the native vegetation contains few endemics. What is probably a high exchange rate of seeds and pollen between the Bahamas, Florida, and the West Indies must retard the evolution of new species. Third, the proximity of the Bahama Islands to each other, and to Florida, and the West Indies must have had an important influence on the evolution of plant dispersal mechanisms. All the native woodland species are well-adapted to dispersal by either birds or wind, and because of this the seed influx to abandoned fields is probably high. And fourth, the seasonally-dry climate and porous limestone surface have selected for drought resistant, pioneer-type species. As a result of all this, the native vegetation is inherently weedy, and man's impact has therefore not been as great as it might have been otherwise.

The most important conclusion reached in the study is that islands vary in their vulnerability. Islands such as Cat Island are very different in this respect from islands such as Hawaii and the Galapagos.

Order No. 73-7181, 311 pages.